Traces between pcb and pcb-based-module - Page 1
Forum section: electronics / general

author: Psi
author_rank: Super Contributor
timestamp: January 28, 2015, 11:30:48 am
content: You know how some surface mount modules come as a PCB themselves. (with the TH pads half routed off and you solder whats left to pads on the main PCB).With that sort of mounting it it ok/bad to have overlapping pcb traces between the two pcbs?So you would have copper tracks in contact but separated by two layers of soldermask.eg.MODULE PCBTRACKSOLDERMASKSOLDER MASK-SOLDER-MASK-SOLDERMASKTRACK TRACK TRACKMAINPCB-MAIN-PCB-MAIN-PCB-MAIN-PCBAlso, what is the correct name for this sort of sub PCB module mounting technique.

author: Codemonkey
author_rank: Regular Contributor
timestamp: January 28, 2015, 11:45:24 am
content: Do you mean put tracks under the module ? if so there's no problem doing that. Just check if its a module with an integrated antenna, there will be a keepout zone where any copper is forbidden (the antenna won't work with a ground plane under it and other copper in the area can de tune it).As far as I know there's no specific name for the modules, just surface mount pcb modules with castellated pad connections.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2015, 12:40:52 pm
content: Board-on-board (contrast with chip-on-board, with the black blob o' goo), the carrier part having either castellated holes (routed-through pads) or internal lands (usually as an LGA).If there's no keepout requirement, it's probably still better to avoid routing on the facing layer, just in case. But if you need to, I doubt it's a problem; make sure vias are tented, and don't try running much voltage, current or frequency/bandwidth/speedy edges through them.Tim

author: Psi
author_rank: Super Contributor
timestamp: January 29, 2015, 11:28:36 am
content: Thanks, i was mainly concerned about the reliability. Two layers of soldermask would be all that stands between potential dead shorts..

author: SL4P
author_rank: Super Contributor
timestamp: January 29, 2015, 11:38:13 am
content: Is overall height critical by 1-2mm?You could either slide a small piece of Kapton tape under/between - before soldering the daughter-board, or raise the daughter by 1mm to guarantee physical clearance... or even a SIL socket to make it pluggable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2015, 12:57:55 pm
content: That wouldn't be advisable... even 0.1mm of extra clearance will screw up automated (paste) soldering. You could guarantee the gaps are bridged by hand, but that would be a PITA.There are such things as sockets for various SMT parts, but they're probably not cheap.For example, you'll see wireless module devkits with them.Tim

author: Psi
author_rank: Super Contributor
timestamp: January 29, 2015, 09:58:51 pm
content: The module will be fitted by hand on a few select units so kapton tape is a good idea, thanks

author: Codemonkey
author_rank: Regular Contributor
timestamp: January 29, 2015, 10:01:04 pm
content: For what its worth, we've produced literally thousands of devices with a module of this type just soldered directly to the board with traces running under it. Don't believe we've had any problems whatsoever.

author: Psi
author_rank: Super Contributor
timestamp: February 05, 2015, 10:33:08 am
content: Thanks blueskull, yeah, this module has no RF stuff, so no antenna, but i will be sure to keep tracks isolated so none overlap except gnd