What about hand soldering QFN's or oscillators with less than ideal side... - Page 1
Forum section: electronics / beginners

author: alank2
author_rank: Super Contributor
timestamp: February 24, 2016, 04:17:44 am
content: What about hand soldering QFN's or oscillators with less than ideal side conductors?Today I was looking at this part:http://www.digikey.com/product-detail/en/FT232RQ-REEL/768-1008-1-ND/1836403The first thing I noticed is that you can see some side conductors on this, but if you look at them under the microscope, you'll notice that they do NOT wrap around the bottom which I find odd. So my first question is, Q#1 - Are some QFN's made that wraparound and others not?Q#2 - If it was the only device available for a particular part (in other words you had to use a QFN) and you didn't have hot air or solder paste, could you get by having longer pads that extend underneath and hand soldering them? Would that work? Would it only work if the conductor wrapped around? Would it even work then?Q#3 - Can you handle a thermal pad by having a through hole via so that you can solder the center pad from the underside?I know it seems like I'm really wanting to hand solder a QFN, mostly I wonder if such a thing can be done or not...Q#4 - Let's say you have an oscillator that has some side conductor exposed, but mostly its pads are underneath. Is the amount of solder/connection you will get from the small side area enough? Will the solder also wick under and around such a joint? What if you use liquid flux first so the part is somewhat sitting on top of a little bit of flux liquid?

author: ataradov
author_rank: Super Contributor
timestamp: February 24, 2016, 04:32:16 am
content: Quote from: alank2 on February 24, 2016, 04:17:44 amThey all are connected, but sometimes do not wrap because of plastic flashing. Those side tabs are not intended for soldering, so no guarantee here.Quote from: alank2 on February 24, 2016, 04:17:44 amIt works OK, but you won't be able to solder an exposed pad this way. If you are designing a new board, then put a big plated hole underneath the pad, so you could solder it from the other side.Quote from: alank2 on February 24, 2016, 04:17:44 amYes. But make it a big diameter hole, not just a regular via. I've seen this done on mass produced devices, nothing wrong with that.

author: KL27x
author_rank: Super Contributor
timestamp: February 24, 2016, 05:31:31 am
content: If you extend the pads on the pcb, then yes, it is possible to hand solder even unexposed pads. Not fun, but if you only want to make one proto, then you can try it. Pretin all the pads on the chip and the board and check to make sure it sits fairly level. Don't try to flow the whole chip and press it down, flat. That will cause bridges. Just level it wherever it wanted to sit on the solder bumps, so you can visually verify the filet on each pad. Reflow one pad at a time with a fine point, because you can't see bridges and messing this up will waste a lot of your time.If you mess up, you can remove smaller chips by flowing solder all around the sides, thanks to the extended pads and the gap between chip and board. Then clean up with solderwick and try again.Also beware the pads that don't wrap around the side. Some of them separate from the IC very easily, and that's that. It's as if the pad is just glued onto the outside of the chip with nothing left when it falls off (but maybe just a microscopic bond wire).

author: janekm
author_rank: Unknown
timestamp: February 24, 2016, 06:17:47 am
content: Quote from: alank2 on February 24, 2016, 04:17:44 amThat's like asking if I don't have a spoon, could I eat soup with a fork. Use the right tool for the job. Getting PCBs made specifically to solder components the wrong way is just nuts. You can use a $3 gas pencil torch off eBay if you can't afford the $20 hot air tool, I've tried it for a laugh. Get a stencil made, it doesn't cost much these days, and your assembly will be so much easier. If you can't do that for whatever reason, you can use the solder balls used for re-balling BGA parts to deposit a consistent amount of solder on each pad of the QFN (takes forever to place all the little balls but it does work pretty well). But really that's making your life harder than it needs to be again...