Phil
Welcome to board talk. This is Phil Zarrow and Jim Hall, the assembly brothers of ITM consulting. We are here to help you solve your SMT problems, whether they be material, equipment processes, procedure, methodology, whatever. We're here for you. And who are we here for today, Jim?

Jim
Our questioner is from Glenn V. His question: is there anyway to know if QFN components have sufficient wetting when doing non-destructive PCBA inspection. Can you provide guidelines for qualifying an assembly house for QFN assembly especially lead free.

Phil
The only way to authenticate acceptable wetting is with x-ray.

Jim
We should say if you are fortunate enough to have a QFN that has a single row of perimeter leads and they have castellations around the side you can pretty well confirm wetting by noting the wetting up the side of the component or out on the edge. Or if you can extend the pads out that can help you, but to know underneath the package you need a good x-ray machine with oblique angle viewing.

Top down alone is going to give you some indication, but not a really good indication of the wetting.

Phil
So with regard to your second question, what to look for when you are qualifying a house, I can tell you one thing we look for matter of factly. When we are working with clients, to help qualify a board shop, we look to see if they have a good x-ray machine, just as Jim just described. Not one of those ancient things and not one of those little cheapy quickie machines.

Jim
We're talking about a fairly sophisticated system with reasonable resolution. It doesn't have to be micro-focused, but very good resolution and reasonable oblique angle viewing.

Phil
Oblique angle viewing is a must and it's been out for quite a few years now and it's a real must have on x-ray, and definitely something to help qualify shop.

Jim
One simple question that I think you might ask when talking with an assembly house about QFN's, ask them what their experience and procedures and philosophy is with window paneing because most QFN's have a large central ground or thermal plane.

The strategy for printing paste on that is a very complex. Most people find that some sort of window paneing gets the best coverage with the least number of defects. If you ask that question and the people look at you like you're coming from outer space, then they probably don't have much experience with QFN's.

Phil
Or they're from outer space. Another area of qualifying the shop, what is their experience working with the devices and who are their references?

I think that pretty much covers that question. 

Jim
And remember, don't solder like my brother.

Phil
And don't solder like my brother.