EEVblog #1158 - How To Create PCB Mod Boards - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: December 15, 2018, 09:50:41 pm
content: PCB mod boards are useful for a whole range of applications and scenarios from production to upgrades, repair, and hacks.Dave shows you several examples and talks about creating manufacturing panels for them, along with castellations, V-Scoring, routing, and manufacturability.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: December 15, 2018, 10:57:30 pm
content: Cool video. With our customers it's rare to see them making an extra board for a mod, most of the time it's just a bunch of bodge wires ;^)Also Dave, how are you planning to roll the 121GW mod boards to those that already have the DMM?

author: johnlsenchak
author_rank: Frequent Contributor
timestamp: December 15, 2018, 11:57:42 pm
content: Nice t-shirt !

author: thmjpr
author_rank: Regular Contributor
timestamp: December 16, 2018, 12:10:11 am
content: So in the video D6 and D7 are shown, presumably two TVS diodes. Doesn't match up with the schematic released in Jan which has D13 (22V Bidirectional TVS SM6T22CA), and Q1 Q2 in parallel with the TVS.https://github.com/tpwrules/121gw-re/blob/master/pdfs/121GW%20EEVBlog%20Circuit%20diagram.pdfWonder if they changed back and forth from having one TVS, to two, then none.Continuous power dissipation of SM6T: ~1W, 2x2SC3326: 300mW (?).

author: tsman
author_rank: Frequent Contributor
timestamp: December 16, 2018, 01:45:34 am
content: Quote from: ANTALIFE on December 15, 2018, 10:57:30 pmAFAIK there are no plans to do that. The board install will mean recalibration as well. Might be able to get the copper shield though as that is easy to install. There was some minor discussion about both parts in the EEVBlog 121GW Discussion thread from page 25 onwards.Quote from: thmjpr on December 16, 2018, 12:10:11 amThe TVS was replaced with the 2 transistors on this mod board. It isn't on the schematic yet.

author: cdev
author_rank: Super Contributor
timestamp: December 16, 2018, 03:52:34 am
content: Dave, thank you for this! I've been thinking about doing something along these lines to one of my cheap SDR dongles.Are any 'daughter boards" on the way for you?

author: palpurul
author_rank: Regular Contributor
timestamp: December 16, 2018, 06:18:07 am
content: Thanks for the great video Dave!I have a question about this technique. When you upgrade your product like this. Does it have to undergo EMI/EMC testing again?

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: December 16, 2018, 06:54:31 am
content: Quote from: palpurul on December 16, 2018, 06:18:07 amOh yes. When you do final EMC testing everything has to be locked down, if you decide to change a single track or even use a different brand component you will need to go though EMC testing again. Though I gather that hardly anybody does the test again for the latter example

author: tszaboo
author_rank: Super Contributor
timestamp: December 16, 2018, 08:22:27 am
content: Quote from: ANTALIFE on December 16, 2018, 06:54:31 amNot everywhere. In EU, EMC certification doesnt exist, there is only precompliance testing(part of CE marking). If you can give a good reason why you dont need to go through the testing again, there is no need for it.

author: Unixon
author_rank: Frequent Contributor
timestamp: December 16, 2018, 08:51:18 am
content: Oh, finally this topic showed up, cool.I had to design these mod board few times. The fun part is there was no easy way to solder a mod board back to the device and what I really had to do is to convert an existing pattern of SMD pads on the host board to a custom SMD footprint of a daughter board implemented as open pads on the bottom layer thus resulting in an LGA type package with a very weird footprint. Some pads can be tiny making it hard to place and align a daughter board correctly for reflow soldering. However, finally this kind of solution doesn't even look like a mod, especially if the daughter board in thin, but rather looks like a nice custom hybrid that "has always been there".

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 16, 2018, 01:21:51 pm
content: This video about about Mod PCBs is quite good.This special circuit for the 121GW is a complete fail, anyhow, as it does not protect the circuit in Ohm/Diode mode any more.See my analysis here:https://www.eevblog.com/forum/blog/eevblog-1157-transistor-zener-clamp-circuit/msg2043871/#msg2043871Hint for Dave: This protection circuit in the Fluke 87 works quite differently (especially at 5V clamping voltage) , and I also doubt, that it was very effective there, either.Additionally, Fluke used selected transistors.Frank

author: cdev
author_rank: Super Contributor
timestamp: December 16, 2018, 01:35:44 pm
content: One thing that has helped me is scanning a device (both sides) at a fairly high resolution in my flatbed scanner. It also has a transparency adapter and when the situation seems to demand it I've added at least two scans with that on, too, so then I have four scans.The transparency adapter scans need to have the device bright and the outer areas dark so I've used a piece of cardboard to place the device in a hole in the middle before those scans to mask off its edges. So then I have four scans with two of them showing some of the internal detail from the backlighting. They can be overlaid in Gimp. It might make sense to make even more exposures (bracketing) with multilayer boards if that enables better visualization of invisible trace internal detail by trans-illuminating the PCB. (bracketing)It might even make sense in some cases to try HDR-processing those scans.

author: jnissen
author_rank: Regular Contributor
timestamp: December 17, 2018, 09:26:51 pm
content: In the 90's at IBM I created a number of mod boards. We called them "Flexible Applique Rework" or FAR boards. The ones I was familiar with were applied to the workstation processor boards I was developing. These were processor boards that had high pin count pin grid array discrete chips (Fixed point, floating point, I cache, D cache, clock, etc...). As we ramped up the clock rate the signal integrity became more and more of an issue. At one point the bodge wires were so numerous we created the FAR's to add termination resistor packs to the back of the board. They would allow us to add Thevenin split termination to the 100's of high speed nets that were troublesome. Was rather fun creating the odd shapes and trying to minimize stub lengths. The boards often looked like swiss cheese with a spider web of wiring between some discrete termination resistor packs that were SMT mounted to the FAR's. The board rework technicians loved them since it eliminated at times tens of hours of detailed rework time. Obviously these only saw limited production use as this was more often found in development level PCB's.