Desoldering SMD ICs "the wire method" - Page 1
Forum section: electronics / general

author: kalel
author_rank: Frequent Contributor
timestamp: November 10, 2017, 07:03:22 am
content: An interesting method for desoldering ICs without chipquick or hotair by a Youtuber:

author: hamster_nz
author_rank: Super Contributor
timestamp: November 10, 2017, 07:27:58 am
content: Nice.I lifted a CP340C USB/Serial SOIC last night with the more brutal 'flood the pins with solder, with a little bit of leverage' method. I will try this next time.BTW, what temp should you use on hot air for this sort of thing? I've been using about 290C to lift RHOS passives... Melted a few LEDs that I wanted to reuse,, but without a preheater anything cooler didn't seem to work.

author: tautech
author_rank: Super Contributor
timestamp: November 10, 2017, 07:51:20 am
content: Quote from: hamster_nz on November 10, 2017, 07:27:58 amFlood the pins with Pb solder like you did with the IC but this time to dilute the Pb free solder and lower its melting point. Then heat a larger general area while gently tapping the LED's until they free from the pads. Then recover them with tweezers.I recently removed some SOIC 18's using this methodology and BTW use a small hot air nozzle.~50% airflow and 60-70% heat.

author: Ian.M
author_rank: Super Contributor
timestamp: November 10, 2017, 07:56:16 am
content: Intersting video although it really doesn't do justice to the technique. It works for any SMD package with *NO* thermal pad, with exposed pins with outwards facing pads that you can thread a fine wire behind, for the full length of one side. So no BGA, QFN, PLCC or other J lead packages. Adding fresh solder to the pins is rarely necessary and just increases the thermal stress on the board. The desolder braid technique shown is absolutely lousy.The trick with desolder braid is to *ALWAYS* cut off the 'dead' solder-saturated end so it doesn't leech heat away from the work, but when you cut it off, bend the braid to find the limit of the saturated end and cut it leaving a short bit of saturated braid - about half a bit's width - so you get good initial heat transfer to the joint you are desoldering. If you are starting from a fresh braid end, tin it then trim it. A 'dry' braid end doesn't conduct heat through the braid very well - most of the heat flow is up the braid away from the joint.Flux is also critical. Without it the solder wont flow up the braid well, and you may even have problems getting the pins to reflow quickly enough to avoid pad damage. Good quality desolder braid is loaded with a lot of solid flux. Cheap braid usually isn't. Dip the end in liquid flux if you have to.What wasn't shown was anchoring the end of the wire. It needs to be soldered to a strong (large area) pad about 45 deg angle out from the corner of the chip, at least 1" away to give you working space for the last pin. Stripping the varnish on the wire end with cutters is dumb - you'll probably notch the wire and then it is likely to snap when you tension it, with the possibility of ripping the pad of the pin you are currently lifting off. Use a lighter flame then fine sandpaper.Then there's the choice of bit. The braid is flat so the working face of the bit should be flat also for best thermal contact. Hoof is probably the best choice unless you've got the access angle to get a chisel or knife bit straight along the row of pins. Conical is bad - as the contact surface is very small and curved. The only worse choice is long conical.To minimise the risk of pad damage, before desoldering the last side of the chip, it can help to tape down the opposite side so the chip cant skew when you get to the last couple of pins. If you are reusing the chip, sit it on the bench on the edge of a business card as a spacer under the body, and bend the pins back down again with a steel ruler or other hard straight edge.