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Ok, so I goofed... accidentally spec'd a 4mm footprint for a 3mm real-time-clock-calendar IC.

Rather than scrap these boards, I'm thinking of designing a double-sided PCB "adapter" to go from the 3mm to 4mm sizes, and sandwich this between the device and main PCB. Hand-reflow using hot air. Device is low power so no thermal conerns.

Has anyone tried SMT conversion like this? Envision any pitfalls? Anything to be aware of? Do I earn the "bodge of the year" award? :)

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    \$\begingroup\$ How many boards are we talking about? \$\endgroup\$
    – uint128_t
    Commented Jul 2, 2016 at 20:31
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    \$\begingroup\$ About a dozen boards. \$\endgroup\$
    – rdtsc
    Commented Jul 2, 2016 at 20:36
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    \$\begingroup\$ @rdtsc Are these board particularly precious (lots of layers, heavy copper, blind and buried vias, and so on)? \$\endgroup\$
    – Nick Alexeev
    Commented Jul 2, 2016 at 21:23
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    \$\begingroup\$ @NickAlexeev 4-layer, external ground plane, 1oz external copper, 0.5oz internal, blind vias, no buried or micro vias. \$\endgroup\$
    – rdtsc
    Commented Jul 2, 2016 at 21:58

2 Answers 2

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[This is more of an extended comment than an answer.
I'm posting this as an answer, because I'd like to add a picture.]

You are not the first person to do that sort of goof. At least, your footprint is larger than the actual part. If it were the other way around, it would have been more painful.

Yes, correct-a-chip adapters are made and used once in a while. There are even companies that specialize in such adapter boards (Adapters Plus is one I have talked to).

If I were in this situation, and this were a one-or-two-off prototype, I would wire the IC in a dead bug configuration. As you've already mentioned, it's an RTC, so no high speed or thermal issues.

enter image description here (source: Screaming Circuits blog)

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  • \$\begingroup\$ sixteen manual solderings, hard ones, instead one hot air touch... \$\endgroup\$
    – user76844
    Commented Jul 2, 2016 at 20:48
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    \$\begingroup\$ The OP's situation is actually opposite from your picture -- 3mm chip on a 4mm footprint. If the pads reach, it's possible that at least half of the connections (the center two on each side) will be made anyway, and then some "creative solder blobs" might do for the rest -- unless the pads are wide enough to short to the wrong pins. \$\endgroup\$
    – Dave Tweed
    Commented Jul 2, 2016 at 21:16
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Don't hurry, there is a better way. If pitch is close to the original, use kapton to mask the thermal pad, then just solder your QFN above it. probably it will work fine without thermal pad connected to ground.

Adapter board is ok too, but it will be very delicate, since you will have to make really tiny half-holes on the sides.

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