Defect of the Month with Bob Willis – Solder Balloons

Hello, my name’s Bob Willis and welcome to Defect of the Month. Every month I take the opportunity of looking at your process defects and hopefully provide you solutions. This month I’d like to look at Solder Balloons. Now that sounds a little strange and you may not have heard that defect term before. It’s basically a defect that I have experienced over the years and coined the phrase Solder Balloons.

I first experienced this when working for Nokia in Finland and the UK. A small section of the online video shows a balloon being popped when touched with a soldering iron. So what are balloons, they are a balloon of solder, which is typically sitting on a through hole or via. You can see this after selective soldering, wave soldering and possibly after reflow soldering depending on the process you are actually using. The solder balloon can be a balloon or football which has formed a round shape, but it could be blown apart or just started to pop. If you can imagine a balloon blowing up and you capture it in high speed, you see different formations depending on the point of solder solidification.

 

Solder balloons can be a problem if you are accessing parts of the board for testing purposes, any uneven surface is not good. If you are running double sided reflow, for instance, then having a solder balloon on the opposite side of the board and then trying to print solder paste on second stage assembly, that would be a major problem as well.

   

The images above show different examples of solder balloons that have popped

So how do balloons form? Well, it’s unfortunately one of those old topics outgassing from the printed circuit board. First of all in most instances you don’t want to soler vias, a via should be capped.

   

Three image of plated through holes being tested for outgassing.

In each case vapor can be seen escaping as bubbles on the right hand side of the hole

So why try and solder it in the first place? There are some reasons that people want to solder vias, but we’ll deal with that a little bit later. If you have an open via and you are going to wave or selective solder or solder vias during reflow soldering one of the things you don’t want is outgassing from the Printed Circuit Board (PCB). Now, outgassing as I’m sure you’ll know, is from moisture inside the printed circuit board. During the soldering process water vapor escapes through the plating, be it thin plating, poor plating, or voids in the plating. You can see it if you test a board. Many of you will be experienced or seen the many videos that I produced over the years showing this particular phenomena happen.

You can also see balloons as part of a BGA soldering operation on vias. Now what we’re actually saying is that if outgassing occurs from the via, that gassing can create a different size joint on a BGA termination, which is sitting on the via. Now, back in the very early days of BGA technology, we did some experiments with soldering of BGAs directly to vias. On some occasions we did get this outgassing phenomena. And you can see quite nicely in these X-ray images, the impact it had on the size or size variation. I am still waiting for a old friend of mine to remind me why we did these experiments in the first place?

 

 

However we can also see a couple of microsections, which show perfectly good solder joints on vias. It’s just another experiment that we were doing back in the day, but again highlighting the potential issue where gassing from virus would be an issue.