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Old 09-08-2018, 10:28 AM   #1
dicky96
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Default QFP128 desoldering / rework question

Hi guys

I have to change a QFP128 chip for a repair I am doing. I've replaced 48 pin QFP chips (such as AS-15) before a few times but never a QFP128

With the AS-15 I basically add some 60/40 solder to the pins all around to reduce the melting point, apply flux and use the hot air station to unsolder.

I then use drag soldering with 60/40 and use flux plus desolder braid to clean up the shorts between pins and a jewellers loop for close examination of the work

I was thinking maybe of trying the same technique on the QFP128 but maybe to use something like Quick Chip low melt solder

Just to make things more complex this is a double sided PCB with a lot of SMDs on the other side underneath where the qfp128 is

Any suggestions?

Rich
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Old 09-08-2018, 11:35 AM   #2
CapLeaker
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Default Re: QFP128 desoldering / rework question

cut off all the legs with an exacto knife or dremel tool, then take the legs off the PCB with lots of leaded solder and solder wick? It all depends on how much room you got.

Last edited by CapLeaker; 09-08-2018 at 11:36 AM..
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Old 09-08-2018, 12:01 PM   #3
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Default Re: QFP128 desoldering / rework question

I agree with the above, most damage occures trying to remove a bad ic all at once, just be carefull not to cut any traces
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Old 09-08-2018, 05:17 PM   #4
dicky96
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Default Re: QFP128 desoldering / rework question

To be honest I used the technique of cutting off all the pins a lot when working on through hole with internal ground plane / layers / vias and found that is the best way of working on those boards

With SMD DIL or QFP I found that coating the pins with flux then applying a liberal amount of 60/40 to all sides of the chip with a soldering iron (so effectively I have one area of solder on each side, all pins shorted together) then using the hot air station to desolder works very well

But as I mentioned i never tried on something with so many pins as QFP128

What got me wondering about using very low melting point solder alloy was videos such as this one https://www.youtube.com/watch?v=ezYcYF94_k4

That looks like a good bit of kit - a bit expensive but not stupidly so.... if you do this for a living. But I would think a cheaper way to heat the PCB would work just as well, its mostly down to reducing the melting point of the solder

I never tried this Quick Chip stuff before though and it seems expensive to me. Anyone used that or can suggest a cheaper alternative? There are products such as this on ebay https://www.ebay.co.uk/itm/Low-Melt-...4AAOSwEK9UEZBr and others that seem similar and are a lot cheaper but are they any use for this sort of work?

Last edited by dicky96; 09-08-2018 at 05:19 PM..
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Old 09-09-2018, 07:47 PM   #5
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Default Re: QFP128 desoldering / rework question

I would be hesitant to cut the legs... Too easy to cut any tracks that run from the pad under the chip.
Chip Quik is a little expensive but it works and it is a lot cheaper than replacing a damaged PCB or at a minimum repairing some pads and tracks.
I suspect that some of the other low melting point products work also, but the Chip Quik kit https://www.chipquik.com/store/produ...ucts_id=210001 comes with good flux and a proven product. It is enough to do quite a few chips and good to have on hand.
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Old 09-10-2018, 01:09 AM   #6
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Default Re: QFP128 desoldering / rework question

I used to use a generic low melting point solder which was the same as quick chip, which I purchased by the roll until I invested in a half decent hot air rework station (Hako FR802) when I am back at the workshop later today I will see if I can find what solder I used to use.
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Old 09-10-2018, 10:33 AM   #7
dicky96
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Default Re: QFP128 desoldering / rework question

Hey that would be useful info if there is a cheaper alternative that does the same job.

Actually CPC/Farnell Have the chip quik kits in stock for just over 10 + VAT and free postage on orders over 8 in the UK. So I think I will order some and give it a go - my missus is back in the UK this week so she can bring over to me

One thing I'm a bit confused there is a SMD1 Chip Quik kit and a SMD1NL Chip Quik kit for very similar price, the NL being the lead free one.

As Chip Quik is meant for desoldering not for soldering, is the NL kit supposed to be better for removing components from lead free boards, in as much as it works better on higher melting point lead free solder, or is it just intended not to leave traces of lead on the board after rework - if you were bothered by that sort of thing?

Rich
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Old 09-11-2018, 06:15 PM   #8
llonen
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Default Re: QFP128 desoldering / rework question

Right you are looking for commercial solders that contain the following

51%In / 32.5%Bi / 16.5%Sn (60C eutectic)
66.3%In / 33.7%Bi (72C eutectic)
57%Bi / 26%In / 17%Sn (79C eutectic)
54%Bi / 29.7%In / 16.3%Sn (81C eutectic)

This site are the one I have used in the past

https://www.somersetsolders.com/sear...%20Temperature

https://www.somersetsolders.com/tin-...der-paste/p195

A little more expensive than I recall, but they used to do smaller quantities. The main problems I used to find with this stuff amongst others it used to leave a hard to remove deposit which I found would interfere with subsequent soldering / rework.

Hope this is helpful

Last edited by llonen; 09-11-2018 at 06:18 PM..
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Old 09-11-2018, 08:01 PM   #9
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Default Re: QFP128 desoldering / rework question

Just use your hot air station to unsolder the chip. If board is "extra tough", put it on a grille atop your kitchen's stove top / burner, and put the heat to low (both gas and electric burners are okay). Let the board heat on the bottom for a minute or two, then blast the chip with your hot air station from top. The solder should melt a little quicker and the chip should come out easier. Just make sure you don't try to pull the chip with the solder still not melted, as that can pull traces out. One thing I do to avoid that is I give the chip a little nudge when I think it has reached temperature. If the chip moves easily on all corners, it's ready to go. If any corner feels stuck, keep heating a little more (or crank up the temperature a little bit).
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