I’ve had PCBs on order for my Bluetooth sensors for about a month now. They finally arrived today.
I got these made at Futurlec. Their quoted tolerances are quite large by modern standards, but since I’m not building a complicated board, I figured this would be OK. This is one of the tighter-packed boards that I’ve worked on, so I’m probably pushing their manufacturing abilities a little.
The board is double-sided with solder mask and silk-screening on both sides. Some parts of the board do require the minimum tolerances specced by Futurlec.
Lesson 1: Don’t run copper right to the edges of the board.
When the boards are cut, the bare copper will be exposed. I haven’t investigated the issue much, but I expect it’ll allow oxidisation and probably bad things long-term.
Lesson 2: Don’t rely on precise hole positioning.
Nothing on these boards is
bad, but there’s noticable inconsistency between boards. They’re usually offset by a constant amount. Some are near-perfect, some have a fraction of a millimeter offset.
The board has a mini-USB socket and I’ve specified the socket with little mounting posts. There are holes drilled to accomodate these. Of course, offset holes means offset USB socket and possible pad alignment issues. I don’t think it’ll be a problem, but certainly something to watch out for next time.
Similarly, the silkscreen is offset slightly. Probably best not to rely on this being perfect.
Lesson 3: Better to have too much solder mask than too little.
The pads between the main CPU are tightly spaced and not all of them have solder mask between them. Since I’ll be hand-assembling the first few, ease of assembly is fairly important to me. I left the pad-to-mask margin at the Protel default, but I’ll reduce it in future. Of course, too little will probably lead to parts not sitting flat on the board. Hmm.
Lesson 4: Where possible, use thicker tracks than the minimum.
I should have predicted this after experiencing the same issue when I was etching boards at home. Bends in tracks will tend to get over-etched and hence be thinner. The risk is that they’ll get so thin that they’ll break.
Lesson 5: Don’t rely on tight grid patterns.
I covered most of the backside of the board with a gridded ground plane. This was set up to be at the minimum tolerance allowed by Futurlec. Most boards are perfectly fine, but a few are ‘underexposed’ - there are solid grid squares where there should be a hole.
Lesson 6: Don’t be lazy about rebuilding the ground plane
In a few places, I’ve got vias pushing through the ground plane (intending to connect to it). I should’ve explicitly connected rather than relying on the grid to ‘coincidentally’ connect. Again, the tracks are getting a bit thin at some points, and I have a sneaking suspicion that some of these are power tracks.
Overall
These are all minor quibbles. I don’t think they’ll make any substantive difference when I actually build the thing. Most of this comes down to “be generous with tolerances”. Don’t rely on everything being perfect; allow a little slop in the design.
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