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I was wondering what is the history for having convex and concave resistor arrays.

Out of this EEVBlog entry and this Stackexchange question, the advantages of convex resistors are clear to me. What makes me wonder and is also the reason for my question:

  • Why are there 2 variants developed in the first place?
  • And why do they continue to co-exist if, apparently, convex has mostly advantages over concave ones?

Summary of convex resistor array advantages from the EEVBlog article:

  • 5-sided vs. 3 sided termination gives better solderability
  • A larger gap between terminations reduces solder bridging
  • Better view for visual inspection
  • Larger terminations increase reflow soldering self-alignment
  • Better part availability (70% of the market)
  • Lower cost in general
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Concave arrays were once somewhat easier to make than convex. They could also be made smaller for the same number and type of elements. Some types of pick and place machines work better with concave arrays. AFAIK, they have always had reliability issues in comparison with convex and discrete parts. With modern prices almost negligible, most designers and companies have generally focused on convex parts since the cost difference is trivial.

There will always be some applications that need that extra miniaturization, or have highly aggressive cost goals for extremely high volume low end consumer goods, or haven't changed their design for 20 years. There's also certain niches or designers that prefer convex for unusual or sometimes even irrational reasons. Thus, the convex parts will likely be available for the foreseeable future.

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