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We have some PCBs that were designed to drive high-power ultrasound transducers. There was an issue with the boards early on that resulted in several of them blowing diodes.

I have since fixed the issue on the remaining boards and am trying to repair the dead boards to solder on some new diodes.

However I have run into an interesting issue that I have never seen before, which is that the lead frames of the QFN packages seem to have metallurgical bonded themselves to the PCBs. This is presumably because they are copper lead frames.

The board is 4-layer with 1oz copper and a large number of thermal vias - there is normally a large heat sink on the reverse side which I have removed it during the rework attempts.

Below is an example of a diode that I've tried to remove. To heat, I am using a hot plate (Weller WHP200) to bring the board up to ~150*C and then a heat gun to bring the diode up to ~350*C. However on all five diodes that I have attempted I seem to be only able to remove the package - the lead frame seems to be stuck fast. Even heating to ~400*C doesn't seem to allow it to budge.

Attempted Desoldering - Separated Package From Lead Frame

I also have a 120W soldering iron to hand (Weller WXP120), and have tried it out as @Sphero suggested. However even leaving it on the copper slug for a good couple of minutes at 380*C (board hot enough that solder on the RS1 pad in the picture was wet), the slug won't budge.

Has anybody come across this issue before? Do you have any suggestions for how to remove the lead frame?

I don't mind experimenting on the boards as if they can't be fixed, they are scrap anyway. So if you have any ideas you'd like me to try, I'm all ears.

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  • \$\begingroup\$ Are you using an actual hot plate? Those aren't great for pre-heating. Because you are bringing the board up by 100°C or so, but you also attach a huge chunk of metal to the board that is 200°C below the temperature of your airgun, creating a huge thermal mass with too low a temperature. Actual board pre-heaters are much better, since they use IR or hot-air to pre-heat from the bottom, which defeats that draw back. I have had these issues purely on thermal mass at customer sites with hot-plates, never with my own lab's pre-heaters. \$\endgroup\$ – Asmyldof Jun 29 '18 at 12:42
  • \$\begingroup\$ @Asmyldof it's a Weller WHP200 hot plate - basically radiant heater. \$\endgroup\$ – Tom Carpenter Jun 29 '18 at 13:01
  • \$\begingroup\$ Somehow mechanically crush that slug into small pieces with a small diagonal cutters? \$\endgroup\$ – GAttuso Jun 29 '18 at 13:30
  • \$\begingroup\$ @TomCarpenter Just suppose I said nothing then. There's a lot of people saying "hot plate" and meaning actual hot plate. Because those only cost 40 euro at Farnell, and Farnell sells it, so it must be good, right? So I had to check. But as I insinuated, and Spehro answered, it's not metallurgy, I'm high-90%-confident it's just heat issues. -- Read remainder below and dropped to 40% confidence level that it's not metallurgy. \$\endgroup\$ – Asmyldof Jun 30 '18 at 1:21
  • \$\begingroup\$ @Asmyldof no worries. I totally understand where you were coming from. \$\endgroup\$ – Tom Carpenter Jun 30 '18 at 8:24
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You'll need a h*ll of a lot of heat to get that solder to melt, that's by design, all those vias and probably 2oz or greater copper thickness is designed to spread the heat.

An air gun probably isn't going to be enough. There are specialized rework tools, but for a few you might want to try a BIG soldering iron, say in the 120W range, after preheating it to 100'C.

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  • \$\begingroup\$ 1oz copper on the board, but we did place a lot of vias on this board - there is normally a large heatsink on the reverse side (I've removed it during this rework). I have a 120W soldering iron to hand (Weller WXP120), and just tried it. Even leaving it on the copper slug for a good couple of minutes at 380*C (board hot enough that solder on the RS1 pad in the picture was wet), the slug won't budge. \$\endgroup\$ – Tom Carpenter Jun 29 '18 at 13:09
  • \$\begingroup\$ @TomCarpenter: Have you tried adding leaded solder and watch if that solder stays molten, alloys with the existing one, or solidifies? \$\endgroup\$ – PlasmaHH Jun 29 '18 at 13:12
  • \$\begingroup\$ Along with @PlasmaHH's comment, do you have any low-melting point alloy such as "Fast Chip"? \$\endgroup\$ – Spehro Pefhany Jun 29 '18 at 13:18
  • \$\begingroup\$ @PlasmaHH I've tried with some leaded solder, and no change. The solder has appeared to mix with the existing solder remaining molten, but still no luck removing the slug. After cooling the solder is very shiny if that helps. \$\endgroup\$ – Tom Carpenter Jun 29 '18 at 13:22
  • \$\begingroup\$ @TomCarpenter: Remaining molten is what counts. If it is and a pair of pliers won't get it off there, it is bond to the pad in some other way. This is weird but not impossible, it may have welded in one way or another, so soldering is not an option. I have had to remove some things with a mini (cnc) mill. \$\endgroup\$ – PlasmaHH Jun 29 '18 at 13:33
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The lead frame is basically just a solid chunk of copper, and in the process of trying to desolder it, I ended up just tinning the copper slug.

It dawned upon me that as it is copper an a decent heat conductor, I might as well just try to solder a new diode straight on top of the old lead frame.

Soldered On Top

A little bit crooked, but seems to have gone on OK. Managed to build up a bump of solder on the smaller pad to match the height of the copper slug. Then used the hot air gun and flux to get the solder to connect up nicely to the diode pads.

Doesn't really answer the question of how to remove the lead frame, but it does get me to the stage of having a diode reattached. Just need to power it up and give it a spin.

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  • \$\begingroup\$ +1 Looks feasible enough. \$\endgroup\$ – Spehro Pefhany Jun 29 '18 at 21:05

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