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Those Darn Plastic Packages

Our plastic packages are not just made out of plastic. The “plastic” part is epoxy resin, and it is plastic in the sense that it can be molded and formed while hot and under great pressure to define any shape.

The resin shrinks potentially tens of percents as it cools and cures in the mold and remains somewhat plastic, creeping over time in response to mechanical stress. To keep the package dimensions stable, about 50 percent of the plastic's volume is filled with silica particles, essentially quartz bits. Into the resin are also mixed optically dark pigments to keep light from affecting the chip's electrical behavior, and other stabilizing agents.

Unfortunately, silicon has piezo-sensitivity: junction voltages, betas, thresholds, and all manner of characteristics shift with mechanical stress. Even the silica particles can cause very large but very local stresses when they bear directly down onto the die's surface. The silica chunk size is about that of an individual transistor, so a random distortion of matching can occur if a particle touches a matched device. We trim our op-amps at wafer probe to as low as 10uV offset voltage, but when packaged the offset blossoms to a few times that value, and randomly. Worst, the superbly low temperature coefficient of offset or other parameters demonstrated in die form are greatly degraded by package stresses. To solve this, we attempt to trim all the parts we can after packaging, especially references. We also use various die coatings to attempt to mollify the stresses.

There is this war going on between engineers and the packages. There are a few mold compounds and die-attach epoxies called “low stress.” Well, the low-stress compounds are somewhat gentler, but not by much.

A hazard is that the high temperatures of soldering can de-laminate the plastic from the leads, die attach paddle, and die. This is not too problematic for non-precise components, but de-lamination will shift the stress that the package places on the part, causing calibrations of the die to shift. We fight this by adjusting die paddle dimensions carefully so that there is enough plastic wrapping around the die paddle to anchor itself as a unit, and we sometimes put features in the die paddle and leads to help the plastic adhere. In the course of characterizing a part on its way to production release, we also subject it to a few package reliability gauntlets. Even though the package usually has little to no de-lamination, it does shift slightly when heated or cooled dramatically, the effect we call “thermal hysteresis.”

The worst thing you can do is load up a package with moisture and then solder it to a board. The plastic used throughout the IC industry absorbs water from the air. Yes, really. If a part is exposed to the ambient in a humid location for several days it will absorb moisture, and when soldered the water quickly turns to steam and can, in the worst case, pressurize and crack the plastic. We experimentally expose parts to an 85% humidity atmosphere at 85C for a couple of days, then perform a solder equivalent heating. We use an ultrasonic method of visualizing delamination, then determine what moisture level the part must be handled at. The weakest packages are shipped with desiccant to keep them dry before soldering. The most robust need no accommodation at all.

The industry has got the physical reliability procedures down pretty well, but we engineers in precision work still have issues. For one, it has become clear that what the customer will perceive as long-term drift of an accurate part is mostly just variations in humidity. We have determined that our references drift only single ppms per 1000hr. when humidity is controlled, several times more than when not. We've checked competing parts with similar results (although our references don't seem to go for random walk drifting, and those of some other manufacturers do, slightly). This is verified by placing parts in ceramic packages with solder die attached. All the materials are solid, and the long-term drift is very low, even with humidity variations.

Why does the IC industry use these plastic packages? Well, they're rugged, really rugged. They keep the die and its bond wires intact throughout the handling and the soldering processes. They are chemically inert, mostly. They tolerate the stress of shrinking solder pulling their leads in all directions. They can be formed in a very wide range of shapes and lead counts. Finally, they're less expensive than any other solution by far. Our customers like that a lot.

So we're soldiering on, testing various combinations of mold compound, coatings, and die attaches. We're looking to develop plastic cavity packages, where there is a void around the die. These are expensive packages, and not widely available. Our circuit design methods and silicon processes are advancing nicely in the field of precision analog circuits, and improve those packages we must.

12 comments on “Those Darn Plastic Packages

  1. Michael Dunn
    March 29, 2013

    Wow, I just learned more about packaging than in the past 30+ years!

    Gives me even more appreciation for sexy ceramic packages and classic TO-99 cans 🙂

  2. eafpres
    March 29, 2013

    Hi Barry–great post.  I had a bad experience with a plastic encapsulated surface mount antenna component many years ago; after we launched it and it was out there for a couple years, the lead free thing came along and everyody re-did their reflow profiles.  Well, our material could no longer take it, and we started getting pictures of blistered parts on boards.

    Did you have any “fun” experiences when the soldering temps all jumped up?

  3. Brad Albing
    March 30, 2013

    This is why should read all the good things that are posted here. Always something new to learn – as an engineer, keep learning or fall way behind and be left in the dust. There are a lot of people posting things here and they are from a very wide range of backgrounds (by which I refer to industries, life experiences, and even geographical).

  4. Barry Harvey
    March 30, 2013

    Thanks, Scott

    Silicon is very hard;  silicon oxide is even harder, even deposited.  Everything in the die is hard, excepting aluminum wires.  Thus I figure any external stress travels right through the unyielding layers with very little compressive relief.  So all components, in the silicon or above it, are exposed to package and die attach stress.

    So yes, your isolated poly resistors will demonstrate piezo changes.  High sheet-rho poly is the worst, low sheet-rho poly being less sensitive.

    Even capacitors have a modest stress sensitivity.  So sorry, you could well have ~100ppm oscillator shifts after packaging.

  5. Barry Harvey
    March 30, 2013

    Fun…like more delamination, shifts, ugly surfaces, having to re-learn how to solder…No sir, I didn't like the change to lead-free.  Not at all.

  6. SunitaT
    March 31, 2013

    To keep the package dimensions stable, about 50 percent of the plastic's volume is filled with silica particles, essentially quartz bits.

    @Barry, very informative post. I never knew that we face so much of challenge to maintain the package dimiension.  Learnt a lot about packaging through your blog.

  7. amrutah
    March 31, 2013

    @Barry, thanks for the blog post.

       “We trim our op-amps at wafer probe to as low as 10uV offset voltage…”  

    I have faced this kind of problem in one of my designs.  For Reference system, when I probe at wafer level and trim the device it operates fine, but once packaged we saw a shift of the voltage for the same trim settings…  The mechanical stress induced offset or Package shift.

  8. amrutah
    March 31, 2013

    @Barry, Thanks for the post.

    “We have determined that our references drift only single ppms per 1000hr. when humidity is controlled, several times more than when not.”

      I come from a place very near to coast where the humidity levels are of the order of 85%-90% most of the year… The effects of humidity you have covered in the blog explaining how it effects the package and how it effects the shift of reference due to humidity is just good.  Any reference papers you recommend on this??

  9. Barry Harvey
    April 1, 2013

    Sorry, I'm not much of an academic.  I mostly survey the people around me (like packaging engineers) to get some collective wisdom.  Sometimes this doen't work, like when no one agrees. 

  10. antedeluvian2
    April 1, 2013

    Barry,

    fascinating post!

    I was wandering if there has been a “recent” change in the plastic/epoxy used. It seems to me that ICs only started arriving packed together with silica-gel (and humidity sensitive dots) in perhaps the last 6-7 years. I have some pretty old ICs (~1970s) that seem to work fine every time I use one when I am prototyping and need a part quickly. Of course they never are used for anything in production, not least beacuse they are through-hole technology. They have been through dry and humid climates, including being shipped by sea. 

  11. Brad Albing
    April 2, 2013

    I also noticed that a lot of my old DIP ICs' package material seemed to be a different material – rather more tough and impervious to soldering.

  12. Erickk
    April 3, 2013

    The use of humidity control measures (silica gel, sealed bags, humidity indicator cards) is due to the shift to surface mount assembly.  In the old days (through hole assembly), it was the PC board that was heated by the solder wave, and the IC package itself experienced only a mild thermal shock.  In SMT assembly, the reflow oven heats the IC package itself comparatively rapidly.  If the plastic has absorbed moisture, the package can rupture due to the rapid heating, just as popcorn does due to moisture trapped in the corn kernel.  Therefore, it is necessary that parts be stored and handled under controlled humidity.  The data sheet for each part now lists a Moisture Sensitivity Level (MSL) that defines the necessary precautions.  If parts have been exposed to too much humidity, they can be salvaged by a slow bake process, which gradually removes the absorbed water without danger to the part.

     

    The modern epoxy packages (black material) are actually quite moisture resistant.  Those of us who are older will remember packages for ICs and transistors molded with a gray silicone pastic.  Those devices exhibited terrible moisture moisture absorbtion, and would fail in humid conditions, due to swelling of the plastic and internal corrosion  I believe it was National Semicondutor that solved this problem, by promoting the Epoxy-B system that is the forerunner of modern package materials.

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