Welcome to Defect of the Month. Every month, Bob Willis provides an example of a particular defect and hopefully some solutions. Bob has made over a hundred different defect videos over the years, so hopefully one will solve one of your process problems.
A common process defect that we see quite often in manufacturing is the tombstoning of chip components. Now here’s a couple of examples. The first I show the lifting of a component during reflow soldering that we would traditionally refer to as tombstoning.
In the second example, you can see the same effect taking place, but in this particular situation, the component only partly lifts, which on some AOI inspection systems may not be spotted. Now tombstoning predominantly is affected by the process, the design, and these are some of the reasons why you might see it in manufacture.
Now the pad size for chip components has to be right. If we’ve got uneven pad distribution, or sometimes the pad separation distance based on the termination of the components, this can contribute to lifting. If you’ve got some other interconnection to one side of the passive component, again, it may significantly affect the time of reflow and where you get reflow not happening simultaneously on each termination, this can be a contributing factor. Probably one of the biggest contributing factors to tombstoning is the volume of solder and progressively as we started to use less and less solder paste in terms of thickness on the surface of smaller and smaller parts and boards, we’ve generally seen less tombstoning take place.
If you’ve got solder paste, that’s got poor tack qualities, we may find that during the placement, the component is not held in place sufficiently, possibly on one end. And this may be the reason why, even if we’ve got simultaneous reflow on both terminations, the part may lift. Anything underneath the component, which can generate movement, e.g. if we’ve got solder paste due to slumping underneath the part that capillary action may be the root cause of why a component lifts during reflow, because there’s energy underneath the part, as the part moves into the reflow process and the wetting starts to occur on the terminations.
Solder mask underneath the component can vary in thickness. And if there are copper traces underneath the solder mask, again, that affects the thickness of the mask. So we get a Seesaw effect which can occur, and certainly I’ve seen this in manufacture. When you go below 0402, you really don’t need solder masks between the terminations. You are probably less likely to actually have copper between those terminations of the passive part. So again, why have the solar mask there in the first place. You will generally find if you compare convection reflow with vapour phase or convection reflow in nitrogen that with nitrogen and vapour phase, with a poor design, you are more likely to see tombstoning occurring on boards. So we’ve got to think of ways of significantly improving the process. One of the simple tricks that I’ve used many times with convection in nitrogen is to reduce the nitrogen content.
You’re slowing down the wetting characteristics. You are saving money, but more importantly, in a short term, you’re reducing the lifting of the part, which is a key issue. We’ve also seen solder ability play a part. If we’ve got variations, in solder ability between each termination, even if every other aspect of the reflow process is perfect, you might see the component tends to wet to one termination quicker than another. Hence that lifting occurring.
