On Friday, August 10, 2018 at 12:23:46 AM UTC-4, Jeff Liebermann wrote:
On Thu, 9 Aug 2018 12:34:54 -0700 (PDT), John-Del
wrote:
I have an HP34C that I bought new back around 1980 or so to replace
my stolen TI SR-51A. It had always been a little flakey but a quick
thump brought it back around. A few years later I decided to have
a look inside when it required more physical persuasion to behave.
I took a picture of the front keys in case they decided to jump out
and get mixed up (they did), but I was surprised to find the half
dozen ICs *not* soldered down to the flexible circuit board.
Yep. It relies on the pressure provided by a foam pad to make the
connection. Bad bad bad idea:
http://www.hpmuseum.org/hp34.htm
They maintain connection by the foam cushion beneath the flex
circuit that maintains pressure between the flex circuit and
the ICs. I removed each IC, cleaned all the pins and the flex
circuit lands, put a tiny bit of dielectric grease on each IC
pin, and reassembled. It behaved itself for quite some time.
Right now it's been in storage for many years but every once
in a while I wonder if the correct solution would be to tack
solder them down. I'm sure the foam pad looses a bit of rebound
over the years.
What do you do when you run across flakey IC contacts in an HP?
If the foam pad was in good shape (springy), I would add a 2nd foam
pad to give it more pressure. This added pad is rather thin. I
should repalce both, but can't find a suitable sheet of foam. I now
have access to a laser cutter, so I might be able to cut some foam
that will work.
I think ordinary soldering is a bad idea and have never tried it.
However, if you do decide to try it, I suggest you use low temperature
180C bismuth solder paste, liquid flux, and a temperature controlled
toaster oven.
https://www.ebay.com/sch/i.html?_nkw=low+temperature+bismuth+solder+paste
Something like this:
https://www.youtube.com/watch?v=cClPqIJwhLs
On the toaster oven, this is how I do BGA reflow on HP JetDirect
cards, which use the same bismuth solder paste:
http://www.learnbydestroying.com/jeffl/pics/repair/BGA%20reflow/index.html
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
I'm going to dig it out of storage, charge the battery, and see how it behaves. If it behaves, I'll leave it as is. I didn't attempt a resolder back then because I didn't want to modify it and wasn't sure of the flex circuit's response to heat.
In later years, I've seen flex circuits (I think they're Kapton) that take soldering heat just fine.
I do keep solder paste in stock because I do a lot of smd IC replacement, and mostly use a hot-air station to keep the heat localized.
Thanks for the info.