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If the contact pad is larger than the footprint, or has a different shape but is still large enough to make full contact with the SMD, will this lead to problems during reflow soldering?

I am designing a metal core PCB on which I will be mounting LM561c samsung diodes. I am striving for a high thermal dissipation factor, to preserve the effeciency of the diodes.

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    \$\begingroup\$ Maybe related: electronics.stackexchange.com/questions/17710/… \$\endgroup\$
    – Dampmaskin
    Apr 1, 2018 at 16:38
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    \$\begingroup\$ That's really unanswerable. Edit a picture of the part and the proposed footprint into your question. Problems may be most likely to result from things like imbalanced footprints, for example if you have an SMD resistor where one pad is thermally linked to a plane (without thermal reliefs) and so cools at a different rate. \$\endgroup\$ Apr 1, 2018 at 16:48
  • \$\begingroup\$ Thanks, I am a DIYer whos fallen down the rabbit hole. Regarding your point about the contact being linked to a thermal relief: on a standard MCPCB, wouldn't this not be an issue because the dielectric layer covers the entire face of the board, beneath the copper contact layer? \$\endgroup\$ Apr 1, 2018 at 17:18
  • \$\begingroup\$ It is the thermal conductance and heat spreading that is the issue. How much shared power per area is it and is it exposed convection flow or forced air and how will you verify Tjcn rise? by pulsed V diode temp? \$\endgroup\$ Apr 2, 2018 at 16:45
  • \$\begingroup\$ It should not be less than 2 Sqin/watt free air \$\endgroup\$ Apr 3, 2018 at 3:07

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In statistics as well in Contract Manufacturing DFM design for Manufacturability the probability of defects in soldering increases with the degree of mismatch to IPC footprints for the desired soldering method < RElow vs Wave solder has different footprints and layout orientation recommendations.

In solder reflow, and contract manufacturing, 99% of the defects are related to poor solder and many or most of these are design related to board layout.

Without details on your actual part, and pad design , it is impossible to assess.

I advise you to read about IPC solder pads and Mfg's recommendations and/or consult with EMS builder for DFM analysis or use your mentor and software to do the same.

DFM= Design for Manufacturability.
DFT= design for Testability
DFC= Design for Cost
DFX = Design for Excellence. ( all of above)

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  • \$\begingroup\$ I appreciate the detailed answer. I am building with lm561c Samsung LEDs \$\endgroup\$ Apr 1, 2018 at 16:58
  • \$\begingroup\$ How is it soldered? \$\endgroup\$ Apr 1, 2018 at 17:49
  • \$\begingroup\$ I advise you to use metal clad PCB if you have them close together otherwise MTBF reduces from poor heat transfer and Tjcn rise. \$\endgroup\$ Apr 1, 2018 at 17:55
  • \$\begingroup\$ Hi Tony, I intent to use MCPCB, and most likely reflow soldered, as it is the method described in the products technical documentation. As a last part to the question: would you advise designing the contact by outlining the shape of the footprint, or by adding another layer of copper on top of the trace that is in the shape of the footprint? \$\endgroup\$ Apr 1, 2018 at 21:50
  • \$\begingroup\$ I would use the recommended solder pads using Sinkpad MCPCB but depends on how many W/sqin or sq.cm. you intend to have. The top layer copper is already insulated over the MCPCB, so don't add any more sinkpad.com/what-is-sinkpad.php \$\endgroup\$ Apr 1, 2018 at 21:56
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Solder creates surface tension during reflow between the part and the pad. This surface tension can be controlled and apply different amounts of force to the part. The footprint also needs to contain all of the solder in the solder paste that will flow onto the part. Usually the footprints are larger to allow for solder to be applied to the part as the solder is applied via a stencil in a thin layer, and the volume of solder around the part may need to be larger.

If your only doing short runs and low volume, then none of this really matters as you can fix the parts after reflow if they are tomb-stoning or becoming misaligned during reflow. If your doing larger runs then the manufacturer can help you tune your footprints to allow for manufacturability, because they don't want to fix parts.

I usually do low volume (under 100) and I have done a similar thing with a voltage regulator. You can always put solder mask and expose the same footprint while keeping the same area for the recommended footprint. Or you could create a very large trace around the footprint. If additional heat 'spreading' is needed, you can also create vias to a different layer also ( if there is room).

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Check the datasheet for a manufacturers recommended footprint, copy that into your layout software and you shouldn't have any issues. You can connect the footprint to as much copper as you want so it doesn't really effect the thermal properties unless there's an optional pad and you omit it.

Just keep in mind that if the pads have uneven thermal dissipation then it might cause re-flow issues.

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  • \$\begingroup\$ Hi Drew, Can you please link me a to a resource that details this import process? Since the Recoomended PCB Solder Pad diagram is in a PDF, I'm not exactly sure how I would import just one cut of this file. \$\endgroup\$ Apr 3, 2018 at 1:48
  • \$\begingroup\$ Hi Chris, you have to re-create the footprint in your layout software using the measurements from the PDF. \$\endgroup\$
    – Drew
    Apr 3, 2018 at 21:27

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