Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, who by day, go as mild-mannered consultants for ITM Consulting; truth, justice and the American way. But we are here at Board Talk up in Mount Rialto in the Board Talk cave to talk to you today about problems to limit situations and process technique. And what do we have today?  

Jim
Phil, there are common themes that show up and I'm going to read three independent questions that were sent on the same topic.  

This comes from B.S.; hope that's no joke because it's certainly a legitimate question. We have been stumped by occasional opens at the corner balls of some BGA components after reflow. We have experimented with many minor adjustments, yet this occasional problem continues. Do you have any suggestions?

The next is from A.S., maybe a brother or a sister. We are having a problem with BGA components bridging. The problem is specific to one corner ball location on one BGA, but it's common to a significant number of circuit assemblies. What could be causing this consistent BGA bridging problem? 

From D.W., We recently reworked a BGA component and the corners all bridged. This was a new BGA component that had never been installed on a circuit board before. Can these components be removed and reused if we remove the warp? The plot gets thicker. Would you consider the component to be scrap? What causes the corners of the BGA component to warp and how do we prevent it in the future?  

Well, ladies and gentleman, this is a news flash: BGAs warp during reflow.

Phil
Yes, they do.  

Jim
Virtually all do, certainly plastic ones, I mean, may be not ceramic; particularly the ones that have a complex internal structure and they are not thermally balanced. And as my brother, Phil, would want to say: it's a fact; don't fight it, live with it.

Whether you are -- and of course when you think about a BGA warping, the corners are going to have the largest displacement causing open and potentially bridges because of them either warping up and lifting above the circuit board or warping down and pushing into the solder paste therefore causing bridges.

The best technique to try to minimize warp is to slow down your heating and cooling processes. You ramp up during your preheat section and you cool down during your cooling section. Now of course, in cool down, you don't want to go too slow because you don't want to create a coarse-grained structure. So it's again, as in many things with electronic manufacturing, it's compromised, it's trade-off.  

Phil
Now I am going to open up another moisture barrier bag for a can of worms. Moister sensitive devices, plastic and BGAs are big culprits. What level MSD is this component and are you adhering to the JEDEC standards on this, in terms of your exposure time, bake-out time. If you have a sloppy program, now is the time to get it together.  

Jim
One experiment you could do with the particular components you know are bad is bake them immediately before reflow. And see if that reduces the problem. 

The other things of course are getting a good solder paste that tolerates head-in pillow and other type defects that is specifically formulated to deal with these issues.

Make sure your printing, in as much as the aperture design, your stencils, if you are seeing open some people overprint the corner balls of their patterns, to get a little extra paste to minimize the chance of having it open

If you have seen bridges you might want to try minimizing the aperture; I haven't honestly had any experience with that. But BGA warping is a fact of life and you need to deal with it, and heating and cooling at the minimum acceptable rate is the most common strategy to deal with it.  

Phil
But most of all fundamentally, before you start any experiments, make sure you have got an MSD program in place.  If you don't, you are not alone and that's no excuse.

You have to have the MSD program going. Get with it. You know what is it doing, you are paying for it. And beyond that as far as paying for things -   

Jim
Don't solder like my brother.  

Phil
Don't pay my brother to solder for you either.