There are certain types of LGA’s and QFN’s that use the pad layout using solder resist defined pads. The problem with this kind of footprint is that the slight height gap between the solder resist and the pad can lead to the creation of voids when reflowed onto a PCB.
I have personal experience of this at a CEM that I worked for as a Production Engineering Manager, this issue caused a lot of second stage inspection and rework to the point that I wrote an internal work procedure purely for one manufacturer of parts and this was flagged to be implemented whenever a Bill of Materials had this part listed.
The works procedure would have been a lot easier if I had of known what I do now that I work for Retronix. It would not have needed to be a 3 page document, just a simple instruction on the MRP system to send these parts to Retronix for Solder Bumping before they were due into a kit.
The addition of a solder bump to each pad avoids the headache of multiple adjustments to the solder screen and reflow profile to try and minimise the voiding percentage. It will also mean you have a vastly improved solder joint with no more voiding than you would see on standard BGA. The option of solder ball attachment as well means that you have the possibility to raise the height of the LGA/QFN to help with cleanliness as the gap allows your cleaning process to flush completely under the part. This slight lift from the board can also be useful for heat dissipation allowing the part to have an air gap between it and the PCB.
We utilise our laser ball attach to add these solder bumps or solder balls to the pads, the advantage of using the laser attach for this procedure is that the part is not exposed to reflow temperature as the part with stay at ambient for the whole procedure. We work closely with a large amount of component distributors and can receive the parts from them directly before undertaking the work and shipping directly to you to avoid the need for two part numbers on your system and in your stores.
If you would like more information on this or would like some samples created to run trials, then please let me know:
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