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I am looking for a way to stack 2 PCBs on top of each other, with the top PCB flush with the bottom one (similar to castellated modules). However, I am looking for a solution that can be easily disconnected and can be automatically assembled by an inexpensive fab house. The PCBs also need to be somewhat precisely aligned.

I had a look at mill-max press fit pins and samtec bottom insertion sockets with alignement pegs, which could be useful for the bottom PCB (female) but not sure how to connect the top PCB (male)

Edit: some more details The connector is for low-speed protocols (e.g. 10MBps SPI). Ideally should support at least a hundred mating cycles. Mating height on the other side is less important

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    \$\begingroup\$ On top and bottom of your PCB sandwich, how much build height can you accept? \$\endgroup\$ Commented Dec 9, 2024 at 8:46
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    \$\begingroup\$ The choice of connector depends on mated height, number of pins, how often it should be removed, requirements regarding ruggedness, RF and speed concerns etc etc. Your specification is too vague to pick any particular connector technology. \$\endgroup\$ Commented Dec 9, 2024 at 9:52
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    \$\begingroup\$ Mezzanine connectors is the standard solution: they are cheap:ish, small and compact but not rugged. They are meant to get attached ones and maybe just removed once or twice throughout the product's life time, perfect for consumer electronics. For more rugged connectors something like 2mm "bottom entry" socket/header strips is cheap & reliable and can handle lots of matings. But they take more board space since need to drill holes through one of the boards. \$\endgroup\$ Commented Dec 9, 2024 at 9:53
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    \$\begingroup\$ @Lundin, Sorry for the lack of details, I've updated the question \$\endgroup\$ Commented Dec 9, 2024 at 14:42
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    \$\begingroup\$ What is the purpose to have zero space between the boards? Especially when the height of the total stack is unimportant. Could the OP just mount one board on the other and have a flat flexible cable joing the two? A hole and a slot (with pins), a Kelvin or Maxwell clamp to provide correct alignment between the boards. What make the relative locations inportant and not the relationship between the chassis and the top board? \$\endgroup\$ Commented Dec 10, 2024 at 8:58

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Your requirement that the boards be flush against one-another (zero mating height) cannot be met with standard board-to-board connectors because all board-to-board connectors are sandwiched between the two boards and space the two boards apart. Even dual-beam connectors (the flattest mezzanine connectors) raise the the daughterboard at least 0.5 mm off the motherboard. Single-piece interposers between the boards also have a non-zero thickness.

To stack the boards with 0 mm spacing you need through-board connectors with no housing between the boards. Any housing would need to be above the daughterboard or below the motherboard.

Here are some options using shroudless strips:

Through-hole pins on the motherboard and an SMT bottom-entry shroudless female strip on the daughterboard.

Pins and female strip

A shroudless male header mounted on the bottom of the motherboard and an SMT bottom-entry shroudless female strip on the daughterboard. Male and female strip

Here is a solution using loose PCB terminals. There's nothing below the motherboard and just the protruding pins above the daughterboard. Mill-max makes bottom-entry sockets: 8874

enter image description here

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    \$\begingroup\$ That's the solution I was considering (bottom entry sockets). In your example, how are the male headers attached? Are they press-fit? \$\endgroup\$ Commented Dec 9, 2024 at 14:44
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    \$\begingroup\$ A little detail here: one should consider PCB thickness. If the current requirements are made assuming standard 1.6mm PCB, we can perhaps switch to 0.8mm and then we have suddenly freed 0.8mm stacking height. Which could perhaps be used for SMD mounting the socket strip on the motherboard. Or switching to some mezzanine flavour. (100 mating cycles as per OP's edit might rule out mezzanine completely though?) \$\endgroup\$ Commented Dec 9, 2024 at 15:09
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    \$\begingroup\$ Aren't there risks involved with mechanical stresses during (repeated) insertions which are not quite in the usual direction (pushing from the bottom, away from the solder, rather than from the top, towards the solder)? \$\endgroup\$ Commented Dec 10, 2024 at 14:29
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    \$\begingroup\$ @DavideAndrea Yes, I was thinking more about the bottom entry female strip, but good to know about those loose sockets, I wasn't familiar at all with those. \$\endgroup\$ Commented Dec 10, 2024 at 16:16
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    \$\begingroup\$ @DavideAndrea I wondered about it. For some reason I have the feeling that the insertion stress must be higher than the unmating one, though I honestly cannot give any decent explanation for that. Possibly the fact that there are chances of slight misalignments during insertion, and the fact that insertion must cause the connector to move out, and not just the friction, which should require a little bit more force? \$\endgroup\$ Commented Dec 10, 2024 at 16:27
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Search for mezzanine connectors or board to board connectors

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    \$\begingroup\$ OP wants the boards to be flush. Mezzanine connectors space the boards apart at least 0.8 mm. \$\endgroup\$ Commented Dec 9, 2024 at 13:13
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Take a look at the following product portfolio from Molex:

https://www.molex.com/en-us/products/connectors/board-to-board-connectors/c-grid

I suggest this product line:

https://www.molex.com/en-us/part-list/0039?general_componentType=PCB%20Receptacle

In terms of costs and variety of products, Molex performs much better with respect to the 2 manufacturers you mentioned.

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  • \$\begingroup\$ Could you be more specific in your recommendation please? Linked page has a lot of different stuff .. \$\endgroup\$ Commented Dec 11, 2024 at 10:07

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