EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: johm on June 25, 2016, 02:36:11 pm
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Hello! I am facing a very small / high density design (22mm x 38mm) and have some questions:
- Has anyone ever had trouble ordering a cheap PCB to china with 0402(in), DFN footprints or even modest BGA's such as http://www.digikey.es/product-detail/en/ams/AS1130-BWLT/AS1130-BWLTCT-ND/3028667 (http://www.digikey.es/product-detail/en/ams/AS1130-BWLT/AS1130-BWLTCT-ND/3028667)?
- Did you request ENIG, or HASL is good enough (budget sensitive project)?
By default, Altium design rules for smart-prototyping.com define 6/6 mil track/space buy don't speak about solder mask sliver. Which is the main problem I'm encountering with fine pitch devices. I have to reduce the solder mask sliver parameter to 3 mil to avoid design rule errors.
Any insight will be highly appreciated, thank you.
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Hi
What you have is a 0.5 mm pitch BGA. That is not what most people would call "modest" in terms of layout rules. There are no fixed absolutes for any of this. It's all a matter of what your assembly process will let you get away with. That of course *assumes* you don't have standards flowed down to you that require IPC standards.
When we do that sort of part, we do it with ENIG and often are forced to 4/4 rules. The bigger issue than the traces is generally the via sizes and things like via in pad. The part you have shown is indeed a small pad count so that may let you get away without as much crazy stuff in that area. The cost for the smaller vias and the tighter trace width / spacing seems to go up in lock step. If you go for one, you get the other as well.
Bob
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0402: Works well enough for hobbyist use on chinese HASL boards. You may encounter tombstoning issues.
.5mm BGAs: Eeh. Your biggest issue will be fanout - you cannot, cannot do via-in-pad on a 0.5mm pitch BGA without plugged vias, and AFAIK no cheap chinese board fabs do it. Not that I suggest unplugged via-in-pad for BGAs at all, but a 1mm BGA has more margin. You will also really want ENIG if you don't have fanout issues. For prototype work, HASL will probably work - but be prepared to do some rework.
DFN is fine. 0.4mm DFN is probably going to be a bitch; otherwise, I have good experience with 0.5mm DFN and QFN on cheap HASL boards.
Solder mask sliver: You will generally find that the chinese fabs will leave no slivers between fine pitch pads; all pads will be NSMD. This does result in more bridging, but careful pad and stencil design will help a lot.