RE: Adventures in Board Bring-up Chapter 1: 8-Channel USB Audio Interface

michaelkellett — Sat, 23 Feb 2019 10:46:00 GMT

Just looking at you February update.

It seems to be coming along - good work.

My tip for removing small two terminal surface mount parts is to use two small tipped soldering irons , one in each hand, working under a magnifying camera if possible.

Its way better than those horrible heated tweezers that you can buy and much better than hot air, because when you get good at it you can move parts from one place to another in a single operation.

(Ideally, you wouldn't solder parts in the wrong place to start with, but it's all too easy when hand assembling prototypes !)

RE: Adventures in Board Bring-up Chapter 1: 8-Channel USB Audio Interface

michaelkellett — Thu, 08 Nov 2018 10:43:54 GMT

I know two good ways of attaching QFNs (short of spending loads of money).

If you are going to use a prototype technique it is a good plan to have a hole (or holes) in the big centre pad - this gives the solder somewhere to go and allows you to inspect the soldering.

The pcb should be (ideally) immersion silver or ENIG finish (not HASL - it's too lumpy).

Apply solder paste (preferably tin lead for the forgiving wetting and melting point characteristics).

If you have a silk screen machine or jig then that's by far the best way to do it, but if not, use a compressed air type syringe dispenser and if you haven't got that apply it carefully with a pin. It doesn't matter if the paste bridges the pins but try not to bridge the pins to the centre pad. Use decent solder paste if you can afford it.

To reflow the solder use a hotplate (at 215C for leaded solder) - if you have access to a reflow oven then use that of course. Just plonk the board (gently) on the hotplate and wait until the solder paste melts all round the QFNs, then give it another 20 seconds or so.

Remove from hotplate.

Once the board is cool you'll see lots of shorts between pins (the more so if you didn't use a solder stencil) but you can clear these with a pointy soldering iron and a watery consistency flux (not the junk that comes in little tubes for hobby stuff). I use Warton Metals Future 315.

I really, really do NOT advocate removing pins from a chip - you'll almost certainly screw up the seating plane by making lumps on the bottom of the chip. Excess heat should not be a problem - just don't use excess heat. Your hotplate or reflow oven should be at save temperatures and a needle point soldering iron at 360C can't get stuff very hot.

In the past I've used hot air (from above and/or below the pcb but it's much harder to control than the hotplate. My hotplate came from China (via Amazon) and cost about £140. The solder dispenser thing came from China as well (direct) and was cheap (£100) but is pretty rough. You need a compressor as well to drive it.