Solder Preform

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arena are primarily the ones on lead (Pb) in solders, that were formerly covered by section 8.a/ and 8.b/ of the old, outdated Annex II to Directive 2000/53/EC, and are now covered by this new legislation.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 (March 3rd) I start my 25th year at Indium Corporation.  It seems like just yesterday that the minor league hockey team I was doing PR for folded and I was forced to look for a new job.

I knew after the first week that I had made a huge mistake.  Going from a world of writing and promoting and ticket selling to day after day of science and chemistry and math couldn't be right for me.  But I needed the job so I stuck it out.  For 24 years so far!  I'll bet I am not the only one with that story to tell.
And it turned out that the move was right for me.  Because Indium is a great place to work with great, smart people but also because every day is different from the day before.  I knew nothing about Indium or indium (the metal) before I started here (other than they were one of the hockey team's season ticket holders and our sports intern's mother worked here).  But I have sure learned a lot since that fateful first week.  Especially about indium metal.  Terms like Solder Preform, Burn In, Thermal Interface and Hermetic Sealing have replaced road trip, slap shot and overtime.

Over the next few weeks I will share with you some of the things I have learned during my tenure here at Indium.  I hope you find them as interesting (and hopefully usable) as I do.

Shelf life means different things in different situations.  Post something on the internet and it will last forever.  Leave fresh fruit out on a hot day and it won't make it through the afternoon.

Jim Hisert recently wrote a blog post on Solder Shelf Life where he teams up with Eric Bastow to discuss Solder Shelf Life.  As they state, Solder Paste has a very defined shelf life because of the flux component.  The shelf life of Solder Preforms, on the other hand, is defined by the solder alloy's propensity to form oxides on the surface of the metal.

How do you minimize Solder Shelf Life issues?  There are several ways:

1) Order quantities that are reflective of your usage. It is attractive to get a large-volume price break, but you need to be able to use the product when the time comes.
2) Request that the solder preforms be packaged in quantities that you use them.  Getting a year's worth of preforms in one bottle may be cheaper, but the constant opening of the jar will only cause the remaining parts to oxidize and become unusable. If you consume 120,000 preforms per year, consider having your order shipped 10,000 pieces per month to assure a fresh supply of material.
3) Store preforms in their original, unopened containers, in a nitrogen dry box.
4) Once you have opened the jar, keep the lid on while it is at the work station.  At the end of the day, return the jar to the nitrogen dry box with the lid off so the nitrogen can purge the oxides that may have begun to form.
5) Consider tape & reel packaging.


 

预成型焊片，Solder Preform，是焊接材料中的一种。最近，Indium公司的美国技术经理Paul Socha的技术文章”Solder Preform Basics” （预成型焊片基础知识）, 在SMT杂志上刊登了。 很快在Indium公司的技术文章下载专区里，也可以下载到。

焊片，通常可以给锡膏印刷的过程起到加锡的作用。除此之外， 如果某些大的元器件焊接需要定量的锡膏，或是通孔(through-hole)的元器件脚的焊接需要比较多的锡膏，焊片都能够定位定量的提供焊接材料。

焊片的外面，还可以含一层薄薄的助焊剂，flux-coated preform.  助焊剂的含量通常在3%或以下，能够有效帮助清除焊接表面的氧化物。

预成型焊片，在北美很多的军事/医疗/航空航天等精密元器件的焊接，都有广泛应用。 Indium公司能够提供各种尺寸，形状，和金，或是包装的solder preforms, 我们的技术团队，更是一直致力于为大家解决焊接/工艺问题，提供最佳的方案。

Cheers! 

Pic: Indium Corporation

 
PS: 很幸运，生活中此后的新篇章我都将和小帆一起写：上个月感恩节，我们订婚了。
 

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.    

Ignoring the solder selection as part of your design process is risky business. 

As Terry Costlow, the IPC online editor of EMS Now noted in an article ‘Controlling the Explosion of Lead Free Solders’, the choice of the right solder alloy can affect the manufacturing process, the cost, and the field performance of the product.

Initially it was thought that the move to Pb free solders was just a matter of changing reflow profiles but major issues such as tin whiskers, brittle intermetallic layers and other concerns soon pushed solder selection into the forefront.

With over 200 published alloys and over 300 custom alloys shipped each year, we have seen the need for considering the solder design first.  Before you settle on a solder you have to consider:

·         Surface metallizations

·         Operational temperature of your product or device

·         Form of the solder you want to use (solder paste, solder preform, solder wire, etc.)

·         Temperature of subsequent soldering steps

·         Thermal coefficient of expansion

·         Tensile strength

And these are just a few of the considerations.  Let us help you make the right selection.  Contact us at: askus@indium.com.

Feel free to discuss solder selection with our industry professional, Dr. Lasky on November 11^th, IPC is having a materials conference: Engineering for Compliance in Irvine, CA.

0402 or 0603 solder preforms deposited in paste for pin in paste assembly

Pin-in-paste is the technique of intrusive soldering through-hole components to a circuit board using reflow soldering.  Typically, solder paste can be printed and this will provide enough solder for complete barrel fill.  Occasionally, more solder is needed than can be printed, and solder preforms can be used.

Phil Zarrow and Jim Hall of ITM Consulting have a recorded discussion forum called "Board Talk" in which they discuss common board assembly issues and solutions.  One of the topics they have discussed is called, "Through Hole Reflow - Pin in Paste."  Here, they discuss the typical techniques used for soldering through-hole components to a circuit board.  These include printing solder paste around the through-hole in the board, inserting the through-hole connector, and reflowing the assembly in a convection oven. 

They mention that one way to apply more solder is to overprint the solder paste.  For many applications, this is an excellent technique.  The overprinted solder paste will wick into the barrel during the reflow stage.  This is a natural process because the over-printed area is typically a non-solderable solder mask surface outside the plated through-hole annular ring.  The solder de-wets from the solder mask and surface tension pulls it into the pool of solder, onto the annular ring, and down the plated barrel of the board.

Occasionally, overprinting solder paste still does not provide enough solder to fill the through-hole.  On these occasions, a good alternative is solder preforms.

There are a couple types of solder preforms to consider.  The first is a standard 0402 or 0603 size preform supplied in carrier tape.  These preforms are 100% metal compared with solder paste, which is only approximately 50% metal by volume.  To incorporate preforms, print solder past around the through-hole.  Place a preform just outside the annular ring, but touching the solder paste.  Just like overprinting, the solder preform supplies extra metal, but this time, the metal from the solder preform will wick down the through-hole barrel and provide much more solder than the overprint was able to.  The flux in the solder paste will be enough to remove oxides from the preform, so no additional flux is needed.

A second type of preform to consider for the most difficult-to-solder connectors are integrated preforms.  These preforms are custom-designed for each connector type and include an array of washers.  One washer will fit over every pin.  Flux is applied and the connector is inserted.  The assembly is reflowed in a convection oven, and more solder is available to fill the barrel than could be supplied using solder paste at all.  This is the most manual technique, however, may be the only option to achieve a complete barrel fill without re-designing the board.   

I'm excited about the May 2009 issue of SMT magazine, which will be highlighting photovoltaic soldering applications.  Not only does it combine soldering and PV (which is our niche), but it will be including an article written by Karl Pfluke of the Indium Corporation.  (Abstract shown below)  Karl's one of my personal heroes, he's a veteran technical engineer and leader in solar applications.

Photovoltaic Stringing in Solar Cell Module Assembly

By Karl Pfluke

Indium Corporation

"As more and more contract manufacturers (CMs) look for opportunities to diversify and fill factory space, photovoltaic (PV) solar cell module assembly seems to be the popular choice.  Commonly referred to in the solar panel industry as the "back end process", the assembly is done in lines and has some similarities to SMT. 

Solders, solder pastes, solder wire, solder preforms, and fluxes are used for interconnects in PV assembly.  Since RoHS and WEEE initiative does not apply to this industry, many manufacturers use SnPb solder for interconnects, Sn60 and Sn62 being the most popular. However, SnAg is occasionally used, and some manufacturers are exploring the use of SAC alloys (SnAgCu), specifically SAC305.

One process, "cell stringing" or "tabbing", uses products quite familiar to the SMT industry.  Tabbing ribbon, a precisely straight and flat piece of copper that has been coated with a tightly controlled thickness of solder, is used to connect multiple PV cells. 

This article discusses the common solders, reflow technologies, and materials used in PV cell stringing for solar module assembly."

Nice work Karl!

~Jim

The Insulated Gate Bipolar Transistor (IGBT) module is rapidly taking over from MOSFET technology as the solid-state power switch of choice. Die-attach to the direct-bonded copper (DBC) substrate is often done by using a solder paste, or occasionally a specially-shaped solder preform.

Prime drivers for the IGBT die-attach solder assembly process are:
- High thermal and electrical conductivity
- Ultralow voiding (<0.5%)
- 100% cleanability
- Pb-free

This obviously means a wise choice of materials (substrates and solder paste) and both the correct reflow equipment (vacuum soldering is typical, to ensure low voiding) and a 100% effective cleaning process before wirebonding, are all key factors in optimized assembly.

The Indium8.9-LDA solder paste has been developed over the last year, in association with industry leaders in the soldering equipment, cleaning and module-manufacturing fields, to meet the specific needs of global IGBT module manufacturers.
We have both the material, and the data to prove its usefulness.

Yet another example of Indium Corporation leading the Pb-free charge in the Power Semiconductor industry.

Cheers!
- Andy

Seth Homer is a Product Support Specialist for Engineered Solder Materials.  He’s like a Green Beret or Navy Seal of solder preforms.  He is another contact here at Indium that you can use to make your soldering application more successful and efficient.

You can read more about Seth here: http://www.indium.com/blogs/Engineered-Solders-Blog/bio/.

Here’s what he has to say about preforms for die attach applications…

Jim:  You have a trained eye for solder preform applications, how often do you see opportunities where a customer can improve their process by using a different form of solder?

Seth:  Although my world is engineered solders, and I am somewhat bias, I have yet to visit a customer who has no interest or need for a preform. The advantages are many….

Repeatability in solder volume

Repeatability in flux percentage in situations where flux is required

Ease of placement where automation is a possibility (Tape and Reel or Waffle pack)

In situations where paste is the current process, switching to a preform offers a stronger joint, a cleaner joint (less flux), and less voiding due to flux residue.

In situations where a solder joint needs to be fortified, a small preform added to the paste offers increased reliability (Solder Fortification)

Jim:  Strong, clean, void free solder joints are especially important for high power die attach applications.  Aside from equipment changes, what can customers expect if they change over to a preform application from a paste application?

Seth: That would depend on a lot of things; in all honesty there are some applications where paste is the best option.  [Like a drop-in replacement for polymer die attach materials, die attach paste utilizes the same equipment set.]  Any process/application change will demand optimization, but once those variables are addressed, I think most of our customers will see yield increases as well as reliability increases.

Jim: One important concern for an engineer switching from a dispensed material to a solid (preform) material is packaging and presentation to die-attach equipment.  Can you give me a rundown of packaging options for solder preforms for HVM production and prototyping?

Seth: In HVM situations, automated placement is a must. We have two primary options for HVM packaging for an automated process. Tape and reel, or waffle pack. Tape and reel offers high speed placement, flexibility, high volume packaging on a compact reel, and an environment which prohibits damage to individual parts. Waffle pack offers many of the same benefits of tape, but lends itself better to manual placement if an automated process is not needed.

Jim:  If our readers are interested, what is the best way to get started evaluating preforms?

Seth: The best way to begin is to define what you want to achieve, and then bring your request to me or one of our application engineers. This gives us an opportunity to understand what you’re trying to achieve, and make suggestions to help if needed. This will make the transition to an engineered solder much smoother, and rewarding.

Seth can be reached at shomer@indium.com

最近在做我们Indium公司的产品在LED应用上的市场调研分析。哈哈，又一次大长见识了。  

对了，公司的同事Amanda有她个人关于TIM的blog: http://www.indium.com/blogs/TIM-Blog/index.php

Pic: Jordan Ross with Indium Corp.

An inexpensive way to get intricate solder preform prototypes is to cut them from solder ribbon or foil.

I just got off the phone with a number of material suppliers and I feel like I have hit a wall.  I want to order a few custom machined metal blocks to fit into a piece of my test equipment but the quotes I am getting back are far more expensive than I am willing to pay since these are one-time use samples.  To make this evaluation possible, I am going to order standard solid blocks of metal.  I will ask my machine shop to form the blocks into the special shape that my project requires.

This technique is a larger scale of what I encourage others to do when they are in the process of evaluating preforms.  Simple geometries are not typically a problem, however the set-up for an intricate preform design can be quite expensive.  If you are unsure whether the preform will work in production, it is a good idea to test some prototypes.  An easy way to do this is to order a solder ribbon and cut the sample preforms from it.  This will speed up the time it takes to get the samples, and will typically be less expensive.

The application note on practical suggestions for solder preform design has some other tips for inexpensive solder preform prototypes.

The process of applying a solder thermal interface material typically includes applying flux to a solder preform and reflowing it in a sandwich between two substrates.  When a preform comes flux coated, its a no-brainer how to apply the flux to the preform. ItҒs already done for you.

There are so many great types of flux available for specific applications, that you might want to use a specialty flux formulation matched to the substrates you are soldering to and the reflow temperatures you will be running.  This flux formulation is commonly a Tacflux and the key to good flux application is not to overdo it.  A little bit goes a long way.

Ok, so it happened again. Another urgent request was brought to me for action. This time it was a customer who had performed a Red Dye Penetration test on a Ball-Grid Array (BGA) that was attached to a board using our paste. A single BGA on a single board was the evaluation (Sample size is a WHOLE other topic of conversation!) The picture is one that I took of the pad that showed no wetting. There are a number of causes of why the paste did not wet only these two pads. Including poor paste transfer, BGA contamination and board contamination. Paste transfer is certainly an issue that is at the forefront of every BGA issue. The first issue that I think of is a contaminated stencil. If their cleaning process were incomplete or sloppy, then this would definitely cause some issues. Especially if the stencil apertures were not completely cleaned out or if the solvent used was not completely removed. Solvent left in the apertures when paste is introduced would wreak havoc on reflow. But, in this case there was sufficient paste printed, because the analysis of the solder joints confirmed that the spheres of the good joints were the same volume as the suspect BGA solder joints. Had the paste had issues with transfer efficiency, those spheres would be comparably undersized. As with Head-in-pillow, the solder spheres on the component may also play a role in the situation, where again, the increased silver content or higher than normal oxide contamination would hinder good wetting. But, would this have shown itself as non-wetting to the pad? Probably not, but rather as head-in-pillow, or a "cold" solder joint. So, where does that lead us? Directly to board manufacturing and storage, or direct pad contamination. Board manufacturing issues would show itself, I believe, more widespread than just two localized pads. Especially since these are regular production boards. This is the same for poor board storage, where an oxide layer had may build up on the pads. But, again, it would probably be more widespread. Therefore, my money is on contamination. We have all been to production facilities, and very rarely do I see any of the operators using gloves. In fact, at one facility, where they were placing solder preforms by hand, we brought up the idea of using gloves. Their defect rate dropped to <1%. As a matter of fact, we do not permit the wearing of silicone bracelets, as the silicone has shown to rub off and contaminate some testing. The silicone actually prevents the wetting of the flux and solder! Even our body's natural skin oil is an inhibitor to wetting. Which is exactly what I believe happened here. More information may be found at Online Help: Indium Knowledge Base.

Have you ever had to hand-place solder washer preforms on the pins of a connector and found it to be very labor intensive? Indium Corporation has a product called InTEGRATED Preforms which enable the user to place multiple preforms at one time in less than 15 seconds.

InTEGRATED Preforms are joined in a matrix by fine, precise strands of solder which, during the soldering process, melt and flow to adjacent pads to give complete preform separation.

In addition to time saved, the quality of the joint is assured because only one preform is delivered to each joint eliminating the chance that a pin will be missed or two washers placed on the same pin. Each scenario will create a quality problem that reduces the yield and increases the time spent on rework.

Unique preform designs and complicated configurations can simplify difficult soldering jobs. An example is a thru-hole connector with multiple rows of long pins that are difficult to reach in the center with hand soldering. InTEGRATED Preforms eliminate the need to reach these remote areas. The connected washers can be placed on top of the board, under the connector. After reflow, equal volumes of solder are delivered to each of the pins in the connector.

In order to get the full benefit of InTEGRATED Preforms they must be uniformly fluxed on both sides including the connecting strands. Also, they need to be flat in the application and uniform heat must be used to reflow them.

Many printed circuit boards have mixed technology. The surface mount components can be placed in the paste first followed by the thru-hole components using InTEGRATED Preforms. The board can then be reflowed and cleaned once, eliminating time consuming steps.

To learn more about this time saving product patented by Indium Corporation, please contact the Applications Engineering Staff at Indium Corporation.and we would be happy to design an InTEGRATED Preform to fit your solder requirements.

For additional information regarding reflow methods using InTEGRATED Preforms, check out the June 2007 issue of SMT Magazine.

Engineered Solders have a long history at Indium. They have been called many things that made sense to us because of the way we made them. But when we stopped to think about the way our customers were using them, we realized the most important part was the ENGINEERING part. Companies call us with a problem and we help them ENGINEER a solution by creating a specific shape of solder (preform) or a length of solder (ribbon or wire) that will help them.

So as another outlet to reach people with soldering questions, we will be using this blog to address the latest topics in electronics, medical, aerospace, defense and other markets that rely on soldering and bonding for their products.

The team that will be contributing to this blog has nearly 60 years of combined experience with these products, so we hope it will be useful to you. We hope you enjoy the blog and we look forward to your feedback!