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I'm going to install a heatsink on a KBU810 bridge rectifier. As the picture shows, the leads are closer the face with markings on it.

KBU810

I wonder if one of the two faces is the preferred site for installing heatsink, maybe because the semiconductor inside the package is closer to it. I wasn't able to find any related information in the datasheet.

Update:

Pictures from front and rear sides:

front side

rear side

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    \$\begingroup\$ Writing side up would be normal. \$\endgroup\$
    – user16324
    Commented Sep 6, 2021 at 18:25
  • \$\begingroup\$ Some heat-sink arrangements provide a thermal path to both sides. If it is not explicitly stated in the data sheet or obvious from exposed metal (less likely on a bridge rectifier) it might be possible to test. Glue an identical thermal mass to both sides and then run a reasonable current through the device and measure the temperature of the thermal masses and see which one increases in temperature faster. \$\endgroup\$
    – KalleMP
    Commented Sep 8, 2021 at 7:13

4 Answers 4

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If one side seems to be metallic, I would mount the heatsink on that. Otherwise, I would mount it on the side that does not have lead markings etc.

Leave this side uncovered and put the heatsink on the opposite side.

enter image description here

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  • \$\begingroup\$ By "lead numbers" you mean the signs "+", "-" and "AC"? \$\endgroup\$
    – apadana
    Commented Sep 6, 2021 at 18:38
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    \$\begingroup\$ Yes. I should have said "lead markings" \$\endgroup\$
    – user80875
    Commented Sep 6, 2021 at 18:46
  • \$\begingroup\$ Some brands have the text on the other side though... \$\endgroup\$
    – Jeroen3
    Commented Sep 6, 2021 at 20:26
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    \$\begingroup\$ @Jeroen3 at that point, the logic is "neither side is obviously better, so leave the useful information visible". \$\endgroup\$
    – hobbs
    Commented Sep 7, 2021 at 15:07
  • \$\begingroup\$ @hobbs I will test the difference when I have time. Have a few lying around. \$\endgroup\$
    – Jeroen3
    Commented Sep 7, 2021 at 19:50
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I doubt there will be any significant difference. Maybe I should measure this.... Text side is also not guaranteed to be same side between the brands making these types of diodes.
Just do a google image search.

But here you can see a leadframe photo, which suggests the thick plastic side is the side with the metal since the leads come up a bit.

enter image description here

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Normally you’d want a minimum thickness of plastic between the lead frame and the heat sink. I'd suggest that the markings should be against the heat sink, based on this illustration from a Vishay datasheet for a 25A bridge in a similar GBU package.

enter image description here

In general, when dealing with second-sourced products, or copies of copies, it’s sometimes useful to look up the original datasheets for information that may be omitted from cloned datasheets. Of course you can’t depend on anything that isn’t in the manufacturer’s datasheet (and sometimes not even that) but it can still be valuable. The congruence of original and clone manufacturer’s specs is a kind of twilight zone.

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  • \$\begingroup\$ Are the letters not slightly raised, therefore (potentially) creating an air gap between heatsink and case? Unless your thermal pad is thick and soft enough to bridge it. \$\endgroup\$
    – Michael
    Commented Sep 7, 2021 at 6:01
  • \$\begingroup\$ @Michael aren't the letters usually etched with a laser? I would assume they would generally be sunk down, not raised. maybe packages with dyed markings are slightly raised but i imagine that would be thin enough to be dwarfed by other irregularities \$\endgroup\$
    – user371366
    Commented Sep 7, 2021 at 6:20
  • \$\begingroup\$ I don’t think that is significant- run your fingernail along a chip and see if you can feel anything. Anyway, you would not likely use a silicone thermal pad, they are pretty terrible thermally (but cheap, and provide insulation), you’d just use a bit of thermal grease to fill the inevitable gaps from roughnesses and differences in flatness between the two parts. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Sep 7, 2021 at 6:27
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    \$\begingroup\$ @SpehroPefhany . Congratulations on achieving 300000 reputation. Always accurate. Always helpful. \$\endgroup\$
    – Marla
    Commented Sep 7, 2021 at 19:03
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I wasn't able to find any related information in the datasheet.

This is on page 2 of the data sheet: -

enter image description here

It's telling you to options. The first option is use big solder copper pads and the 2nd option is to mount a heatsink and, as Charles as pointed out that will be on the non-written side of the device.

And this is on page 1 of the data sheet: -

enter image description here

This information allows you to calculate how big a heatsink you need for such and such a power dissipation.

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    \$\begingroup\$ Yes, I had seen this information. I decided to mount a heatsink, but didn't know which side is better. \$\endgroup\$
    – apadana
    Commented Sep 7, 2021 at 8:04
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    \$\begingroup\$ This does not answer OP's question, which is not asking about how to determine the size of the required heatsink, but is rather which side of the device to install it on. \$\endgroup\$
    – J...
    Commented Sep 8, 2021 at 10:29

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