Die Attach

Maria Durham, Indium’s new Technical Specialist in Semiconductor and Advanced Assembly Materials, has been doing some research on indium lead (In/Pb) solder alloys. We chatted about her findings this week. 

 [Andy C. Mackie: ACM] Which indium/lead solder alloys are most common, and what are their properties?

[Maria Durham: MD] Firstly, the use of lead-(Pb-)containing solders in some soldering applications is restricted due to local environmental and RoHS compliance, but there are still many applications where they are  allowed. Many military, aerospace, and industrial equipment uses, as well as many applications related to vehicles, are exempt. The table below shows the most common indium/lead (In/Pb) alloys (pink) and their properties, sorted by liquidus temperature; the higher of the two melting points (solidus and liquidus) seen for non-eutectic alloys. In blue are three comparison materials.

Indalloy 205 is the most commonly used, probably because it has the closest liquidus temperature to the tin/lead eutectic (183°C), 63Sn/37Pb (Indalloy 106). This means it can be reflowed using a standard Sn/Pb eutectic profile. The next most common alloys that are used are Indalloy7, 204, and 206.  Besides the melting range, indium has comparable thermal and electrical conductivity to standard materials.

[ACM] What makes indium-lead (In/Pb) solders so attractive, and why have we seen a recent resurgence in their usage?

 [MD] One main attraction to using indium/lead (In/Pb) solder alloys in soldering to precious metal surfaces is that, unlike tin-containing solders, they do not leach gold. That is, gold does not dissolve in them to any appreciable extent. During discussions at Semicon West in 2011, one of our California customers reported going through 8 simulated reflows with Indalloy 205 in contact with a gold surface with no loss of joint strength and no joint embrittlement. That is pretty impressive. Note that embrittlement is often caused by gold-intermetallic formation. It has been noted that even at 250°C, 50In/50Pb dissolves Au at a rate 13 times slower than it does into 63Sn/37Pb, although this, of course, is a kinetic, not a solubility limit, study.

The higher melting Indalloy 164 (92.5Pb/5In/2.5Ag) has the lowest coefficient of thermal expansion (CTE) of all of the In/Pb solders and is able to withstand the higher temperature excursions that can be seen in step-soldering type applications (where a very high melting solder is used to form the first joint, followed by a next lowest melting alloy, and so on). This is seen in applications such as power electronics assembly, where the first step solder is often used for die-attach either as a solder paste, wire, or preform. The high melting point helps the solder withstand the operational temperatures associated with under-the-hood electronics, in applications such as engine control modules, where Indalloy 151 (92.5Pb/5Sn/2.5Ag) or Indalloy 163 (95.5Pb/2Sn/2.5Ag) are most commonly used. In/Pb solder is excellent on very rigid structures such as ceramic-to-metal or ceramic-to-ceramic. The desired solidus / liquidus temperature range can be adjusted by changing the indium:lead ratio, making it very easy to “dial in” the alloy to a specific reflow process.

Another attraction to using In/Pb solders is that they exhibit good fatigue resistance in thermal cycling from -55°C to 125°C.  In testing, the 50In50Pb solder joint fatigue life is about 100 times greater than that for 63Sn/37Pb.

 [ACM] What fluxes are used in these applications, and how are they formulated differently?

 [MD] The fluxes most compatible with the lower melting point (<200°C) indium-containing solders are NC-SMQ-80 (solder paste) or the lower-tack TacFlux® 012 (suitable for use with wire, preforms, and spheres). These are no-clean fluxes, specifically formulated for lower temperature reflow.  Under appropriate low temperature reflow these fluxes leave behind benign residues that do not need to be cleaned off (“no-clean” flux), although they are often cleaned off in most practical applications, usually to ensure reliable wirebonds absent of flux spatter.

 To learn more, please contact us.

 Cheers!  Andy

最近有一家供应商突然中断提供了银锡焊接材料，所以有些客户朋友们马上向Indium公司资讯。还好，Indium一直为大家提供银锡材料。

Sn96.5Ag3.5是221°C的共晶材料。这种银锡材料很早以前就被分层焊接(step soldering)和在汽车工业上使用(automotive industry ), 是最早的无铅材料之一。Dr. Ron Lasky的“Happy Birthday RoHS”博文中有更详细的介绍。

SnAg银锡合金有以下几种特点：

• 高热传导性：High thermal conductivity (33W/mK)
• 低的张应力（ tensile stress ）： 5800psi
• 良好的热循环性能thermal cycling：-55 到125 ⁰ C
  

Cheers,

Acknowledge: Dr. Andy Mackie with Indium Corporation; Dr. Mackie's blog post "Tin/Silver Solder Paste in Die-Attach (Sn/Ag)"

Pic: Google Image

• High thermal conductivity (33W/mK)
• Higher melting point than SAC alloys (221C)
• Low tensile stress, so suitable for large die (5800psi)
• Excellent thermal cycling properties (-55 to 125C)

1.  Preform (a specially-shaped solder piece) with TACflux® used to hold the preform and die in place
2.  Solder paste, which holds the die in place with no extra materials added 
3.  Soft solder die-attach wire, a fluxless type of solder wire, which is melted onto the substrate metallization under an inert cover gas, and the die directly mounted onto the molten solder pool, then allowed to cool.

At the time of writing, the price of silver (Ag) was approaching the USD$50/tr.oz. (Troy ounce) level, and threatening to go higher. With 1 Troy ounce being 31.1grams, this makes the cost of pure silver ingot close to USD$1.60/gram.

Image from goldsilveroz.com

Materials costs are therefore a major consideration for anyone using silver in any form. Naturally, we are now seeing a few Power Semiconductor packaging houses evaluating the possibility of moving away from silver-filled epoxies for die-attach. The alternatives they are considering include the adoption of solder paste (or solder in some other form: wire / ribbon / preforms) versus a silver-filled epoxy.

Here are some thoughts on the Power Semiconductor assembly pros and cons, based on using solder paste as an alternative to silver-filled epoxies.

Good news (+)

+   Reduced materials costs
+   Improved pot-life / shelf-life *
+   Improved high temperature thermal-cycling
+   Strong, metallurgical joint formed between leadframe (substrate) / joining material / die
+   Improved thermal conductivity
+   Faster throughput (more units per hour, UPH)**
+   Easy clean-up ***
+   Does not wick onto NiPd surface to cause poor wire bondability

 * Although it is true that solder pastes are stored under refrigerated conditions, they do not require the -40C storage that is typical of silver-filled epoxies. 

 ** The dispense of solder paste is very rapid and can be done using multi-dot dispense heads. It undergoes rapid temperature reflow, versus the slow cure needed for metal-filled epoxies, which can be up to typically 1-3 hours, depending on the volume of silver epoxy.

 *** Because the solder paste flux does not cure like a polymeric material,  tubing and other conduits for the solder paste are easily cleaned out using common solvents, or can be simply purged with flux.


  ==================

Bad news (-)

-   Capital costs #
-   Adoption time / new process learning ##
-   Needs a solderable die surface
-   Voiding increase ####

 # The main cost-drivers here are:

- Reflow: Specialty reflow equipment is required for high temperature solders, such as
Heller or BTU reflow ovens

- Cleaning: If wirebonding is required after the reflow process, standard cleaning equipment and cleaning chemistry (aqueous or solvent-based) will be needed to remove flux residues

- Gas: Forming gas (H2/N2) or simple nitrogen may be needed to assist reflow.

Note that increasingly, for clip-bonding (non-wirebonding) applications using the new ultralow residue solder paste Indium9.32, even cleaning may not be needed, as the residue has been found to be compatible with compatible with a number of molding compounds in the industry.

 ## By partnering with a company like Indium Corporation with many years of experience in die-attach soldering, the ramp-up time can be significantly reduced.

 ### A solderable surface is usually a sequence of Ti / Ni / (Ag or Au) plated layers. The thickness of the silver (Ag) or gold (Au) precious metal layer is usually limited to 100nm (0.1microns). Compare this to a standard silver-epoxy bond line thickness (BLT) of 0.5-2mils (12-50microns).

 #### Acceptable voiding of less than 5% of the total die area is fairly easily achieved with good quality substrates and die-finishes.

  ==================

In closing, I am indebted to my friend and colleague Sehar Samiappan (Indium Corporation Area Technical Manager - South East Asia) for his insights.

Contact me to discuss this further.

Cheers!   Andy

A quick trip to discuss roadmapping with one of the world’s top processor manufacturers, and a visit to discuss Pb-free power die-attach materials, left me with a few hours to spare at LAX.

This time around I was trying to work out how much package-on-package (PoP) solder paste we would expect to see for a waferlevel CSP (WL-CSP) or a BGA dipped to half height. The need for some deep thought was driven by a customer who asked at what point a PoP dipping paste needs to go from a type 4 to type 5, 6, 7 and so on (however you define them), based on the PoP/CSP pitch or ball diameter. Good question.

To start with, in order to get consistent quantities of paste on each sphere, the PoP paste metal loading needs to be well below the point at which rheopectic behavior can expect to be seen (that is, much less than 50% by volume of solder powder metal). By doing this, you pretty much guarantee a “monolayer” of solder paste powder particles (radius r) coating the CSP or BGA sphere (radius R). Figure 1 shows the kind of result that is typical for a good paste: in this instance our halogen-free PoP paste Indium 9.88-HF.



Figure 1: 0.4mm pitch CSP with PoP paste

If the metal loading is too high, even at time zero, you will start seeing large variations in the amount of PoP solder paste adhering to the surface of each sphere (bump), even on adjacent spheres: the small amount of paste that is picked up during the dipping process adheres to the main solder sphere in uneven clumps. This is why standard type 4 printing solder pastes just don’t work in PoP applications: not only is the particle size too big – the rheology is all wrong.

If R>>r, then a reasonable first order approximation is that you can treat the sphere surface as planar and so model the number of solder particles based on a series of hexagonally close-packed particles (Figure 2 gives the definitions).
 

Figure 2: Definitions for the PoP paste dipping process

Using the same model of solder powder particle size as in the discussion on waferbumping paste, you can calculate a couple of potentially useful things:

i/ The maximum number of solder powder particles on each solder sphere (bump)

ii/ The mass of solder paste adhering to each soldersphere

The first (i/) is useful for establishing the inherent variability due to the finite size of the solder powder, and I’m going to suggest another Mackie rule of thumb of a minimum 150 solder powder particles per solder bump, based on the maximum allowed particle size (diameter). The table below gives  the result of this rather simplistic analysis:



Table: Effect of Package Bump Diameter on Solder Paste Type Needed

A 400micron bump should therefore be fine even with a type 3 dipping paste, whereas a 200micron bump will need a type 5 paste.

I look forward to someone proving this wrong. The second (ii/) is helpful, because we can easily use it to test the theoretical mass of PoP dipping paste against what we actually find. Note that this is just simple geometry: it doesn't tell us how much paste is really needed to resolve issues such as the 60 - 90micron bowing we are hearing about from our customers, even with the more rigid PoP packages currently available.

Cheers!  Andy

最近小忙，少讀書了，也少和大家分享了；不過工作之餘，翻看了一下《A Seat at The Table》一書，覺得裏面有些道理也蠻有啓發的。比如説此書中一直圍繞這個主題來展開了論述“Today, the only thing your customer cares about is value.”

就這個觀點，再對照一下Indium公司的兩個主要系列產品：

²       電路板組裝焊接材料(Solder Materials):  這裡也要分產品而論。對於技術含量較高，工藝使用要求較多的焊錫膏(Solder Paste)材料，重視成品可靠性的客戶們會更多的關注產品帶來的“價值”。 如果只圖便宜的材料，但是用起來“錯漏百出”的，最後還是事倍功半：返工，復修，廢棄率高（特別是浪費貴的不能翻修的板子），產出率低，總體成本也自然高了。 對技術含量較低，工藝已經“模式化”的產品，像錫棒(Solder Bar)，錫綫(Solder Wire),  性价比會更關鍵……在目前日益高漲的金屬原材料市場中，Indium公司考慮到客戶們的成本壓力，也推出了性能可以和SAC305錫棒媲美的有成本優勢的Sn995錫棒。

²       半導體封裝材料(Semiconductor Materials):  整個半導體行業應該算是一個高成本，高投資，高回報（運營得好的話）的三高行業。半導體封裝材料也像是其中的經絡血脈吧，連接各個部分，讓整體最後順暢無阻的工作。半導體各個部分的材料都不便宜，設備更是不菲；對材料性能的表現要求和驗證都很嚴格，畢竟都投資那麽多，不能“功虧一簣”嘛。所以客戶們一般會十分重視產品的價值。 Indium 公司目前提供的半導體材料有：Wafer Flux, Wafer Paste, Micro Spheres, Flip-Chip Flux, Substrate Paste, Ball Attach Flux, Die-Attach Paste/Wire, PoP Fluxes, etc. 

Indium公司還為大家提供散熱界面材料(Thermal Interface Materials)，工程焊料(Engineering Solders)，薄膜光付太陽能板製造材料&太陽能板組裝焊接材料(PV Solar Materials)，和銦金屬及其化合物等。 這些材料使用在比較領先的應用中，新興行業，或是細分市場中，客戶們都十分重視產品和服務能給自己帶來的價值。

Cheers!  

Pic: Indium Corporation

PS: 前些日子看了中央4台的《第三屆漢語橋在華留學生漢語比賽》，感慨不已！除了感嘆這些留學生們對“那麽難”的漢語的精湛掌握，對中國文化和歷史的了解，甚至對中國的熱愛；更感慨的是，這些活動也説明了祖國的強大！現在越來越多的留學生們來中國學習，想進一步了解中國，和中國人民交流；中國話也在慢慢傳播到全世界！以前中國學子們苦讀英語，考TOFEL, 雅思，GRE什麽的；現在金髮碧眼的學生們也在場上比拼誰更了解我們的“四書五經”了，哈哈！

My friend and colleague, Eric Bastow, got back last month from an IPC standards meeting with some interesting news for those of us who supply and use solder paste. Here I’m talking about everything from standard SMT printing and Power Semiconductor die-attach solder paste (type 3 and 4) down to PoP and waferbumping solder pastes (type 5, 6, 7 etc). I had heard that there were some changes in the way the powder types are categorized and wanted to learn more. Here is what we discussed:

[Andy Mackie] What is the current status of solder powder “type” designations from the new IPC J-STD-006B (Oct 2009)?

[Eric Bastow] In the original J-STD-006B (Oct 2008) and its two amendments, a solder particle size distribution (PSD) table, Table 3-1, was included as part of the standard to define the different powder “types”: 3, 4, 5 and so on. However, this table has been removed from the version published exactly 1 year later (October 2009) and also, somewhat confusingly, called J-STD-006B. This latter standard refers the reader to the old J-STD-005 (Solder Paste) for powder type determination by PSD, tables 2A and 2B.

[Andy Mackie] So how are solder powder types currently (May 2010) defined by the IPC? 

[Eric Bastow] The responsibility for defining the powder size distribution for the respective types now goes by default to IPC Task Group 5-24b, which maintains the J-STD-005 and its amendments and associated documents. This standard and its amendment were created in the early 90’s, and then published in January 1995, when even type 4 paste was uncommon at best, so its relevance now in the second decade of the third millennium is rather questionable, particularly given the enormous changes in solder powder manufacturing methodology and analytical characterization that have occurred in that timeframe.

[Andy Mackie] I understand that there are even some concerns about the test methods used to define the PSD.

[Eric Bastow] Yes, very much so. It is interesting to note that the original J-STD-006B Table 3-1 recognized that “Types 5, 6 and 7 are shown as general industry accepted size ranges for development purposes. Current listed methods for measuring these particle sizes may not be accurate enough for exact size and range distribution”. That initial sentence is very revealing about the tentative nature of these “type” definitions. These same concerns were raised at the J-STD-005 meeting at APEX in April 2010, and I also raised issues about the relevance of the test methods (see below) that were in use.

Note that none of these addresses the possibility of pure solder powder being the sample.

[Andy Mackie] How did you and Indium Corporation drive the Solder Paste Task Group (5-24b) into the next phase?

[Eric Bastow] We realized that using 15year old test methods and standards for solder powder based exclusively on extraction from solder paste would raise serious concerns with our customers. As a start, Indium Corporation suggested round-robin testing amongst the various solder powder suppliers. The testing will involve the use of the in-house measurement techniques of those suppliers on representative powder samples from each of those suppliers, to see what sort of data scatter is observed. We helped the task group to recognize that defining the particle size distribution of the various types, especially the finer types, does not make much sense without first determining a reliable and repeatable method of measuring the particle size.

Once that is complete, we can begin to define what we mean by each powder type, and also if there is a need for such “hybrid” categorizations as type 4.5.

[Andy Mackie] Eric: thank you, and please keep up the good work.

===

The interesting thing is that it will not affect the way Indium Corporation supplies or manufactures solder powder and paste materials according to our customers’ needs: just how we define them.

Cheers!   Andy

Solderspheres or solder spheres, or even solder balls: whatever you call them, Indium Corporation has been making them for years and has rightly acquired the reputation for doing whatever it takes to meet our customers' unique needs.

Unique Alloys:

Hard to find alloys (like multipart alloys; low-melting alloys and even gold/tin (80Au/20Sn)) are our bread and butter. As "Indium Corporation" it should be no surprise that we lead the world in our ability to supply low-melting indium-alloy solder spheres, as well as other forms of these alloys, such as engineered solders or solder pastes.

Unique Quantities:

We don't want you to buy more than you absolutely need. If you just want 100 spheres, we can easily do that: if you want more - we can do that, too. But remember that, because each customer's need is unique, our prices may be higher than our competitors, especially for more standard alloys. Some customers also have unique inventory-control needs, so we work with many customers to ship on-demand by retaining a buffer stock of spheres here at Indium.

Unique Sizes:

Our current dimensional capabilities as of this writing are from 80microns to 0.062inches, or even bigger. Generally, the bigger the sphere - the less spherical it is (within the limits of surface tension and viscosity), and we can't control the laws of physics, so instances where a very large amount of solder is needed, a preform may have better dimensional control. Also, notice that we won't ask that you order in a specific unit of diameter measurement, like the mil or the micron or the millimeter: we're a global company - just tell us what you need.

Unique Packaging:

Often needed for more delicate alloy spheres, we can offer specialty overpacking that eliminates oxides from the atmosphere around the solder spheres, essentially stopping oxidation in its tracks. It's the same technique we use to package our soft solder die-attach (SSDA) wire: a technique that showed that the very reactive wire was still "as new" 3 years later. We also offer spheres in tape & reel packaging (see image) for 24mil, 35mil and 62mil diameter spheres.

Unique Tolerances:

Just as a case in point, a MEMS customer of ours had a need for a low-melting indium-alloy solder sphere with a tolerance of +/-5microns (+/-0.005mm) for a sphere with a 350micron diameter. That demands a tolerance of just over 1% - pretty demanding, but we did it.

Our standard tolerance is +/-1mil (1 thousandth of an inch, or 25.4microns), but as you can see, we have the capability to go to much tighter tolerances using three proprietary manufacturing techniques.

Other Needs:

We are also seeing people asking for doped-alloy spheres; low-alpha emission solder spheres and other things that we could never have dreamed of...

So please just let us know what you need. We'd be happy to help out, and if we can not do what you ask - we'll let you know why.

Cheers!  Andy

I mentioned in a previous a blog posting that the primary driver for halogen-free electronics is ostensibly environmental, but that the confusion about “which halogens and which molecules and what level?” has seemingly decoupled the laudable desire for an improved environment from the reality and made it more of a marketing tool. All this notwithstanding, there remain some instances where the performance of the final product itself can be directly impacted by the presence of halogens, usually as ionic halides. This is the reason why Indium Corporation recently developed what appears, at first glance, to be an odd combination: a high-Pb (high-lead) alloy halogen-free die-attach solder paste, Indium9.72-HF.

The halogen-related failure mode for die-attach solder pastes is the corrosion of wirebond pads on the topside of Power Semiconductor die which are soldered to the leadframe with halogen-containing solder paste. Many manufacturers producing high volumes of identical power devices may also use die-attach (sometimes called “soft solder die attach”, SSDA) wire to attach the die to the leadframes in a fluxless process, but many manufacturers prefer the inherent flexibility of a solder paste-based process for medium mix / medium volume applications.

Long term blog readers will recall that I did a posting on solderspatter (a.k.a. soldersplatter or soldersplash), and that it can be caused by bubbles of solvent vapor or moisture outgassing from solder paste deposits during reflow. In bursting, the tiny flux droplets or solder particles from the surface of the bubble can be propelled quite a distance (several feet). While solder on wirebond pads is clearly a failure from a reliability viewpoint, certain wirebond pad metallizations may also be subject to corrosion from flux. A poorly maintained reflow oven may also drip flux condensate (usually in the exit – cooling – zone), and this too can be a cause of organic materials on wirebond pads.

As long as the bondwire is gold, and wirebond pads are covered in a uniform layer of gold, there is no problem (as long as the flux residue is washed off) since gold is unreactive, even in corrosive environments. Aluminum (Al) or aluminum/silicon (Al/Si) bondpads, however, are potentially reactive. Halogenated materials, such as fluxes and overmolding compounds may react with them to either reduce the wirebond pull strength and/or increase the wirebond junction resistance, leading to localized heating and subsequent thermal-related joint failure. Even covalently-bonded (C-X, where X is a halogen) materials may dissociate at high temperatures: which is how the banned brominated flame retardants work, of course.

The biggest danger of halogenated flux corroding wirebond pads is when:

1/ Completed assemblies (between the reflow process and the cleaning process) are left for a long time before cleaning; particularly if they are exposed to high humidity (high %RH) before cleaning.

2/ The cleaning process is inadequate: either due to poor selection of the cleaning solution, or poor bath maintenance, or inadequate “scrubbing” energy being imparted to the surface to be cleaned, or simply if inadequate time is allowed for cleaning.

Note that even optimizing 1/ and 2/ may still lead to bondpad corrosion.

The Indium9.72-HF paste is available in both type 3 and 4 powder, in the standard high-Pb alloys, Indalloy 151 (92.5Pb/5Sn/2.5Ag) and Indalloy 163 (95.5/2Sn/2.5Ag), and for larger die that need a higher reliability joint, we also offer the Indalloy 164 (92.5Pb/5In/2.5Ag). A Product Datasheet is available for download, of course.

Cheers! Andy

I love watching a good basketball game, and one of my favorite local teams is the Syracuse Orangemen. If you go to a Syracuse home game, notice the shot clock – it was made with Indium Corporation solder. There are a lot of places you can see our products in your everyday life. That smart phone in your pocket, the electrical components in your car, the thermal interface in the computer in front of you. That’s one of the things that makes this job rewarding, being part of so many various applications.

In addition to learning about these different applications, we also get a good reference for what assembly trends are developing, and which material technologies are becoming more popular. 

I’ve watched the halogen-free trend explode and fade, as it was adopted by some large OEMs and their contract manufacturers, but has not spread to most other companies. Another trend that is fading away from the spotlight is Pb-free die-attach solder, since the EU has not found a suitable replacement and has pushed back the exemption deadline. 

A long-existing topic that has had recent mention is solder jetting. The trend towards soldering smaller components is not new or surprising, but for smaller components (01005s and 0201s) we have seen a trend towards dispensing instead of jetting – which seems to suit those applications.

For small component printing, transfer efficiency is critical. Outside of solder paste optimization, “nano-stencil” technology is an upcoming technology that may take-off and improve paste release characteristics. Solder paste is being used in some other creative ways too, like low temperature alloy dipping paste for rework operations. Manycompanies are now using or evaluating specialized solder applications to replace components without fully reflowing the rest of the components on the board.

Integrated preforms are finding their way into more and more applications recently as well. These connected preforms are being used to reduce the need for component pallets and selective soldering operations.

All these applications are great ways that our customers are taking soldering technology to the next level, using materials and assembly methods that were not common before. I look forward to learning how you’d like to use solder in your application!

Those of you have been watching this blog for a while will know that I’ve been keeping tabs on the status of the European ELV (End-of-life vehicle) legislation on lead (Pb), mercury (Hg), cadmium (Cd) and hexavalent chromium (Cr^VI). It’s been both galling and heartening at the same time, to find that when I Google “elv legislation”, this (my) blog keeps coming up as one of the top 10 sources on the subject. OK: enough of the bloggy, solipsistic prevarication...

My friend, Geert Willems of IMEC late last week let me know that the EC (European Commission) had given its final decisions on Annex II ("the exceptions"), and pretty much adopted the recommendations of the Öko Institute from their 127 page report of September last year (2009). I have to say my hat is off to Dr. Otmar Deubzer of IZM and Stéphanie Zangl of Öko for the very thorough and logical background to this legislation.

The decisions that affect those of us in the semiconductor (flip-chip) and power semiconductor arena are primarily the ones on lead (Pb) in solders, that were formerly covered by section 8.a/ and 8.b/ of the old, outdated Annex II to Directive 2000/53/EC, and are now covered by this new legislation.

A quick visual summary of the legislation relevant to lead (Pb) in electrical interconnects is given below, and please consult the original document for confirmation, as I may have missed some subtlety of the legalese in my quest for brevity. Also, frankly, subsection 8 (b) led to some Transatlantic confusion over whether finishes on pin connectors and PWB's were covered(?), but I think the below is correct:




Refer to the table below for the timeline for of each subsection/exception:



Note that the last review of exemptions was carried out in 2009, with potential effect by 1/1/2011. This implies that the legislative hammer will potentially fall on each of those usages slated for future review on January 1st two years after the review year. Lead (Pb) for most electronics attach usages of interest to those of us in semiconductor and power semiconductor packaging may therefore be "legislated out" by 1/1/2016.

Basically, the use of Pb-containing solders in solder paste, die-attach paste, die-attach wire, solder preforms, and thermal interface materials (TIMs) in automotive electronics assembly is safe for now, and changes will not be forced on the automotive electronics assembly industry at a time when even current manufacturing practises may be leading to still-unresolved safety incidents.

Cheers!  Andy

Today I made my rounds in the office, collecting ideas for you from our tech guys  - ideas to help you speed the alloy and flux selection process.  The team gave me ideas from the start of the design process all the way up to speeding the order process, and all the steps in between.  These are solder basics, but they can help you get your process up on its feet quicker - if you put together a little information up-front:

1) Call a tech guy early, but be prepared by knowing the specifics of your material needs, like powder size, flux type, and any design requirements.

2) If you’re an engineer specializing in component attachment, get yourself involved with the component or board design team. It may mean extra meetings, but it will save many headaches in the long run after you help the team remember the meaning of “design for manufacture”.

3) Define the details of your application, equipment, and process before selecting a material. For instance, knowing the needle size that you will be utilizing in a dispense machine will speed the powder size selection for die-attach solder paste.

4) Be aware of cleaning requirements and your current in-house cleaning equipment and chemicals before choosing a flux or flux vehicle.

5) Understand the operational temperature of your assembly and the maximum processing temperatures of the components. This will make alloy selection much faster.

6) Don’t get hung up at the ordering process – know what size packaging you need. Do you have equipment that only fits a certain size syringe or cartridge?  Knowing this ahead of time will save you a second call to verify while talking with an Account Specialist.

7) For alloy compatibility and metallurgical considerations, be prepared to lets us know the composition and thickness of your surface finish. This will also save a second call, because it is required information in order for us to get you the right alloy and the perfect flux for your application.

8) For solder paste printing recommendations, know the specifications of the stencil you will be using. Aperture size, stencil thickness, and any other dimensions you can provide will help guide which flux vehicle and powder size we will recommend to you.

9) For preform selection, try using thinner preforms. For prototype situations you can stack the thinner preforms to build solder volume, and it is much quicker to order preforms in 1 thickness as opposed to many thicknesses.

10) Understand your process bottlenecks. By letting us know your material needs we can usually suggest a few materials, but perhaps one of those materials can help eliminate a problem that is slowing your process down.

11) Consider your company’s roadmap for the next 5 years. It doesn’t make sense to select a material and need to select a new one only a year later. Save yourself the time involved in a second solder evaluation and know what the future holds regarding safety/environmental concerns. Likewise, understand the roadmap of your supplier, their future materials, and how their current materials will fit your company’s future plans.

Andy Mackie: What role does gold play in protecting surfaces in SMT and semiconductor assembly processes?

Lenora Toscano: Gold does not form an oxide; it protects the nickel from oxidation or passivation. A clean nickel surface has very high solderability for most solder types, but its oxide is very difficult to remove with standard flux types. Also, gold dissolves almost instantaneously into most solders during assembly, thus promoting superior wettability.

Andy Mackie: What standards exist on the thickness of gold for different electronics and semiconductor assembly applications?

Lenora Toscano: The main application of ENIG (electroless nickel/immersion gold) coating is in chip-on-board (COB) technology, the typical thickness of the immersion gold layer on the HDI substrate being 3-5 micro-inches.

Edge connectors typically require the use of hard gold. Acid gold deposits are used for compliance with MIL-STD-275, which states that gold shall be in accordance with MIL-G-45204, Type II, Class 1. The thickness shall be 50-100 micro-inches, typical thickness is 30-50 micro-inches on 150micro-inches nickel.

On the other hand, for solderable surfaces, typical thickness is 5-15 micro-inches on 150micro-inches nickel.

For wire bonding, in general, gold plating of a minimum of 30 micro-inches on 200 micro-inches nickel works well. Soft gold is generally preferred. Soft gold processes are also used for boards designed for semiconductor chip (die) attachment. These qualities comply with Type I and III of MIL-G-45204.

Andy Mackie:  What are the differences between gold layers deposited by immersion gold and electroplated gold processes?

Lenora Toscano: There are five main differences:

1. The coating thickness is different. Immersion gold is a displacement reaction, gold displaces the nickel on the surface, and is self-limiting as the nickel surface is coated with the immersion gold. Common baths cannot produce thicknesses of much more than 10 micro-inches, while with electroplated gold the thickness depends on current and time. The higher current or longer the plating time the thicker the gold coating.
2. The structure of the gold deposit layers is different. Electroplated gold is denser that the naturally porous immersion deposit.
3. The hardness is usually different. Electroplated gold often has other metals introduced into the plating that make the deposit harder.
4. Porosity is different. Immersion deposits have more porosity that electroplated deposits; it is the nature of the plating system.
5. Deposition composition (purity) varies with additives in the bath. Immersion gold baths contain gold as the only plated metal, while electroplating systems may introduce small amounts of other metals.

Andy Mackie: How thick does gold have to be to fully protect the underlying surface, and what are the trade-offs as customers attempt to reduce their gold costs?

Lenora Toscano: Per IPC-4552 ENIG specification, 1.97 micro-inches is the recommended minimum at +/-4 sigma from the mean, with 3 – 5 micro-inches being typical.

The immersion gold deposit is porous by definition. It does offer very good protection to the underlying nickel, but over time the porosity of the deposit results in the passivation of the nickel surface and the wetting forces will be reduced. Of course, this process should take years to occur, but if the gold coating is too thin (below the minimum requirement), it will occur sooner and affect the solderability. 

Andy Mackie: What advantage does gold have over silver or other metals?

Lenora Toscano: Again, gold has good tarnish resistance and solderability after storage because it does not form an oxide or hydroxides, so it is unaffected by temperature and storage conditions that might reduce the shelf-life of the other finishes. It meets requirements for lead-free (Pb-free) assembly while offering a coplanar surface that is both solderable and aluminum-wire and gold-wire bondable.

Gold has good electrical conductivity, and produces a contact surface with low electrical resistance. Electroplated gold is also an excellent etch resist.

Electroplated silver is not widely used in the printed circuit industry. Under certain conditions or electrical potential and humidity, silver will migrate along the surface of the deposit and through the body of insulation to produce low-resistance leakage paths. Alkaline cyanide baths for silver electroplating are highly toxic.

Immersion silver is susceptible to problems if not correctly stored and even packaged. Packaging materials that contain sulfur or allow exposure to air will result in tarnishing of the surface (sulfide, sulfate, and chloride formation). High levels of surface contamination can detrimentally affect solderability.

• 焊接点牢固 
• 十分抗腐蚀
• 特别高的热传导性能
• 与别的贵金属相兼容
• 出色的抗热疲劳特性
• 优异的润湿性
• 高熔点（2800 C）

Eutectic Gold Tin (AuSn) with a composition of 80Au20Sn is a unique material.  This particular alloy of gold tin (AuSn) is considered a solder because it has a melting temperature of 280ºC, which is lower than the 350ºC transition temperature into braze materials.  Still, there are some similarities between this solder alloy and braze alloys. The most obvious is the hardness of the gold tin (AuSn) alloy. With a tensile strength of 40,000PSI, this solder is much more rigid than the tin solders most are familiar with. The strength is more closely compared to the silver brazes which melt above 500ºC. 

With that strength has come some unique manufacturing difficulties. For many years, one obstacle for implementation of gold tin (AuSn) as a solder preform or wire, was its availability in thin forms or fine diameters. The gold tin (AuSn) is extremely hard and it became brittle as it was handled through manufacturing and would crack if it was pressed too thin or fine.  

Luckily, in the 40+ years since eutectic gold tin (AuSn) was first used in electronics manufacturing, processing techniques have come a long way.  Today, gold tin (AuSn) solder can be made into dimensions much smaller than the soft solders, allowing it to be used in applications which require the highest level of precision.

Typical dimensions and tolerances of gold tin (AuSn) can be found in the below chart.

This chart as well as more detail on gold tin (AuSn) applications are available in the paper titled, “Process and Reliability Advantages of AuSn Eutectic Die-Attach,” presented at IMAPS 2009.    

Materials to be used in packaging of high power semiconductor devices are often chosen by their coefficient of thermal expansion, or CTE. For instance, substrates such as AlSiC, Molybdenum, and Tungsten are chosen to mimic the coefficient of thermal expansion (CTE) values of the materials they will be attached to so as they expand and contract, the substrates do so in tandem, minimizing the mechanical stresses at the interfaces between these areas, or their CTE mismatch.

The coefficient of thermal expansion (CTE) of indium does not match many materials, yet it is chosen commonly as a solder thermal interface material between substrates with as dissimilar substrate properties as silicon and copper. 

How can indium bond together silicon with a CTE of 2.6PPM/ºC and copper with a CTE of approximately 17 PPM/ºC, then undergo years of thermal and power cycling, and not show degradation of thermal performance?

The answer is in the strength and malleability of indium. Indium is the softest metal which is stable in air. Although the CTE of indium is 29 PPM/ºC, the tensile strength of indium is 273PSI, which is very soft, and the shear strength of indium is 890PSI, which is significantly higher. In an application where indium is soldered to a back-side metallized die and a copper integrated heat spreader, there is significant CTE mismatch. 

However, assuming the interfaces of these solder joints is sound with minimal voids, the bulk indium will bend and stretch along with the contraction of substrates and will not crack.        

Dr Jennie Hwang

I recently had the opportunity to discuss several issues in Pb-free die-attach and other solder applications with Jennie Hwang PhD, DSc, and world-renowned consultant in solder and  electronics assembly processes. 

Cheers! Andy

Figure 1: 15% Voiding with air reflow

Figure 2: ~0% Voiding after vacuum reflow

Figure 3: Multiple voids

While at the Semicon West 2009 show in July, I had a chance to sit down with Herr Klaus Roemer of Pink GmbH. PINK is most famous in the die-attach and power module manufacturing world for their reflow ovens with vacuum, but are also known in the medical and aerospace industries for manufacturing extremely high precision, one-off, vacuum equipment for applications as diverse as particle-accelerators for ion bombardment, and large-volume chambers for helium leak-detection. I asked him some questions about Pink vacuum soldering technology.

Cheers! Andy