Why does my new dev board look like this? - Page 1
Forum section: electronics / general

author: redshift
author_rank: Contributor
timestamp: May 11, 2015, 11:13:47 pm
content: I bought the new MSP432 Launchpad from TI to give it a try. It sounds really cool! Seems like you can get a lot done with this using very little power. Anyways, why does my board look like this?The solder mask is a weird speckled matte colour and there seems to be flux residue left around both sets of male headers.There is also two solder bridges on the TM4C1294NCPDT. (This is a Tiva C Series cortex M4F used for onboard emulation)What I find even more strange is that the boards in TI's promotional materials look the same. (See:User's Guide,Getting Started Video)I checked the datasheet of the tiva mcu and pin 7,8 and pin 51,52 are pairs of positive power pins that look like they can each be tied together. So it seems like these bridges are intentional. Is this a common practice? Why not just attach the pins with a short trace?

author: Pack34
author_rank: Frequent Contributor
timestamp: May 12, 2015, 12:24:49 am
content: There is a trace connecting the two together. This sort of thing happens during assembly if you connect the pins directly together instead of going out away from the board and back to the pin. When you have fine pitch components like this, it isn't really possible to get good solder mask in between the legs of the chip.

author: redshift
author_rank: Contributor
timestamp: May 12, 2015, 04:21:27 am
content: Quote from: Pack34 on May 12, 2015, 12:24:49 amWhy does this happen?Quote from: Pack34 on May 12, 2015, 12:24:49 amI don't understand this. The solder mask would have been put on before the solder paste. How would the quality of the solder mask layer be affected by the trace underneath?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2015, 04:39:33 am
content: Your first picture is too hopelessly blurry to make sense of, but it looks like they did a bad job washing them I guess.If you look closely at the pads, you will find there is no soldermask between any of them in the TQFP footprints. The pads are naked on the board, no soldermask to separate the pads.If copper goes between pads with no soldermask covering them, solder is very likely to wick over both. Result, failed AOI (automated optical inspection) -- looks like a short.A lazy PCB designer might route copper horizontally between pads, in this way. Or let the autorouter do it, or a polygon pour object. A responsible designer checks for features like this and takes steps to avoid them (like routing the trace manually, so that it exits the tip or root of the pad, not the side, or adding keep-outs so the autorouter or pour object do not connect horizontally).If the overall impression is poor design and/or build quality, that wouldn't surprise me. Wasn't this the board they were hocking for like 10 bucks, or free, or something? Very likely they're promoting them, at cost, at that rate. Are they going to be the cheapest assemblies possible? You damn well better bet on it.Tim

author: redshift
author_rank: Contributor
timestamp: May 12, 2015, 05:39:23 am
content: Thanks for the replyQuote from: T3sl4co1l on May 12, 2015, 04:39:33 amThese boards are $12.99USD and they're certainly not earning anything from them. So I definitely don't blame them for cutting little corners. The board works fine.But now I'm curious. When I need to attach two adjacent pins with a tiny-pin pitch separating them, why shouldn't I just make a straight trace that causes a bridge? Can the AOI be taught that in this case it's ok? How else could I route this trace in a way that doesn't create acid traps?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2015, 10:34:20 am
content: AOI probably varies by what machine is doing it, and they can probably watch for those sorts of things now. But hey, a low budget assembler might not have current tools, either, so... you'd have to ask them. And yeah, you can add exceptions, but that's more programming or machine time.There's probably a mechanical reason not to do it, like uneven solder fillets, causing a minuscule amount of strain, or... something. But that's the whole point of gull wing leads, so that's kind of a bad excuse.Like I said, best practice is to avoid adding metal in the gap between pads. Instead of a straight trace between, use a "C" shape that exits each pad lengthwise. If they're going to an inner (or opposite) layer, you could use individual vias, or put the via in the "C" shape. (If the via is very near the pads, it should probably be tented on the same side as the footprint, so solder paste doesn't wander into it and leave the joint dry.)Tim