OK, some more on the saga of learning to solder 2.5 x 3.5 mm 0.5mm pitch parts at home.
As a reminder, this is the stupidly small package that I'm working with:
I have stock of three different solder pastes that I've tried out on the same part and board. The first is some old Sn63Pb36Ag2 with type 818 flux that I've had sitting in the fridge for a couple of years; I've lost its pedigree but it's a respectable brand that someone was selling on ebay in economical size syringes that they'd filled themselves with something that normally only comes in giant pots (Multicore, Kesler, something of that ilk). The second is a syringe of fresh Mechanic brand XGZ40 (a common cheap Chinese variety sourced via ebay). The last is a virgin syringe of Chipquik SMD291AX bought from RS. The latter two are straight 63/37 alloys. All are type 3 mesh.
The results:
Old Sn63Pb36Ag2 | Mechanic | Chipquik |
| | |
Worked fine, a tad drossy, best tack | OK, less tacky than the first | Least tacky |
All those results are on boards with an ENiG finish. They all self-aligned equally well, any apparent differences are down to camera angle.
So, my conclusion? The best feel, the easiest to use was the old solder but the results are a bit drossy - it's too old. The Mechanic and Chipquik produced comparable results but the Mechanic was a little bit easier to print and slightly tackier. The mechanic is also about £5 a syringe and the Chipquik £15. So I'm going to stick with the Mechanic for the time being and keep my eyes open for some quality high tack paste with some silver content at a reasonable price. All were good enough to get the job done.
For the sake of experiment I also ordered some HASL(leaded) finish boards at the same time. All the advice I've ever received as "
Use ENiG for QFN packages" so I wasn't too hopeful for the results. I was wrong, the HASL boards were as easy to place these tiny QFN packages on and get finished results as good as the ENiG boards. No pics because they're not really helpful, you can't distinguish the results from the ENiG board done with the same paste. Now, this is a one-off, but the takeaway is that fine pitch
will work on HASL - your mileage may vary. Why this matters is that the ENiG finish option alone costs significantly more than the boards most of the time from people the likes of JLCPCB (£12.25 extra for ENiG on a £1.44 100x100mm board).
Final point. The HASL board I had shipped with plain airmail, the ENiG boards with DHL. The HASL boards only took 3 days more to reach me and the cost was a fraction of using DHL. I'm sticking with the jet snail mail in future unless it's a genuinely urgent job, three days less isn't worth the £10 it would cost on a 100x100mm board order.
For the record, JLCPCBs turnaround times, order to delivery to my door so far.
4 layer HASL board (jet snail), 17 days.
2 layer HASL board (jet snail), 10 days.
2 layer ENiG board (DHL), 7 days.