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I want two PCB's to be soldered with each other in 90-degrees style.

I think that the base PCB should have a slot with the width equals to the top board thickness. The top board should have a flange with SMD pads. Looks simple:

enter image description here

Questions:

  • How do I calculate the slot width? Let's say the top board is 1.8mm thick with two 70 um copper layers. This will give me the total stack fhickness of 1.94 mm. Is it OK to mill the slot with 2mm cutter?
  • Should I make the slot plated or not? Or should I make the slot with "castellated" style edges to allow the solder flow through the slot?
  • Is there anything more I should think of doing things like that?

UPDATE

Forget to mention: I need these connections to withstand a lot of Amps. Like 30 or so.

Regarding the forces: I did not think about it. Moreover - I will have a heavy component on the top board (relay). So I can become a problem... Now I think that I could make additional fixtures on the top cover to prevent the top board from bending. However this is not a solution is the board will be soldered with an inclanation.

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    \$\begingroup\$ I am not 100% certain but I think when PCB fabs say their board is 1.8mm thick they mean that 1.8mm is the total thickness of the copper-clad they start with, including 1oz copper. It sounds like your board has 2oz copper, so you should just add the thickness of that additional 1oz (35+35=70μm total) to the thickness. Your PCB fab may have different definitions; ask them if you're unsure. \$\endgroup\$
    – Hearth
    Commented Mar 12, 2019 at 15:59
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    \$\begingroup\$ Do you intend to wave or hand solder? talk to EMS process engineer about stiffeners as solder is not best solution mechanically with high lever torque.. UV cured Polyurethane is used for stiffener to prevent vibration in large format parts, I think is reliable, but hard to repair. I use cheap 24 hr cure stuff at home. Strong and ductile. \$\endgroup\$
    – Tony Stewart EE75
    Commented Mar 12, 2019 at 16:41

4 Answers 4

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I want two PCB's to be soldered with each other in 90-degrees style.

In general, this type of mounting is NOT recommended.
Using just solder to provide mechanical strength is unwise in the extreme. If the board was mounted flat (parallel) to the base board, then castellated half connections are viable since the forces on the joints are lower. These types of installation usually have a minimum of double sided tape providing extra mounting support for the daughterboard.

Soldered connections have a brittle interface, so using them in any situation where force is applied is poor engineering.

You have given no idea of the mass or size of the board to be supported at right angles to the baseboard. This will have an impact on how effective this mounting might be.

You can provide more strength for the soldered joints by using right angle wire/pin connections:

enter image description here

Even here (where the intermetalics span the plated through holes) you have to be careful about the mass, lever size and vibration environment for your boards.

Double row headers help considerably, you can see many options here:

enter image description here

While the pins have considerable holding force, depending on the mass of the daughterboard you may need other mechanical restraints to support the boards.

Update: since you now note that the connections carry considerable current. I'd suggest that a 'solder only' connection is completely unsuitable. The resistivity of solder is an order of magnitude more than that of copper pins.

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    \$\begingroup\$ It might be worth mentioning how these compare to card edge connectors, too. \$\endgroup\$
    – Hearth
    Commented Mar 12, 2019 at 17:01
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    \$\begingroup\$ Forgot to mention: I have tens of Amps flowing through these cinnections. I'm not sure that pin headers will not get hot if I will put 2-5 Amps peer each pin. Moreover soldering providing the fixatin when pin headers - not. \$\endgroup\$
    – Roman Matveev
    Commented Mar 12, 2019 at 17:09
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    \$\begingroup\$ @RomanMatveev The resistivity of solder is magnitudes higher than that of a copper pin. You absolutely need the pins if it's high current. \$\endgroup\$
    – Jack Creasey
    Commented Mar 12, 2019 at 17:20
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    \$\begingroup\$ That's a thing I wasn't think of. However - being 10 times less conductive - solder conductors will be much shorter and much thicker. So generaly I think that it will not be a problem. I could conduct an experiment when I will be at work. \$\endgroup\$
    – Roman Matveev
    Commented Mar 12, 2019 at 17:25
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    \$\begingroup\$ You need to think of the copper pins as being in parallel with the bulk solder around them . DO NOT rely on bulk solder as a high currant conductor. \$\endgroup\$
    – Jack Creasey
    Commented Mar 12, 2019 at 17:41
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enter image description here

You might consider if you have space for this.

https://www.digikey.com/product-detail/en/harwin-inc/M22-2530605/952-2286-ND/3728250

Final comments

  • Tin plated steel alternates are adequate since there are no air contacts to corrode. ( no gold plating or berryllium copper needed)

  • However dummy socket for physical alignment needed for solder process for smooth engagement on assembly.

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  • \$\begingroup\$ Could you make clear: you meaning to use pin headers as the only fixation? Or it can (should) go with board to board soldering? I need several tens of Amps to flow. \$\endgroup\$
    – Roman Matveev
    Commented Mar 12, 2019 at 17:15
  • \$\begingroup\$ header contacts to sockets are rated for 2A but solder joints can be higher with proper THT holes and matching trace sizes with 2oz Cu, but you did not ask about traces \$\endgroup\$
    – Tony Stewart EE75
    Commented Mar 12, 2019 at 17:21
  • \$\begingroup\$ I'd use poligons, not traces. \$\endgroup\$
    – Roman Matveev
    Commented Mar 12, 2019 at 17:23
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    \$\begingroup\$ It's not linear as heating from neighbour increases local ambient temp, so modelling or benchtest is advised \$\endgroup\$
    – Tony Stewart EE75
    Commented Mar 12, 2019 at 17:31
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    \$\begingroup\$ @RomanMatveev Typical pins of 0.1" headers are 0.64mm square, or 0.41 mm2. They are good for 5 to 7 A each even as infinite wire. Their ampacity approximately matches external 100 mil trace of 2-oz copper. So you can't do much better. \$\endgroup\$
    – Ale..chenski
    Commented Mar 16, 2019 at 1:14
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How do I calculate the slot width? Let's say the top board is 1.8mm thick with two 70 um copper layers. This will give me the total stack thickness of 1.94 mm. Is it OK to mill the slot with 2mm cutter?

Talk to the manufacutrer of the boards to see what their tolerances are. That being said, a standard board is 0.061' to 0.064' (1.54mm to 1.62mm), I would allow for something a little wider, like 0.068' (1.72mm) if you don't plate it.

Should I make the slot plated or not? Or should I make the slot with "castellated" style edges to allow the solder flow through the slot?

I would plate it, it would add more points that the solder could adhere to and create a better joint. Realize that plating usually adds around 0.001" (0.0254mm) thickness to the slot on both sides, and the hole wider.

Is there anything more I should think of doing things like that?

Remember that copper plating is not very 'structural', meaning if you put any kind of force on the PCB, copper plating rips right out. So be careful in the amount of force that is exerted on these two PCB's. Levers also increase force so the larger the PCB's are, the more moment or torque can be put on the joint between the two boards and the more likely there will be failure, so keep the PCB's small. There are better methods of accomplishing what you want than soldering, Like connectors or FFC (flat flex cable) solutions

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You can do this, however the slot weakens the board. You can have pads on both sides of the vertical board as through-jumpers to ease the layout problems the slot will likely cause.

I've even seen this used in consumer products with paper-based phenolic single-sided boards (much weaker) for very light parts. It's a cheap construction, as well as being inexpensive.

One caution- it makes repair very difficult or impossible without the right tools (such as a desktop solder wave or IR rework station).

Personally, I would prefer headers even if you have to solder both sides.

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