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I and my friend have designed a 0.8mm 4 layer PCB for our project. The top and bottom layers are for signal traces and the middle layers for power and ground. All the components that we use are SMD type and also we have few components on the board which contain plastic.

We did some experiments on soldering the components to the board and have failed a few times. The solder paste which we are using is lead-free solder which melts at 217 degrees celsius. Every component which we are using has a max peak reflow temperature of 260 degrees celsius.

The first time we tried to solder the components we used our hot air soldering station at 260 degrees celsius but we weren't able to solder the components at all, the solder paste wasn't melting. Then we tried higher temperatures, but the stone plate(where we had our PCB placed) seemed to absorb the heat and the board was losing temperature quite quickly.

Then we did some research and saw articles where people were using more than 300 degrees celsius, sometimes 350 and even 370. When we tried these temperatures the soldering was kind of successful, the solder paste melted and everything was soldered correctly. But the plastic components, buttons and the connector, have melted... Which weren't melting at 260 degrees celsius.

Also, we tried soldering some components on 2 layered PCB and everything was soldered within seconds even at 260 degrees celsius. We think that means that our PCB has way more thermal mass because it has 4 layers and also entire ground plane connected to every layer via many vias.

The last thing that's left is preheating the board evenly, so 260 degrees celsius would be enough. We don't have a hotplate, nor infrared heating bed. Considering our board properties and that the hot air soldering station is the only tool, can you guys give us some tips on how to preheat the board, what temperatures to use for preheating and soldering, and maybe which nozzles are better for such purposes?

Thanks in advance!

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    \$\begingroup\$ Are you unable to solder it by hand, with a soldering iron? If you must use a hot air gun (if you have BGA parts for instance), try soldering only the parts that don't melt with hot-air reflow (leave the melty parts off the board), then solder the plastic parts by hand. \$\endgroup\$ – Hearth Nov 3 '20 at 14:18
  • \$\begingroup\$ Sounds like the root cause is ground vias that aren't correctly drawn in the CAD. If you just smack 'em directly to the ground plane, then you create a PCB which nothing can be mounted on, since the whole ground plane will act as a very effective heat sink. Ground vias need to be "star shaped" with multiple small traces going out from them. \$\endgroup\$ – Lundin Nov 3 '20 at 14:24
  • \$\begingroup\$ Thanks for your reply @Hearth. Yes, we think it's a good idea to solder more vulnerable parts by hand but in our case, it will be a bit difficult. Because of the size constraints, our board is very small and dense with a bunch of 0201 sized components close to each other. Using a soldering iron to solder SMD SD card holder, 24 pin 0.5mm FPC connector, and few SMD buttons will be hard. If preheating the board will help us lower peak temperatures it will be the ideal solution. \$\endgroup\$ – Dada Kräuter Nov 3 '20 at 14:33
  • \$\begingroup\$ Thank you @Lundin for your reply. Yes, we use many vias and we have them directly connected to the ground plane without thermal relief. When we did research no one was using thermal relief for vias when via stitching and that's why we didn't do it either. So you think that thermal relief will make a big difference? \$\endgroup\$ – Dada Kräuter Nov 3 '20 at 14:40
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    \$\begingroup\$ Yes it will make a big difference. You don't need to use thermal relief on vias that are just used for the purpose of stitching, only on those that are directly connected to the ground pin of a connector, diode etc. Anyway, you should probably post your PCB layout as a separate question and ask for a design review. \$\endgroup\$ – Lundin Nov 3 '20 at 14:43
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You've designed a board with very small components that will be very hard to hand solder and then decided that you must use hand soldering. This is a bad idea, when you design a board you should lay out components such that you can assemble it effectively. But since you have the boards and probably want to try and use them, I think you have two options:

  1. Find someone with a reflow oven or build a basic one. This will raise the temperature enough to reflow the solder but not too much that you melt plastics.

  2. Hand solder it. This will be harder, but is still possible. My advice would be to use a soldering iron for as much as possible. 0201 components are not easy to solder, but can be done with a fine chisel tip. Then, rather than reflowing the entire board with hot air (which is really hard without a hotplate), turn up the heat on your hot air, get a small nozzle and reflow just the individual components that you cannot use an iron on. Switching to lead solder will make this easier since the temperatures can be lower, giving you more margin for error although lead free can certainly be used as well.

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  • \$\begingroup\$ 1. We thought about that but unfortunately for now it's not a possibility. 2. Yes, hand soldering will be possible but we have more than 100 components and it would take so much time... I kind of think that it is possible to preheat the PCB with a hot air soldering station itself. What if we'll set the temperature to around 200 degrees celsius and heat up the board evenly for a few minutes, then change the nozzle and crank up the temperature to 260 degrees celsius. Will that do the job? As we only have a few PCBs left, we are afraid to experiment and that's why we have posted here. \$\endgroup\$ – Dada Kräuter Nov 3 '20 at 18:13
  • \$\begingroup\$ @DadaKräuter Did you order SMD stencils with the PCBs? Otherwise, hand soldering 100 components on a single PCB should be less than a day's work, though you might need a microscope for the 0201, particularly to review the work afterwards. 0.5mm FFC connectors etc can be tricky, so start with those before anything else. \$\endgroup\$ – Lundin Nov 4 '20 at 7:48
  • \$\begingroup\$ @Lundin we did order a stencil also, otherwise putting solder paste for all of the pads would be a nightmare. :D \$\endgroup\$ – Dada Kräuter Nov 4 '20 at 9:57

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