HP65 display issue

[teenix]

I whole heartedly agree with those points, especially the time element.

I've been using a soldering iron for 50 odd years now and still learning things especially with surface mount components - mostly that I need new eyes. I did learn in my early teen years, don't solder in bare feet. The iron slid off the table and the tip landed between my toes. Sizzling your toes like sausages is not much fun. I remember my father thought he knew how to solder better than me and got me some acid to dab on the PCB connection to help the solder wet - obviously watched a plumber once. It worked well of course except for the stinging nasal passages and a ruined project after a short time. I was too young to know the dangers myself, but that little bottle of acid soon disappeared into the garden.

I've used a Letter Clip on some DIP IC's including the HP displays which contacts all the pins. I have a temperature controlled de-soldering gun and is usually on the PCB for 2 seconds or so which is enough time to melt and remove the solder and move the pin away from the side of the plated hole (if there is one), so it comes out easy. Longer time can be required depending on the oxidization of old solder and how much of it has leached onto the pad on the other side of the PCB and up the side of the pin. Re-soldering before de-soldering can help here.

To my mind, the IC pin is so thin in the PCB hole that most of the applied heat will be on the solder. I haven't noticed any problems having the clip on. A worse condition is any large copper surfaces on the PCB drawing the heat away. This heat does travel up the pin of course, and the letter clip is warm after all pins are de-soldered. This suggests that a lot of the heat transferring up the pin (not all) has been trapped by the clip.

Its hard to say how much heat will hurt these old ICs, but a lot will be working on their last legs (oops - bad joke) and any excess heat won't help. I've got a partially dead HP display, maybe I should see how long it takes for the iron to ruin any of the remaining LEDs.

cheers

Tony