Techniques for desoldering a PGA chip - Page 1
Forum section: electronics / repair

author: Peacefrog
author_rank: Contributor
timestamp: January 08, 2022, 12:36:15 am
content: I'm planning to desolder a 128 pin PGA chip with a Hakko 301 gun, and need a bit of help.Before trying the real thing, I decided to practice on a junk board with a PGA chip - but I don't seem to be pulling the the solder clear of the hole, even if I add fresh 60/40 first and hold the board vertically. That said - I'm pretty wary of overheating the joint, and trying to keep the nozzle directly off the pad.Can anyone offer advice?

author: TheMG
author_rank: Frequent Contributor
timestamp: January 08, 2022, 02:12:34 am
content: Preheating the board will help a great deal, especially if large internal ground/power planes are involved.Also, flux helps the solder to flow. Apply liberally, under the IC too if you can pour some in there.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: January 08, 2022, 02:39:38 am
content: Use a hot air gun (station) IMHO the best way

author: coromonadalix
author_rank: Super Contributor
timestamp: January 08, 2022, 03:39:55 pm
content: hot air to desolder a pga lolll you'll burn the board before it gets out, you need to pre heat the board Ie: give the ''pcb traces mass'' some help the get the solder out easily with an solder pumpGood flux will help and add flux many times as needed ... and yes adding some 60/40 can / could helpBut you should practice on old or dead boards before doing soI do pga without any problems, i work on chineeses pcb with rohs solder (shitty) have to refresh the soler with 60/40, and my Hakko desoldering pump is checked tons of time to get it in optimal shape and maximum ''suction power" i use some kleenex in place of the round filter pads in the handle, since they tend to jam very often, and the darn handle ''solder capture spring'' is removedI heat the pins aroud 2-3 seconds max and suck the soldermy 2 centsLots of flux, a good hand and practice make you better ...

author: voltsandjolts
author_rank: Unknown
timestamp: January 08, 2022, 03:45:52 pm
content: Quote from: Peacefrog on January 08, 2022, 12:36:15 amWhich bit do you want to preserve - the PGA chip or the PCB?

author: Peacefrog
author_rank: Contributor
timestamp: January 08, 2022, 05:43:08 pm
content: Definitely the PCB!

author: Peacefrog
author_rank: Contributor
timestamp: January 08, 2022, 05:46:30 pm
content: One thing - I do have a preheater, but there are SIMM sockets / plastic components nearby. Will foil or copper tape be enough to shield them? Is a heat gun pointed to the other side of the board more practical?

author: TheMG
author_rank: Frequent Contributor
timestamp: January 08, 2022, 06:56:26 pm
content: You only need to preheat to between 100-150C. The vast majority of components should be able to withstand this temperature no problem.It really makes a world of difference for ease of desoldering.

author: benj38
author_rank: Frequent Contributor
timestamp: January 19, 2022, 01:15:38 pm
content: In case you did not perform the surgery yet...I highly recommend you use something like ChipQuik SMD removal Alloy. Look on amazon to get a feeling of what is available.I have had great success with this in the past!The main advantage is that the chance of ruining the chip or the PCB are almost nil. Also, you do not need to remove the PCB from the instrument (as you would if you preheat) if you can access the pins with a soldering iron.The alloy looks like regular solder, but melts at a very low temperature of under 200 degrees Fahrenheit (similar to Wood's metal, but not so toxic). By melting it with a regular soldering iron onto the current solder (use lots of flux, it does not wet as well as solder, and flood all the pins making a huge puddle), the resulting mixture stays molten at a very low temperature, and for quite long, allowing one ample time to safely remove the chip.If you have an adjustable temperature soldering station you can do the whole thing with the station set to a temperature well below the melting point of normal solder. This will ensure that you have almost zero chance to damage anything. Make sure you clean all remains of it from the PCB, as you don't want the new soldered chip to have this mixture too. The ChipQuik website has clear instructions.The only disadvantage is that this alloy is quite expensive from ChipQuik (though you do not need much for a single chip, making it very acceptable overall). I believe more reasonably priced alternatives also exist.In fact, I have researched this years ago and figured out what the alloy must be and bought some small bars of it form a non electronics-related source. It was much cheaper than ChipQuik, works perfectly, but less convenient in a bar form than in the ChipQuik wire form. Unfortunately, I have not documented my findings, and forgot the exact generic name of the alloy.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: January 19, 2022, 02:23:05 pm
content: low melt point material composition roughlyBi ~ 65%Sn ~ 17%C ~ 14%In ~ 2%S ~ 1%O ~ 1%