Epoxy Flux

Folks,

Many people have been infatuated by the price of gold in recent months, but the price of silver has also skyrocketed. In 2000 silver was about $3.00 per troy oz. In the eight years that followed, its price grew to $15/oz. Today it is trading at over $41/oz! This price is almost an all time high, except for the time when the Hunt brothers tried to corner the silver market in 1980. The aberration of their efforts jolted the silver price to just short of $50/oz, but it settled down to $11 or so after the Hunts came under margin call and other pressures.

Unfortunately, the dramatic price increase today, does not appear to be an aberration. Although we may hope that it will soon drop to more historic levels, we may not have reason to expect that it will.

Although not as dramatic, tin and copper have experienced significant prices increases as well. The price of tin has doubled in the last year to $15/pound and copper has increased from about $3/lb to $4.50.  These metals are obviously key ingredients in critical electronic materials such as solder pastes, solder bar, and solder preforms.

In addition, oil, which is used for most organic electronic materials such as PWB resins, flip chip underfill, and epoxy fluxes, has increased to $110/bbl - approaching its all time high of $145/bbl.

All of these price increases have a significant impact on the electronic materials supply chain. Although we are used to price decreases in the cost of our mobile phones and PCs, at this point in time, the price of the materials that go into these devices will be increasing.

As one materials supply chain executive commented at APEX, “It’s not like we can be clever and somehow work around the price increase of silver and these other materials, we have to pass it on to our customer, or go out of business.”

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

This week a customer in Asia asked why one of our new epoxy fluxes was not allowing the package-on-package (PoP) device to be picked up from the dipping tray. Obviously, the vacuum nozzle must have sufficient force to extract the PoP package from the PoP flux reservoir (yellow, below).



Those of you who know me also know that I am always trying to reduce things to numbers so, naturally I got thinking about how I would model this from a physical viewpoint and came up with the following:

If the downward force (weight of component plus tack force of epoxy flux) is greater than the upward force (air pressure on the bottom of the component), then the component could not be extracted from the epoxy flux. The figure shows the different variables. Expressing this mathematically, this comes out, in SI units, as:

Downward force = m.g + n.Ft.pi.(d/2)^2

where Ft is the tack force in units of mass per unit area, taken from the maximum tack force determined by the Solder Paste Tack Test from J-STD-005, ANSI/IPC TM 650:2.4.44

Upward force = 101000.A.pi.(D/2)^2

where A is the measure (fraction) of atmospheric pressure and denotes how good the vacuum is (zero vacuum is 0.0atm : hard vacuum is 1.0atm).

There are some uncertainties with this approach: How does the vacuum vary across the nozzle diameter? Does the 5mm diameter probe used in the IPC test equate to a complex CSP (chip-scale package) bottom surface, with many rounded solder bumps or solderspheres? And so on. But, at least the model puts us in the right ballpark. Just to give you a feel for how this works, the second figure shows some results. Note that scenario (iv) is the only one showing problems (negative force balance).

The data implies that you are only likely to see an issue with inability to pick up components from a dipping flux tray if either:

• Components: Heavy (thick / large)
• Vacuum Nozzle: Too small a diameter and/or the vacuum is weak/poor
• Flux: Very tacky (high tack force)

For many of the newer applications, component sphere/bump immersion to just deeper than the bump height (say 100-110%) is desirable. If the customer dips the whole bottom of the component into a standard (non-epoxy) flux, this potentially opens up a lot of issues including reliability (SIR; electrochemical migration); component displacement (skewing) during reflow; as well as difficulty in picking up the component from the tray. The solution to this series of issues, is to choose either a standard flux with a high pre-reflow SIR, such as our PoPflux 30B, or a low-volatile content epoxy flux.

I'll have more to say on epoxy fluxes in a couple of months, as we are currently nearing the end of extensive testing at several customers in Europe and Asia.

Conductive epoxy is a common material choice for bonding components, especially if the assembly process is temperature-sensitive. Tin-based solder paste or preforms with flux are preferred Pb-free bonding materials; however, conductive epoxies arguably provide advantages over these traditional solder assembly materials. 

It has been my experience that these advantages are perceived in the absence of an awareness of the full solder assembly materials product offering. Specialty solders can provide the same advantages as conductive epoxies and then some.   

Some claimed advantages to conductive epoxies include:

·         RoHS-compliance

·         Ease of assembly

·         No-clean

·         Low cure temperatures

Low-temperature solders such as 58Bi42Sn and 52In48Sn are specialty low-temperature solders which have these same properties including processing temperatures below 150ºC. Both of the referenced alloys are Pb-free, can-be used with no-clean fluxes and are assembled using the traditional solder assembly techniques.

It would seem a toss-up between whether to use a conductive epoxy or specialty solder to assemble temperature-sensitive components except that there are additional advantages to a soldered assembly as compared with an epoxy-assembly. These include:

·         Thermal cycling reliability

·         Solder material consistency

·         Reworkability

·         Thermal Conductivity

After you've reflowed solder in contact with a flux, you're always left with a certain amount of flux residue. There are no clear industry guidelines on how you refer to the residue, and new terminology is emerging all the time. If you leave it up to me, here is what I recommend : 

1/ "No clean" flux residues:

- Standard Residue:  >40%
- Low Residue (LR): Between > 10% to 40%
- Ultralow Residue (ULR): Between >2% to 10%
- Near Zero Residue (NZR): Between 0 to 2%

Each % is given as the weight percent of flux residue after a real reflow process, and refers to the fraction of the raw flux, or flux component of a mixture (such as solder paste or metal-filled epoxy). Note that the exact amount of residue will vary with the reflow profile; the mass of flux or solder paste studied; and the rate of gas flow over the sample material, as well as secondary factors, such as the oxygen level in the reflow atmosphere.

Thermogravimetric analysis (TGA) is a pretty poor method for determining post-reflow residue levels. Results from the use of a platinum TGA sample cup with nitrogen flowing over it have been found in our testing to vary significantly with the mass of sample present, probably because the headspace in the cup acts as a "dead zone" for entrapment of vapor: TGA may therefore give artificially high % residue readings, compared to the results on a flat leadframe or other substrate.

From the viewpoint of a standard semcionductor assembl process, now consider the situation of a low-clearance direct chip attach "flip-chip" or package-on-package application, where the flux is essentially entrapped in a "cage" of I/O's, sandwiched between two flat diffusion barriers. As well as issues of flux residue, this also raises the question of how the electrical properties of the flux will be affected, if more of the solvent and other volatiles from the flux are trapped in the residue.

2/ "Water-soluble" (same principles apply for "Solvent cleanable") flux residues:

- Water-soluble: Residues can be truly dissolved in water to leave a transparent liquid: the color of the this rinse liquid is immaterial,
- Water-dispersible: Non-transparent rinse liquid with any hint of translucency or turbidity

I know that the differences here will be very dependent on rinse-water quality and temperature; chemistry of any cleaning agents; stage of bath-life and so on, but to my mind, if the rinsed liquid is not transparent, then the solids from the flux must be suspended as fine particulates. These particulates usually have refractive indices different from the bulk liquid: the result - turbidity. There may be a means of bath-life end-point determination by turbidity or dynamic light scattering (DLS) or a similar technique; possibly in combination with the standard refractive index measurement that is most commonly used.

In conclusion, note that ULR and NZR fluxes are showing increased usage in flip-chip applications, since these types of material interfere less with the curing of underfill polymers. NZR fluxes are becoming critical for copper-pillar bumping applications.

Just my thoughts - let me know what you think.

Cheers!   Andy

Pictured here are thermal profiles for a product packaged in different ways and allowed to warm up to room temperature

To keep your shipments cold, silly…

Aside from the short answer, it is actually a complicated subject dealing with thermal dynamics, shipping logistics, and even a bit of psychology!

Cold packs are used to increase the thermal inertia of a shipped product, not to cool a product.  These thermal ‘shock absorbers’ are used to delay the effects of shipping paste, flux, or epoxy.  This can really help on those hot summer days when a shipment arrives and sits on a dock unclaimed for a few hours.  A sudden peak in temperature like this is called a thermal excursion, and can impact the performance of some materials in severe instances.  Cold packs are used on material shipments that may never see thermal excursions.  Why?  The piece of mind that solder materials will arrive in the best possible condition is worth it to us.  It may be the best thing we can do short of actually hand delivering it to you.

It is only natural to wonder how much of an effect the cold packs have during shipping.  The short answer is that cold packs do help delay the thawing of solder pastes and other materials - although results vary with different packaging sizes, and duration and magnitude of temperature excursion.

It’s okay if a flux of paste arrives at room temperature, and that is what is expected for long distance shipments.  We hope that a paste, flux, or epoxy does not sit out in the sun for the better part of a day, but in that event the cold pack is used to minimize damage to the material.

Michael Qiu of Indium Corporation

Michael Qiu supports Indium Corporation in Eastern China.  I thought an interview with him would be a good chance to see the semiconductor industry from a different perspective.

Jim: How long have you been involved in the semiconductor industry in China?

Michael: I have been in the semiconductor industry in China more than three years. Before that, I spent four years learning IC design, IC manufacture and Package in a University in Shanghai.

Jim: What semiconductor materials are most popular for your customers?

Michael: For some semiconductor package customer, silver epoxy is very popular applied to do die attach. As for high power device, paste/SSDA is used popular for die attach. But for some advanced customers who do BGA, FC and WLCSP business, flux or fine power paste is required.

Jim: What trends do you see in semiconductor package manufacturing in China?

Michael: For [cost sensitive] customers who do Diodes/Transistor package, the manufacture cost will be more and more important.

BGA/FC/CSP package share will increase soon in the near future and more factories will be able to do those packages.

More advanced package like SIP (system in a package) and MCM (multi-chip module) will [become] popular, but this may take several years.

Jim: What topics would our customers in China like to see in the Semiconductor Packaging Blog?

Michael: Our customers always hope to get our professional technical support. They prefer the blog on a particular application or issue. For example, comparison between In and AuSn in LD die attach and use dilute hydrochloric acid to remove oxidation layer on In preforms.

Jim: What is the most exciting thing about the semiconductor industry to you?

Michael: A chip which is smaller than your nail has so many functions.

邱学丞：一个比你指甲还小的芯片能拥有如此多的功能，这太迷人了。

This is the shape of flux deposits left after pin transfer

Spheres attached after pin transfer

Pin transfer is a way of selectively depositing a semi-solid or liquid material (like a solder paste or ball-attach flux).  It is commonly used to apply flux to BGA (ball-grid-array) pads to promote subsequent solder sphere attachment.  Pins are dipped into a reservoir of material where the pins are coated with flux, paste, or epoxy.  Next, the pins are lifted out of the material reservoir and placed down onto the BGA pads.  A portion of material that traveled on the pins sticks to the pads as the pins are lifted away.  As archaic as it sounds, this method is quite repeatable – and used extensively in semiconductor packaging.  This method of application is used because it deposits flux very quickly, and can compensate for changes in substrate height.  

Pin transfer fluxes are specially designed with rheological characteristics to help optimize the amount of material that is picked up and placed on the pads.  Other materials can also be pin transferred.  Pin blocks are used to transfer solder paste and epoxies in a range of applications.  Sometimes only one large pin is needed to transmit flux, this is called flux stamping.  Stamp transfer has the benefit of being simple to set up because there are few parameters that need to be adjusted.

Just in case you haven’t heard of epoxy flux yet, let me fill you in.

Epoxy flux is what I call a “super no-clean” flux. A no-clean flux is designed to entrap unused activator in a semi-solid residue. With epoxy flux, that residue is an actual epoxy – just like underfill. The advantage of this is the compatibility with underfill. Epoxy flux isn’t supposed to be used alone, it is used in place of a normal flip chip flux. After soldering with epoxy flux, you still encapsulate the flip chip with capillary underfill as usual. The capillary underfill can attach to the epoxy flux residue much better because of the similar chemical nature of both materials.

Do you have a 2' x 3' area on the wall of your office or cubical for the latest new and improved Indium Corporation Wall Chart? Click here to email us your name and address so we can send you one.

We have updated the Table of Specialty Alloys and Solders, added new tables for fluxes and improved the graphics. And yes, the periodic table is still there, as well.

The wall chart is a ready reference for a quick check of the melting temperature of an alloy or one of its unique properties. The chart will help to compare different properties of alloys to narrow down your solder selection.

Indium Corporation is a quality supplier of preforms, Integrated Preforms, Informs, wire, ribbon and foil, as well as, solder pastes, liquid fluxes and epoxies. If you need assistance you can contact an Indium Corporation Applications Engineers anywhere in the world. Our location information is on the chart. Call us to discuss your application and we will help you determine which form of solder will give you the best performance in your application. We have very talented engineers that help dozens of companies each week with their soldering questions.

In the upcoming weeks, we will be discussing topics such as:

The proper alloy for the metallizations
Flux selection
Methods of reflow
Engineered solder packaging
Ways to improve quality and uniformity in your process
Step soldering
Indium as a cryogenic seal, as well as, vacuum applications.

Paul A. Socha
Principal Engineer
Engineered Solder Products

Part of having a hobby is to learn as much as you can about that area of interest. Most hobbies have a ‘glass ceiling’ when you discover you are at the point where your research just isn’t yielding the answers you want. At that point you have a chance to learn about things no one has explored yet.

One of my hobbies is also my job. Unlike most hobbies – here at Indium we have access to all types of the latest soldering materials. Spheres, fluxes, epoxies, equipment, paste, and components are all available. Don’t get me wrong, every test I do here has a defined purpose, but I am free to uncover the answers that my customers are looking for (sometimes before they even ask).

Hopefully this blog can be a place where I can share some of these experiences and gain insight from you as to where future efforts should be directed. Let me know what interests you, because that’s what is important to me!

The word Creep is not something an engineer wants to hear when designing a metal TIM into their Laser Diode Stack up. But is creep really an issue, and do we really understand creep when talking about indium? The answer is relative. Attached you will find a stack up diagram I made of a typical Diode stack. This could be a laser application or an LED application. The Die Attach layer is usually a high temp solder or a silver filled epoxy. Can indium be used at this level? The answer is yes, if and only if the temperature of the junction is far away from the melting point of pure indium, which is 156C. Soft solders are just that, they are soft, and in die attach some customers have used indium at this level, but it is not nearly as common as other higher temp solders like AuSn, Sac alloys or SnAg. Silver filled epoxies are really getting better and better but there are some issues with their conductivity and their process. When using a solder as a die attach the die itself can float or move during reflow. In this case some kind of mechanism can be used to hold it in place during reflow to ensure that its alignment is perfect. So to answer the question, solder can be used as die attach in this application, but the alloy chosen will really determine how effective it is and how reliable it is.

In the case of TIM2 (thermal interface material level 2), there are a few more considerations here. Let us first assume that we are going to reflow at this level. Copper at the heat-spreader level will not be a problem, but Nickel at the spreader/sink level will be a problem. Aluminum will also be a problem here. The problem is that these materials are hard to solder to, but it can be done. A high activity flux such as Indiums RSA or Flux number 3 can be used to break the oxide layer that will be present. However a layer of gold on the surface will help assure soldering will be effective. Indium recommends not to exceed 50 micro inches of gold, and recommends that the thinner the better, usually 10 micro inches will do it. Indium the element will actually dissolve the gold or other wise known that the gold will diffuse into the indium. During soldering an Indium/Gold inter-metallic will form. This is a brittle layer and if too much gold is used can induce reliability issues and cracking of the joint. So back to our original question; why would you use indium here and can you use indium here? This is the most common area where creep of indium can be an issue. Creep can be acceptable however. Indium will not creep to the degree that it will come out like pump out or like play-doe. The degree of creep is related to the pressure that is put on it, ie: CTE movement or direct pressure from clamping, as well as the temperature that the interface sees. If the junction temp is less that 20 degrees of the melting point and some movement is allowable, such as in an LED application, this is acceptable. However in a laser application, Indium Corporation usually advises that we do not go with pure indium at this level and rather choose an alloy such as Indium Silver or Tin Silver. The more Silver you add to the alloy the harder the material will become. Consider this. Pure Indium has a conductivity of 86W/mk, mp=156C Eutectic and a tinsel strength of 273 psi. Add 3% silver and the MP goes to 143C Eutectic, the conductivity goes down to 73 but the tinsel goes up to 800 psi. Better yet, add 10% silver and the MP is now plastic from 143 to 237, conductivity goes down to 67 but the tensil goes up to 1650 psi. Then consider SnAg which has a melt point of 221C a conductivity of 33W/mk and a tensile of 5800 psi. What is the best for your application? The answer lies in what is acceptable to you, if CTE is an issue go with less silver, if temp is an issue look at no indium, if conductivity is an issue look at high indium.

When considering a compressible metal at the TIM2 level, the issue now at had is how much pressure you have, the temp of the junction, and the planarity of the surface. Obviously with a compressible interface you no longer need any gold, and there will be no issues with indium in direct contact with nickel. However, copper and indium can form an inter-metallic over time; however the oxide layer on the copper usually keeps this from happening. In fact in our thermal lab we only saw this happening when we actually baked the modules for over 1000 hours at 125C+. Even then the phenomena was nominal. If you do not believe you will rework the interface 4-5 years after its construction I would say that this will not be an issue, actually it will improve the thermal performance and reliability. Rework within 1-2 years will not be a problem. Many of our customers already use indium at this level as a compressible interface but few are aware that they can actually improve the performance if they convert to a Heat-Spring ™. The Heat-Spring is a patented process that allows us to decrease the contact resistance of the metal if the pressure is at least 50 psi. This allows the stack up to use a thinner bond line thickness and improve the thermal performance of the Metal Thermal Interface. So what about creep at this interface? Once again altering the alloy can eliminate the chance of this happening, but converting from a standard indium flat foil to a Heat-Spring will further decrease these chances because your bond line is usually significantly less if you use a Heat-Spring. (On average about .003").

In Summary, the things to consider when using a Metal Thermal Interface or Die Attach Solder are as follows:
• What is the working temp of the interface?
• Is this temperature too close to the melting point of the Metal Thermal Interface?
• Can the device handle the reflow temperature of a higher temp solder such as Gold Tin or Tin Silver
• Is the thermal performance of the interface an issue so if you change the conductivity of the metal thermal interface by decreasing the indium content, you detriment the conductivity of the entire stack up? For example going from 86w/mK to 67w/mK.
• Is creep really an issue? If your device can accept a small degree of movement there is no issue to use indium. If even a slight issue will cause a problem, suggest an Indium Alloy and not pure indium.

In the end, Indium Corporation is here to help you. Please see our web site for additional information including our e-list of alloys to help you choose the best Metal Thermal Interface Material.