Need find solder stencil for 2mmx2mm QFN-14 package - Page 1
Forum section: electronics / general

author: wb0gaz
author_rank: Frequent Contributor
timestamp: July 01, 2020, 01:14:04 am
content: I am faced with (hand) placing and reflow soldering some RF switch ICs that are in QFN-14 package with 0.4mm solder pad pitch, 2x2mm package outside footprint, and 1mmx1mm ground pad.The part involved is skyworks SKY13414-485LF or equivalent (SP4T RF switch). I've not been able find any in-production alternatives with a larger package, so I am searching for tools needed to facilitate hand assembly.I have not been able find a suitable stencil for this package.Any suggestions where I can find a suitable stencil?Thanks,Dave

author: asmi
author_rank: Super Contributor
timestamp: July 01, 2020, 02:34:27 am
content: You don't need stencil for hand assembly of QFN parts. Tin pads with small amount of solder, add flux, place part and use a heat gun to solder it while holding it down with tweezers. To speed things up, buy a good hot air station (I usethisone and can totally recommend it if you can afford), but for small QFNs even cheap ones from ebay/ali will be more than enough. Infact before I got my current station, I used cheap-ass one (I think it was like $99) for 3 years - it just takes longer to bring things to soldering temperature (especially for lead-free solder).

author: wb0gaz
author_rank: Frequent Contributor
timestamp: July 02, 2020, 10:05:15 pm
content: Thank you Asmi, that is very helpful advice.I have a (not as nice) hot air tool and preheat box, so I will use those tools. I do not require lead-free (it is personal project).Until now, my only similar experience was 6-pin 3mm x 3mm device (SAW filter) with PCB lands on 0.65mm pitch (and it also had some pin material along sides of part, to improve inspection). This part is smaller, has no pin material on sides, and only 0.4mm pitch. Unfortunately, this kind of RF switch parts are only made in such small package now.I did not consider tinning pads before, but that is good suggestion (it is such a small part, I hope I can properly handle the part while tinning!) I will get plenty of practice - the PCB requires 8 of these parts!Thank you,Dave

author: langwadt
author_rank: Super Contributor
timestamp: July 02, 2020, 11:11:51 pm
content: Quote from: wb0gaz on July 02, 2020, 10:05:15 pmyou should tin the pads on the pcb, not the part

author: wb0gaz
author_rank: Frequent Contributor
timestamp: July 08, 2020, 03:57:34 pm
content: Thanks, that makes sense!Dave

author: KL27x
author_rank: Super Contributor
timestamp: July 23, 2020, 08:31:01 pm
content: If the part has a central heatsink pad (which is 30x larger than the little pin pads), there are 2 ways you can go.You can get just a tiny bit of solder on that center pad, to prevent a blob.But if you are doing a lot of them, you may eventually discover it's easier to blob that center pad with some excess solder to be on the safe side. As you heat, keep the chip pinned in placed with tweezers. When the solder melts, just squish the part down. The excess solder will shoot out the sides of the chip. Wipe the excess solder up with an iron. (As has been stated, plenty of flux).This is because if you apply too little solder to that pad, it will suck the chip against the board, and the chip won't self-align. You'll heat and heat, and the chip will just sit there not moving, and you will wonder when the solder is going to melt... meanwhile the die is cracking. The only way you will get the chip lined up is by moving it with tweezers, and your hands are not that steady.YMMV depending on the actual part and board, but IME the priority is to make sure all the pads connect. Bridges are not a concern, so long as you use adequate flux, the parts aligns, and you squish the part down to squeeze out the excess solder. The bridges automatically disappear. So too much solder is not necessarily a problem. It adds an extra cleanup step, but it ensures you don't have hidden problems, or that you have to stop, lift a chip and clean things up and start again. When doing a large batch of something, it goes way faster when it works the first try, every time. Redoing and fixing and tweaking things is what makes things take forever.