Slightly OT: TQFP mounting

Peter C. Wallace pcw at mesanet.com
Fri Dec 21 17:37:00 CST 2007


On Fri, 21 Dec 2007, Chuck Guzis wrote:

> Date: Fri, 21 Dec 2007 13:54:16 -0800
> From: Chuck Guzis <cclist at sydex.com>
> Reply-To: "General Discussion: On-Topic and Off-Topic Posts"
>     <cctalk at classiccmp.org>
> To: cctalk at classiccmp.org
> Subject: Slightly OT:  TQFP mounting
> 
> If you feel this subject is too far off-topic, please respond
> privately.  OTOH, it just might help some who are trying to repair
> some of the old gear that uses SMT.
>
> At any rate, I've got some 68-pin TQFP packages that I need to mount
> on a PCB.  I've searched the web and become throughly confused over
> recommendations.
>
> I'd like to stay away from using a hot-air rework iron as I don't
> think I have enough skill to use it without causing some damage to
> the PCB or component. I'd like to use my temperature-controlled
> Weller soldering station as the heat source.  I'll also be using Sn-
> Pb solder rather than lead-free.
>
> Two approaches that I've seen for conventional soldering iron
> mounting involve what I'll call "flood and suck" that involves
> covering all of the leads on one side of the QFP on the PCB with
> solder and then using a solder "sucker" (such as a Soldapullt) to
> remove the excess.
>
> The other approach uses solder wick (solder removal braid), laying
> the braid over the QFP leads and PCB and heating and applying solder
> *through* the braid to the leads.
>
> I'm fairly confident that I could do either, but who's had real
> success with either method?
>
> Thanks,
> Chuck
>
>


I've done the flood method many times, you dont actually need to use a solder 
sucker or wick much if you use a _LARGE_ excess of flux, and tilt the card as 
you solder each side so the solder you are 'flooding' with makes a little wave 
following the soldering iron tip. If you have enough flux and the correct 
angle (pins down and with a tilt to help the wave move), the surface tension 
of the solder will pull the solder bridges apart at the end of the 'wave'. 
Usually only the last two pins on a side will end up with a solder bridge. 
requiring solder wick to remove.


That said, there is a danger in flooding: if you cant get it right in a few 
passes, you may dissolve the copper pads in the excess solder, causing a 
distinct lack of holiday cheer...

Best thing is to practice on cards/parts you dont care about.


Peter Wallace




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