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I reflow my own boards in a toaster oven. When I drop my resistors off the paper tape, I always flip them over to black side up before I place them. Should I bother if it doesn't really matter?

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    \$\begingroup\$ The main reason I flip them over is so that I can see the numbers for post-assembly inspection. That's why I hate the fact that capacitors aren't labelled. \$\endgroup\$ – Dave Tweed Nov 6 '18 at 23:22
  • \$\begingroup\$ Most of the resistors I use have no markings due to size and cost restrictions. \$\endgroup\$ – isdi Nov 6 '18 at 23:31
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    \$\begingroup\$ @davetweed just mark them yourself, can't be that hard \$\endgroup\$ – PlasmaHH Nov 7 '18 at 7:41
  • \$\begingroup\$ No it does not matter, the resistance remains the same. \$\endgroup\$ – Misunderstood Dec 9 '18 at 4:52
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There are several reasons to put the alumina (white) side down. For small production/hobby runs it may not matter.

  1. You can tell if the resistor has been overloaded for whatever reason as the passivation/element color will generally change under high continuous heat.

  2. Depending on the technology, the contact dimensions may not be the same on the top as on the bottom. Pad geometry assumes alumina side is placed into the paste.

  3. It's a little easier to clean which can be an issue in high resistance components. Passivation layers typically have varying heights/roughness, unlike the flat alumina base.

  4. Wear and tear under mechanical stress, passivation layers are not as tough as the alumina substrate so if the board rubs against it it will eventually get damaged (usually only a problem on thin board under mechanical loads).

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  • \$\begingroup\$ What??? Have you ever manufactured a product? Some dimensions may not be the same?? Where do you buy your resistors? And what manufacturer would do that? Wear and tear? Mechanical stress?? Passivation layers??? Really? If your product was designed correctly why would it be overloaded? Got any links to a datasheet to backup what you say? \$\endgroup\$ – Misunderstood Dec 9 '18 at 4:59
  • \$\begingroup\$ fwiw, RCWK0306R100FMEA. insofar as metallization patterns, I have a Rohm ESR series 0805 under the microscope with two different patterns top/bottom (bottom looks much nicer btw). The passivation layer covers part of the top contact point. You won't see that on a datasheet, because the manufacturer intends the alumina to placed facing the board. If you place the resistor face-down the heat dissipating element is in a different thermal environment. Design margins vary depending on the intended environment and performance requirements, so doing this can reduce operating margins. \$\endgroup\$ – isdi Dec 9 '18 at 16:43
  • \$\begingroup\$ for the overload, it's meant as a diagnostic tool, not as part of a design error. \$\endgroup\$ – isdi Dec 9 '18 at 16:46

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