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I would like to design and get produced a device in miniSD or microSD card form factor. This is not actually a memory card, it just uses that connector. There are other devices like that, for example Electric Imp. I'm wondering what type of circuit board and what type of outside packaging is used in devices like that?

microSD thickness is only 1mm. There are plenty of chips which are thicker than that, even in the thinnest possible package they come in. Typical circuit boards are 62 mil (1.62mm), the thinnest I have seen is 10 mil (0.25mm). I'm not sure if a multi-layer board can be made in 10 mil in any case. Are microSD memory cards usually made as system in package (SIP) or do they have a board inside? In non-memory devices, what type of board is used (if any) and how do they solve the device height problem?

In a device like this, are the contact fingers on the circuit board or on the case? If on the case, then how does the board connect to them? How is the case made?

EDIT Bounty started. Please describe: How are such devices built? (wire-bond, PoP, encapsulated in epoxy, WLCSP on thin or flex board, etc ...) If a board is used, what kind exactly? Who can manufacture these? (board, assembly, encapsulation, ...) Any example whitepaper explaining the design or different designs? How to deal with over-thick chips? (for which no thin replacement exists) Thanks!

EDIT The answer below had some good ideas, but I am still looking for a definitive answer. I want a "design guide/white paper" for small form factor encapsulated devices like this, and some pointers to manufacturers. Any good answer will get a bounty.

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  • \$\begingroup\$ PulsarProFX sells 5 mil PCBs. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Oct 7, 2013 at 7:43
  • \$\begingroup\$ How many components do you expect to stick on this micro-sd card sized PCB? Also, I hope you have a wirebonder and the resources to do epoxy encapsulation. You're not likely to be able to put pre-packaged ICs on the board, you'll have to mount the IC's COB-style, and encapsulate them in epoxy. \$\endgroup\$
    – Connor Wolf
    Oct 7, 2013 at 8:42
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    \$\begingroup\$ Most micro-sd cards are made by stacking up the flash memory IC die and a controller IC, and then directly wirebonding them together. There is a teardown of a bunch of micro-sd cards here: bunniestudios.com/blog/?p=918 \$\endgroup\$
    – Connor Wolf
    Oct 7, 2013 at 8:43
  • \$\begingroup\$ google.com/search?q=wlcsp \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Oct 7, 2013 at 10:22
  • \$\begingroup\$ @ConnorWolf: I would like to put 4 ICs, and about 20 supporting components (many of which can be 0201). The ICs do come in WLCSP, except for one which comes in a 0.9mm thick BGA (so, 1mm total thickness is unlikely unless I can grind off a bit from the top of that, haha). I am indeed having trouble fitting this on a one-sided 11x15mm board... microvia in pad anyone? :) \$\endgroup\$
    – Alex I
    Oct 14, 2013 at 22:57

1 Answer 1

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I am not very sure what are your manufacturing powers, but here are some considerations:

  1. You can use flexible PCBs. Produced on polyimide base they can be 0.20mm thick for two layers board and 0.13mm for one layer board.

  2. You can implement the connection contacts as traces on this PCB. As long as the PCB is flexible you can use one side PCB and to bend it this way in order to provide some space for the parts:

    connector 
_________     parts
         \__A__A__A__A______

  3. Some very small parts have to be used, but considering your other questions on electronics.stackexchange.com you already have most of them.

  4. BTW, if it is too expensive, or too hard to put everything in a SD card package, there is another solution, accessible even for an amateurs - put everything in small box that ends with SD card-like connector. This way, the thin part of the design will be only the connector - a piece of PCB with contact tracks:

                      ___________
 connector pcb   / BIG PARTS \
        ===================== |                
                 \___________/
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  • \$\begingroup\$ +1 for putting the main electronics outside of the SD connector (#4), much easier. Many PC MCIA cards used to be designed this way. \$\endgroup\$
    – tcrosley
    Oct 15, 2013 at 17:16
  • \$\begingroup\$ @johnfound: Thank you, all good ideas. My "manufacturing powers" are to find a contract manufacturer, preferably one that has experience in this type of design. But in the short term also some way to prototype... \$\endgroup\$
    – Alex I
    Oct 16, 2013 at 6:12
  • \$\begingroup\$ P.S. I need 4 layer board, this design is absurdly hard in 2 layer. All copper can be 0.5oz though. \$\endgroup\$
    – Alex I
    Oct 16, 2013 at 6:16
  • \$\begingroup\$ @AlexI Well, flexible boards exists in 4 layers. With a greater thickness of course. But I really think the point 4 from my answer is the best solution here. It solves a lot of troubles with only little disadvantages. \$\endgroup\$
    – johnfound
    Oct 16, 2013 at 6:20
  • \$\begingroup\$ @johnfound: yes, slightly extended microSD (and thicker in the extra part) is something I considered of course. I do want the overall design to be very compact, so while if there is just one or two components which are over-tall I could move them to the "box" area, I want to have components in the thin part as well, overall size target might be 25x11mm or whatever along those lines. \$\endgroup\$
    – Alex I
    Oct 19, 2013 at 18:29

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