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I have a design where two boards of the same size will be stacked like pancakes and physically touching. I want to electrically connect them, and I thought of just adding matching solder pads and putting the boards on a reflow oven. The idea is that the pads will melt a bit and form an electrical bridge across them, permanently bonding the boards and making a durable electrical connection. Here's the type of pad I was thinking of using. Is my idea crazy or will it work?

The manufacturing process also requires me to put it in a reflow oven anyway, so I thought I would save some effort by using this technique instead of something like a pin header or a board-to-board connector.

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    \$\begingroup\$ That seems risky -- try castellated mounting holes instead: learn.sparkfun.com/tutorials/… \$\endgroup\$ – calcium3000 Nov 22 '19 at 13:50
  • \$\begingroup\$ Are the pads coated in anything or are they just copper? Copper melts at 1085 °C. If the board has a HASL finish then it might work. Otherwise, you could manually put some solder on them before putting the boards together. \$\endgroup\$ – user253751 Nov 22 '19 at 13:50
  • \$\begingroup\$ The pads will be HASL. The idea is also to put some extra solder on it if needed using a stencil. This way I can do it all in one go instead of doing it by hand in dozens of spots. Much more reliable and consistent. I am hoping to skip the stecil though, and maybe it will work with just the HASL that the manufacturer puts in. \$\endgroup\$ – Ermir Nov 22 '19 at 13:53
  • \$\begingroup\$ Other solutions is mezzanine connectors or just male headers that solder to corresponding holes in the two PCBs. \$\endgroup\$ – Anders Petersson Nov 22 '19 at 13:59
  • \$\begingroup\$ Be careful of flexion, thermal cycles, and differing expansion coefficients. These will slowly degrade the solder connections. \$\endgroup\$ – rdtsc Nov 22 '19 at 14:09
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In theory, you'd expect overlapping pads to solder together, if assembled with solder paste.

In practice, they probably would, but you'd be unable to inspect the joints.

The way this is normally solved in the industry is to use plated through half holes, also known as castellated pads. This image of a module intended for surface mounting is from sparkfun. There are many modules available like this.

enter image description here

The mother board will have conventional SMD pads. The centres of the holes on the daughterboard aligns with the centres of the pads. Solder paste on the board pads will be necessary. On reflow, it forms a fillet up the hole, which creates a solder joint that can be inspected.

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  • \$\begingroup\$ Bingo. The search term might be "castellated pads" \$\endgroup\$ – Scott Seidman Nov 22 '19 at 14:00
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The normal approach is not to join the two surfaces, but to join the edge of the top board to the top surface of the one below. To ensure maximum solder adhesion and provide a plated copper edge to solder to, the top PCB will have castellation edges.

(As a commentator points out, the pads themselves do not melt at reflow temperatures! It's the solder that melts.)

It's not impossible to bond one surface to another with solder in the middle; that's what "BGA" parts are. However, it's harder to get the heat in there, and it's much harder to inspect and fix if it turns out not to work.

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  • \$\begingroup\$ I will put some HASL in the pads instead of just leaving the copper. This has a much lower melting temperature than the copper, which should allow it to bind with the HASL layer on the other board. If not, then I can add some solder paste using a stencil. \$\endgroup\$ – Ermir Nov 22 '19 at 13:55

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