My company is doing a project with a lot of QFN. The question that we are not sure is how to properly solder a QFN on a PCB. QA says The alignment tab on the side of the QFN should have a physical solder from the board, like a leaded chip. After reflow the QFN do have contact with the board already but no solder on the alignment tab. So we have no manually solder. Does anybody who works with QFN have any input on how to treat soldering a QFN?
In the IPC-A-610E standard QFNs are classified as BTC (bottom termination components). Most QFNs do not have a solder-able surface on the outside of the part. Usually the lead that you can see on the outside is copper and classified is not solder-able. On these type of parts no toe solder is required. The only way to truly inspect these type of parts is with an x-ray.
I agree, the only way to "truly" inspect a QFN is with an X-ray. But this might be over-kill for him. He asked about soldering, not inspection. We use QFN's regularly and you can inspect these parts from the side to see if they are soldered. Unfortunately, most AOIs are not capable of solder inspection for these parts. So Bill is correct. He is also correct about referring to the IPC standard for QFN components. This is something your design Engineer should do when laying out the pcb. Refer to it for land patterns and dimensions.
if you want to get some sort of a joint even to allow AOI to inspect you can overprint on on the outer lead by 3 thou or so. This will give you a joint that can be seen from the side for inspection purposes.
We are a PCB design house/CM and we have found that if you elongate the QFN pads to the outside of the part by .025 to .030 you then have the opportunity of doing touch-up manually with a soldering iron should there be a voiding issue. This would have to be done at the CAD/Gerber level before PCB manufacturing.
If you extend the pads you get the effect that the solder below the component is pushed to the outside land which results in a very low standoff of the component, deteriorate the overall quality of the solder joint. Hot air (pencil or some of the semiautomated rework stations) helps with rework. We use Ersascope and x-ray for verifying the solder joint quality (Hi-Rel). Regards, Thorsten