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I made my first batch of 6-layer PCBs. On some of the boards I got a solder bridge on an MCU - I know I need to improve my design, my solder mask, and my mounting method.

Usually on 2- or 4-layer PCBs I don't have any problems removing solder bridges with braid and flux, but on this 6-layer PCB it's nearly impossible. I have a lot of difficulty heating the area and it cools almost immediately. My solder "sticks" on the pin or on the pad and forms aggregates. Do you have any tricks to rework a 6-layer PCB ? Do I need a a heating table? Please help me to find a solution to save my board.

EDIT 2021-01-29
I Buy a cheap IR preheating plate, this save all my boards. I rework them quickly without problem. The 6 layers thermal inertia is impressive. Preheating plate

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    \$\begingroup\$ A photo of the scene of the crime might help. \$\endgroup\$
    – Transistor
    Jan 28 at 21:22
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    \$\begingroup\$ Got a hot-air gun? Use it to heat the bottom of the board while you solder. Your problem is the internal copper planes are sucking the heat from your soldering gun. It can't deliver enough heat to get the junction up to soldering temps. The solution is to pre-heat the whole board. They do sell special heating tables and such, but in a pinch you can definitely do it with a hot air gun (carefully!!!) i.e. images-na.ssl-images-amazon.com/images/I/… These will get the board hot enough to melt the solder if you overdo it, beware of that. \$\endgroup\$
    – Kyle B
    Jan 28 at 21:25
  • \$\begingroup\$ a pre-heater and rework flux gel can be helpful if you have access to one (together with hot air or iron). if using an iron, don't necessarily go for the fine tip, larger tip can actually make it easier. thermal reliefs on pads attached to large ground areas too, usually a reasonable compromise, for future designs \$\endgroup\$
    – Pete W
    Jan 28 at 21:26
  • \$\begingroup\$ Also keep in mind your BRAID is also a heat sink! You should be using the thinnest possible braid you can get your hands on to minimize this. Sometimes I'll even cut thick braid down into thin strips for this kind of purpose. \$\endgroup\$
    – Kyle B
    Jan 28 at 21:27
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    \$\begingroup\$ @PeteW Ok, I will find a pre-heater, thank you \$\endgroup\$
    – rom1nux
    Jan 28 at 21:35
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I usually use a hot air rework tool at the bottom of the board, you would be surprised that even getting the temperature of the board to 80C to 120C can do wonders for soldering by heating up areas of the board that are difficult. (If you increase to 120C on the other side it will cut the heat leak in half)

Sometimes this doesn't always work, so we got a infrared rework station and use that to heat the other side of the board.

Make sure you have good flux (I use this, it seems to work much better to wet solders and break down the oxide layer)

Another thing is if using an iron, use a new tip or a very very clean one as these build up oxides and reduce heat transfer.

(You can drill holes in a 4 or 6 layer board, you have to be really careful and not short layers together, a microscope is needed to inspect the hole if you need to disconnect a mid layer trace, this only works if there are planes or nothing between the fix and the outer layer of the board)

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  • \$\begingroup\$ I order a pre-heater table. Many thanks for the advice. \$\endgroup\$
    – rom1nux
    Jan 28 at 22:01
  • \$\begingroup\$ In a pinch, an electric skillet could work. I've had my eye on the missus, waiting for her to get a new one so the old one can be used for board reheating. ;) \$\endgroup\$
    – rdtsc
    Jan 28 at 22:15
  • \$\begingroup\$ Yeah, anything to get the heat up, I suppose you could also preheat it in a toaster oven also if you were desperate and could rework quickly \$\endgroup\$
    – Voltage Spike
    Jan 28 at 22:17
  • \$\begingroup\$ Yes heating up at 90/100°C solve all my rework problem, many thanks all for your advices \$\endgroup\$
    – rom1nux
    Jan 29 at 17:15
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Braids can sometimes get used, but are tricky for fine pitch. Plus they tend to remove all solder so you have to re-do the pins one by one afterwards, heating them once more. Needless to say you'll need the smallest braid you can buy.

This is what I do when I get bridges across pins on a QFP:

  • Add flux all over the pins.
  • Heat all pins containing the bridge at the same time, using the clean solder iron tip.
  • You can aim at the bridge itself - that is you can place the tip above the feet of the pins rather than at the place where they meet the pad, which would normally be the proper place to aim for when soldering. Often bridges appear since the pads are electrically connected, and heating the pins before the pads can counter that part a bit.
  • Move the tip horizontally away from the part, along the feet of the pin.
  • Superfluous solder should now either wet along the pads, end up on the tip or get dragged out on the solder mask where it can easily be removed.
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  • \$\begingroup\$ Yes, this work well, with a good flux, I solve 99% of my bridge approaching the tip of with hot air in few seconds. But on 6 layers PCB with inner ground plane it's really hard counteract thermal inertia. I solve my problem with preheating plate . Thanks for your advices \$\endgroup\$
    – rom1nux
    Jan 29 at 17:40

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