What are the exposed connections on the edges of some DIPs? - Page 1
Forum section: electronics / projects

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 17, 2022, 05:10:54 am
content: Like a 74LS08 for example. I guess it's the lead frame substrate connection?So what happens if this touches the same exposed connection on another similar logic IC jammed too close on the PCB?

author: ataradov
author_rank: Super Contributor
timestamp: December 17, 2022, 05:33:37 am
content: This is usually die connection. It is better to avoid connecting them between the ICs, although most likely nothing will happen especially for ICs from the same series.Before the molding process those tings are the only way to hold the metal pad that is attached to the die. Picture of a lead framehttps://hackaday.com/wp-content/uploads/2018/10/1920px-dip_zagotovka.jpgI also heard stories that some old ICs had characteristic damage that could be detecting by measuring this die voltage. I don't remember the details though.

author: amyk
author_rank: Super Contributor
timestamp: December 17, 2022, 09:44:04 am
content: The substrate is usually grounded.

author: ataradov
author_rank: Super Contributor
timestamp: December 17, 2022, 05:30:48 pm
content: Some old ICs used a charge pump to generate negative voltage for additional bias. Here is a description of one for 8086 from Ken Shirriff's blog:http://www.righto.com/2020/07/inside-8086-processor-tiny-charge-pumps.html(excellent blog in general)My dad used to work on Soviet-era factory equipment (CNC and stuff like this). This equipment was racks of boards full of ICs. Some they could fix locally, some they had to bring to a service center. And in the service center a person would very quickly mark the "bad" ICs, but they would always hide when they were doing it. So, naturally after a minor bribe, they told that they are looking for that negative voltage.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 17, 2022, 11:47:55 pm
content: I am trying to get a C64 cartridge to work and it doesn't, it powers up correctly but within 10 seconds malfunctions. But each chip tests fine by itself, the only thing I can see is that for whatever reason the DIPs are touching belliies. I'll try slapping one on a socket to eliminate the possibility of these weird exposed connections touching. The only other even worse explanation is flaky vias on the PCB. I am too lazy to want to test that for this particular board...

author: ataradov
author_rank: Super Contributor
timestamp: December 17, 2022, 11:55:38 pm
content: It sounds highly unlikely. Can you actually see them touch?

author: BrianHG
author_rank: Super Contributor
timestamp: December 18, 2022, 03:20:02 am
content: Not only helps hold the IC die in the IC package, but it also helps remove heat from the IC die.Without, only the super thin IC bond wires would carry away the heat from the die to the pins and the tiny IC in an all plastic package would bake itself in the center. At least with the thin flat plate of metal where the silicon sits upon, those wings to the left and right distribute the heat along the plastic case.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 20, 2022, 04:17:30 am
content: Quote from: ataradov on December 17, 2022, 11:55:38 pmYeah they're touching but the mold flash around the package and the fact that these connections seem a bit recessed means it is indeed not a likely problem. Probably a PCB fabrication defect, maybe some vias are bad... these boards were manufactured locally in Montreal a long time ago by a vendor that had some quality issues towards the end of their run ...I will desolder the chips and take a closer look at the vias underneath, could be a broken out annular ring or something wonky.