Advice on Desoldering an IC

[Martin Bishop]

Rob

I would imagine that an 11/24 CPU is at least a 4 layer board, with power planes and hopefully thermal reliefs at pins connected to a plane.  The pins you will be having difficulty with are most likely on the Gnd or Vcc plane.  I often leave those pins for last, once in the groove.  And, I'm not above cutting off the IC body or eliminating the connector body.

To state the obvious the key to desoldering is to get the heat in; now, the solder is what gets it there.  So with a desoldering tool and a suitable ID bit, load it with SnPb solder, put it to the pin and don't start sucking until the via's barrel is clean at completion (adjust for board, pin, tool and Jovian phase).  Additionally, apply some circular motion to the tool / pin in the final stage of heating and during evaculation to preclude sticky spots.  If the board is really tarnished some flux on the pin(s) may be necessary to initiate heat conduction, BGA gel flux would be my pick for this requirement.

If the board really has it in for you a pre-heater may help, although I have yet require to use one for TH desoldering and the pins falling onto the quartz elements might be "unhelpful".  I always use one of SMD PCB assembly with hot air.  See e.g. https://www.pcb-soldering.co.uk/aoyue-853a-pro-quartz-infrared-heater-220v.  Alternatively, you could use hot air to cook the board up; I always heat all the copper in the board rather than the joint - once the board is toasty a little more heat will flow the solder or if you get distracted it may just happen ...  This aproach works well for SMD assy.

HtH; BR  Martin

-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Rob Jarratt via cctalk
Sent: 15 April 2022 18:50
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Subject: Advice on Desoldering an IC

I am trying to remove an IC from my PDP 11/24 CPU, a DS8641. I am really struggling to desolder it. I am using the technique of applying fresh solder and then removing it. But after multiple cycles of this I think I am starting to damage the PCB.

 

I am using a fairly cheap desoldering station (this one https://cpc.farnell.com/duratool/d00672/desoldering-station-uk-eu-plug/dp/SD
01384?st=duratool%20desoldering). Its spec in terms of vacuum pressure is equivalent to that of the professional Hakko ones though. I am also trying a hand desoldering pump. None of these are able to clear many of the holes of solder, although some are doing better than others. Nevertheless, the IC remains stubbornly unmoving.

 

Are there any tips for removing ICs?

 

Thanks

 

Rob