Strange PCB structure - Page 1
Forum section: electronics / beginners

author: rexxar
author_rank: Frequent Contributor
timestamp: February 20, 2015, 09:57:13 am
content: Hey guys, I found this while inspecting the battery inside a Gameboy cartridge. There's a couple sets of traces that come out from the chip and terminate in a hole? My guess is that these traces were bridged for programming or something, then had the hole punched out when they were done. Does anyone know what this is for?

author: German_EE
author_rank: Super Contributor
timestamp: February 20, 2015, 10:10:40 am
content: Your initial guess is a good one, it could also be tamper proofing with a metal screw joining the traces together. We also have the possibility of poor PCB layout, Dave found this in a recent multimeter review with a screw driven through a trace.

author: tszaboo
author_rank: Super Contributor
timestamp: February 20, 2015, 10:17:41 am
content: You see a third trace coming out of that plane, ground based on the silksreen, and leading to the two trace coming from the chip. So I would say, that two pins were grounded at one point, and they drill that grounding out. Maybe that was easier than making test points?I think it is possible, they were mass programming them, like programming a whole panel at the same time, and then do the depanelisation. Maybe this was setting an address, or just pulling down the "ground this pin to program".

author: Rerouter
author_rank: Super Contributor
timestamp: February 20, 2015, 11:22:08 am
content: Or again could have been someone tying any unused I/O to ground, but it could well be programming, as it looks like they where routed to be drilled out,

author: tesla500
author_rank: Regular Contributor
timestamp: February 20, 2015, 11:38:34 am
content: Could this be a feature to ground the edge connector for plating? These cartridges have edge connectors that are much more heavily gold plated than the normal pads, and they would probably have to have all the contacts grounded for the plating operation.

author: amyk
author_rank: Super Contributor
timestamp: February 20, 2015, 02:35:58 pm
content: That is a 44-pin mask ROM that has been configured in 8-bit mode, so no in-circuit programming. The drilled-out portions are to float the high 8 bits and an address input.http://www.macronix.com/Lists/DataSheet/Attachments/1228/MX23L6410,%203.0-3.3V,%2064Mb,%20v3.8.pdfIn the full cartridge image here you can see some other areas with traces cut out:http://upload.wikimedia.org/wikipedia/commons/c/cc/PokemonSilverBoard.jpgMy guess is that they were for testing purposes.

author: rexxar
author_rank: Frequent Contributor
timestamp: February 20, 2015, 05:15:35 pm
content: Quote from: amyk on February 20, 2015, 02:35:58 pmGood catch, D8-14 do go into these holes. D15 seems to serve double-duty as an address line? I do wonder what pulling these lines low is supposed to do. (by the way, this is a pokemon gold cartridge, exact same board as what you posted)Quote from: NANDBlog on February 20, 2015, 10:17:41 amThere's four test pads on the back, but they're for Vdd and the RTC battery, no data.

author: SeanB
author_rank: Super Contributor
timestamp: February 20, 2015, 05:49:18 pm
content: That is from the board manufacturing process. the board initially starts off as bare SRBP material, which is then drilled where they want the pin holes, and then they silkscreen on a conductive ink coat and do a via fill with conductive ink as well, then it is used to electroplate on the copper in an additive process to the desired thickness ( as opposed to starting with a full copper board and etching it off), then they do things like gold plate and solder mask and silkscreen on legends. Then as a final step the boards are milled out with a bit, and at the same time the mill bit punches out these predetermined areas where 4 or more joins were for the conductive ink to make the whole surface contiguous. Under the IC there will be more holes where the thin traces meet as well.This is common on older PCB's where it was used to cut the cost of copper, often you find calculators made this way, with the board being finished by using a punch and die set that both removes it from the large panel and at the same time it punches out the joins leaving them isolated electrically.

author: rexxar
author_rank: Frequent Contributor
timestamp: February 23, 2015, 06:57:45 pm
content: Very interesting! Thanks for the great explanation SeanB

author: Mechanical Menace
author_rank: Super Contributor
timestamp: February 23, 2015, 07:11:42 pm
content: Quote from: amyk on February 20, 2015, 02:35:58 pmMask ROM, mapper, crystal for the clock, and a battery backed SRAM for storage? I'm surprised at that from such a (relatively) recent cart.