Dealing with very old PLCC parts for assembly - Page 1
Forum section: electronics / general

author: blundar
author_rank: Contributor
timestamp: May 31, 2024, 11:14:23 pm
content: I did a little searching and failed to find exactly what I was looking for. If this has been covered, please forgive me / point to link.I bought some *very* old PLCC32 Flash EPROMs that have been discontinued for many years. They appear to be legitimate, being in what looks to be manufacturer packaging - new old stock. However, they're 23 years old and some of the packs have been opened and exposed to humidity/air.Any suggestions for how to proceed? The goal is to apply Pb solder paste to PCBs, pick-and-place parts then reflow them. I have a 5 zone reflow oven that's reasonably well set up. I can make adjustments to soak/ramp times to help the flux do its job if I know what to change in the profile.The QC engineer at my local assembly shop seems to think that I can probably get away with a highly activated flux solder paste, something water wash. He's working on coming up with an exact recommendation because they don't do Lead + fucked up parts very often. Most of their builds that need fucked up parts use RoHS these days.I'm wondering about doing a pre-soak in something like Kester 2331-ZX for a few minutes before placing the parts on the board.I'm currently looking at Kester R562 and HM531 pastes, but they're rated ORM0. Also looking at FCT WS159 paste as it is rated ORH0 - more active than the Kester offerings.Any and all suggestions welcome.

author: loki42
author_rank: Frequent Contributor
timestamp: June 01, 2024, 06:54:42 am
content: why non RoHS? There's low temp lead free if temperate is an issue. Otherwise modern solder is good stuff.I've used some parts from the 70s and standard SAC305 or SN100C no clean worked fine with them.

author: wraper
author_rank: Unknown
timestamp: June 01, 2024, 07:28:32 am
content: Why not check actual solderability before making any decisions?

author: tooki
author_rank: Super Contributor
timestamp: June 02, 2024, 12:19:26 pm
content: How about soldering PLCC sockets to the boards, and inserting the chips into those? Then you avoid any solderability issues with the old chips, and the contact wiping action on insertion will negate any corrosion or tarnishing that did occur.

author: tooki
author_rank: Super Contributor
timestamp: June 02, 2024, 12:22:10 pm
content: And of course you could just manually clean the old chips (e.g. with fiberglass brush or abrasive rubber eraser) or pre-tin them in a solder bath or similar, using a vacuum desoldering iron or wick to remove any excess and/or bridges.

author: coppice
author_rank: Super Contributor
timestamp: June 02, 2024, 12:34:12 pm
content: Quote from: tooki on June 02, 2024, 12:19:26 pmPLCC sockets are a reliability nightmare, even with pristine untarnished parts. Soldering old parts mostly means being thoughtful about fluxes. Making PLCC sockets work well for them is really tough.

author: tooki
author_rank: Super Contributor
timestamp: June 02, 2024, 03:26:42 pm
content: Why is that? Plenty of products over the years have used them successfully, including some recent ones like the Pace ADS200.

author: coppice
author_rank: Super Contributor
timestamp: June 02, 2024, 03:37:29 pm
content: Quote from: tooki on June 02, 2024, 03:26:42 pmPlenty of products have been pretty unreliable, too. Its a long time since I used a PLCC part, but I used to have huge problems with PLCCs in sockets, using a variety of designs of sockets. The wiping action of most sockets does very little to clean the pins during insertion. You need to get things really clean before insertion. They don't exclude the atmosphere very well, so working systems can corrode and see failures after some time. Quite a few sockets don't even hold the parts that well. Shake some products up a bit, and you can find PLCC packages rattling around inside. Why do people even use them? They add bulk and cost, but who expects to have to replace parts in production equipment often enough for a socket to be a win? There used to be PLCC EPROMs. Those are about the only parts where PLCC sockets made sense for production.

author: tooki
author_rank: Super Contributor
timestamp: June 02, 2024, 03:56:23 pm
content: That’s very weird. Many vintage Mac models used socketed PLCCs for various chips (though amusingly, not for the ROMs, which were either DIPs or SIMMs), and in my many years of being involved with those machines, I’ve never heard of the PLCC sockets ever giving any trouble. I don’t think it’s a fundamental problem with the technology.

author: blundar
author_rank: Contributor
timestamp: June 02, 2024, 05:02:36 pm
content: So any recommendations for highly activated solder pastes? Any commentary on what I've found?The manual cleaning suggestion is a good one - thank you.I'm trying to be proactive. I hope that everything solders up nicely - I'm just anticipating problems based on the broker giving me a warning about these parts.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 02, 2024, 10:33:02 pm
content: I wonder why bother?For some newly designe product, the risk of rework and returns outweigs the cost savings of being able to reuse some old parts.If it's for some retro computing thing or other existing PCB, then sure, you make do with what you can.

author: wraper
author_rank: Unknown
timestamp: June 02, 2024, 10:54:14 pm
content: Quote from: blundar on June 02, 2024, 05:02:36 pmAs I said, you should actually check them. It can vary from soldering like fresh component to something that won't wet at all even with active flux.

author: SMTech
author_rank: Frequent Contributor
timestamp: June 03, 2024, 12:34:41 pm
content: I would be more worried about the moisture than solderability. Parts that have not been stored correctly for that length of time are quite likely to have absorbed moisture. You will need to bake them quite severely before use to reduce/remove the risk of them going pop during a reflow cycle. Most paste manufacturers offer a more aggressive flux variant if you find the parts are tarnished/giving poor wetting.