Phil Zarrow: Kim, there's been a lot of attention given, and concern over—and rightfully so—to the type of voids that occur in the bottom termination components, BTCs, particularly in the ground plane. But the voids that also, very much so, keep us awake at night, are the voids that occur in the actual solder joints, either electrical interconnections. What are some of those voids?
Kim Flanagan: You're absolutely correct, Phil. Often we talk about how to Avoid the Void® when it comes to bottom termination components, such as QFNs, but sometimes we need to focus a little bit more on the other types of voiding.
First type of voiding being Kirkendall voids, which are often called champagne voids. They occur along the intermetallic layer on the pad side, and they can affect the reliability of the solder joint as the intermetallic layer is the weakest part of the joint. Those little extra voids in there allow the crack to propagate even more so. They're called champagne voids, because they're actually... they look like little champagne bubbles inside the solder joint.
Phil Zarrow: They're very small, but they could be very nasty, as you said.
Kim Flanagan: Absolutely. Yeah.
Phil Zarrow: Great case. What other types of voids should we be concerned about?
Kim Flanagan: So another type of voiding would be shrinkage voiding, which happens often on off-eutectic solders, so what happens is some of the particles in the solder joint are expanding, or contracting, under heating and cooling, quicker or slower than others, so when that happens, you're creating little pockets within the solder joint, and usually to remedy that you'd want to use a eutectic solder instead, and that helps to have it so all the particles are expanding and contracting at the same rate under heat.
Phil Zarrow: How about our old nemesis, the via-in-pad?
Kim Flanagan: The via-in-pad, right. This happens a lot on BGAs, so when the flux is out-gassing, it gets caught up in that via, and it creates a void.
Phil Zarrow: And of course, with the pitches of area arrays, BGAs, CSPs, flip chips getting finer and finer. Now .4 millimeter pitch going down to .3 millimeter pitch, there's obviously less room for the designer to fan out in the dog bone patterns, so you just got to go with that old via-in-pad.
Kim Flanagan: Yup, exactly.
Phil Zarrow: Yeah.
Kim Flanagan: So You got to watch out for that sometimes.
Phil Zarrow: You sure do. You sure do.
Kim Flanagan: And the last type of voiding is blowhole voiding that occurs during wave soldering, when the solvent isn't completely evaporated in the flux, and it pools up in the holes, and when the hot solder hits it, it explodes and creates a void.
Phil Zarrow: Kim, when we talk about voids, the other concern we have, is it a defect or is it only a symptom? And that's going to basically guide us into action, whether we're going to rework or it's cosmetic, and we should just basically as best practices leave it alone? How do we determine that?
Kim Flanagan: Once the board goes through X-ray analysis, the operator may see something a little weird that may need further analysis. You would take a cross-section of the solder joint, which is a destructive test, to investigate the proximity of the void to the interface of the solder joint.
Phil Zarrow: Kim, where can we find out more information on this topic?