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Why are Contacts not Uniformly Placed on SD Cards?

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If you have ever looked at the contacts side of an SD card, you may have wondered why the contacts are not uniform in size or placement. Why is that? Today’s SuperUser Q&A post has the answers to a curious reader’s questions.

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

Photo courtesy of CLF (Flickr).

The Question

SuperUser reader Rahul Basu wants to know why contacts not uniformly placed on SD cards:

As you can see below, some of the contacts seem to be longer than the others. Also, in the SD card section (second row from the top) the last contact (#8) seems to be thinner and closer to the seventh contact. Why not have all the contacts be the same size and place them at uniform distances from each other?

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Why are contacts not uniformly placed on SD cards?

The Answer

SuperUser contributor Ecnerwal has the answer for us:

The power and ground contacts stick out farther so that power is applied or removed, before or after, the data contacts are connected or disconnected.

The eighth and ninth contacts were added to the MMC format when making the full-sized SD format. There was not room for eight full-sized contacts (maintaining backward compatibility) and perhaps they learned a thing or two about applying and removing power first between the implementation of the MMC and SD formats (or about making it cheaper without an extra switch if they extended the contacts).


Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.

Akemi Iwaya (Asian Angel) is our very own Firefox Fangirl who enjoys working with multiple browsers and loves 'old school' role-playing games. Visit her on Twitter and .

  • Published 01/5/16

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