How Do You Inspect QFN Components?

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 Glenn V. "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? 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 cheap 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.

Thanks to Phil and Jim for sharing this useful advice. No doubt that x-ray inspection is the most appropriate way to inspect QFN for both joints and voiding inspection. There is various type of x-ray equipment for selection in the market, example: 2D or 3D Manual, 2D or 3D Automated, In-line or Offline.

Another crucial aspect to look into is the type of voiding measurement that must be to avoid miscalculation due to x-ray image from different side of the board.

Make sure you select the right equipment for your long-term manufacturing and inspection requirement. Vitrox Technologies is more than happy to share their experience and advice.

Here in Komex we found a way to inspect QFNs since we don't have any Xray machine in line to verify the well soldering.

What we di is to have an over print in the 0.2 mm larger than the pad, with this solder "excess" we get a kind of chanfer on the terminals side, so we can inspect them with the angular cameras of the AOI.

In regards the question from Don Patterson, in order to eliminate the solder balls, it is necessary to change the stencil aperture for those chips, try with an aperture called "inverted home base" this aperture is only for chips.

X-ray inspection, as per IPC-7093, and also x-section analysis for samples previously inspected and selected by x-ray images.

We use a bench top convection oven for prototype SMD boards. Frequently, on the sides of chip capacitors and resistors there are solder balls that must be hand removed. What's causing these to form? Also, at room temperature storage, what is the practical shelf life of a syringe of solder paste, and what symptoms would one see when the syringe is getting too old?

In 2D Xray, the QFN leads look like flat strips with a dark toe. The toe is where excess solder is. The thermal pad in the center has large voids. The joint is hidden and 2D Xray is our only available tool. Can you advise further how we can judge the quality of these joints?

Phil and Jim, great talk and good advise! We made a study with one of our customers regarding the best pad design for automatic X-ray inspection of QFN solder joints. Feel free to contact Viscom for details!