Difficult connectivity issue 1

[RAndres]

I've got a tough connectivity issue. I need to place a BGA with ~100 balls on a round PCB. The board is not much larger than the footprint. It's about 0.1 inch larger radius than the minimum possible given the inscribed rectangular part. The part is .340 x .245 inch. I need to bring about 20 connections from that board to another board which is at right angles. All this is to fit in a cylindrical housing with ID just large enough to fit the round board. I can't find any connectors which would fit, so I've been looking into doing this with flex circuit. The parts won't move relative to each other after installation. I've got some thoughts, but would like to hear from anyone who has dealt with similar issues. I've attached a crude sketch.

 

[VEBill]

I'll ask the most obvious question first. This question is based on the crude sketch.

Can the round PCB be rotated by 90° such that it's parallel to the long axis of the tube?

If so, one might make one larger PCB.

I anticipate that the answer is 'no'.

 

[VEBill]

Next thought.

I've been inside several of my gadgets, including iPhones and Surface tablets, usually to replace deeply buried batteries (which seem to at the very bottom, requiring complete disassembly).

Those sorts of gadgets have plenty of infinitesimally tiny multipin connectors. Even under optical magnification, they're still small. Hilariously tiny.

Problem is, your PCB is extremely tiny too.

One would need to consult the catalogs, and I have no suggestions how to find those.

 

[MacGyverS2000]

Hirose, JAE, and TE all have a number of connectors small enough to work... just depends upon your budget.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[RAndres]

Unfortunately the BGA HAS to be oriented as shown. I've looked at connectors, and the style used in phones and things like that using flat flex are the smallest I've seen. They are almost small enough to work, but not quite. If I pare down to the absolute minimum I need 16 connections, which may not be possible, I still haven't seen connectors under 1.2 or 1.3 mm wide. Maybe I need to dig deeper than Digikey, but they usually show a pretty comprehensive selection.

 

[VEBill]

Wristwatches (electronic, especially the recent Smart Watches) could perhaps be another source of ideas for tiny connectors. I assume that they'd perhaps offer some examples that are the next generation of more-insanely-small than Smartphones. (Speculation alert.)

If it was me, I'd see if YouTube offers any Smart Watch tear-down videos. In case they identify a source of tiny connectors, or similar interconnection ideas.

---

Failing that, you might need to use a flexible Kapton-like PCB somewhere in the path. Perhaps connected to the far side of the PCB and routed around (through a very slim and tiny gap) to where you're headed.

Or, move the BGA part to the other side, and then you'd have the entire back side to make a surface mount connection.

But I expect that you've already got some other unspoken sensor parts on the other side of the circular PCB, on the assumption that you wouldn't have such a stringent positioning requirement on a chip....if it was all by itself.

 

[btrueblood]

How many of these are you building? Hand-soldering a strip of ribbon cable is one alternative, but obviously isn't such a great idea in large quantities.

 

[MacGyverS2000]

You have enough space on that board for ~0.42" (10.7mm) long connector... with dual rows, that's not a bad pitch at all (~0.5mm). Should be plenty of options, whether it's plug-to-plug or plug-to-flex at both ends.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

An obvious option would be a mezzanine board arrangement, to get more space for the connector, by moving some of the unspoken components onto the 2nd board.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[RAndres]

I appreciate all your thoughts, especially Dan and VE1BLL. The BGA is an image sensor, so must face out of the cylindrical housing. The best solution I've come up with to date is a flex circuit between the two boards. The diameter of the round board must fit within the housing's 9.5 mm ID, so a connector would have to have a footprint of under 9mm. I see that I goofed the dimensions of the BGA, it should read 0.240 x 0.245 inches. I entered the diagonal across the rectangle for the first dimension.

IRstuff, mezzanine would be great as I'm more familiar with that than flex, but I haven't found a connector to allow it. If I keep the other board close, I think I can get away with putting all other required components there.

I've been messing with the stackup for flex and outline, and the minimum bend radius is giving me trouble. Requirements I've seen call for 10x the thickness of the flex for single bend during installation. With a .01 thick flex, that's a lot. Currently looking at how much I can trim a secant section off of the round board to allow room for this bend. Width of the flex comes in to play as well.

 

[IRstuff]

Bear in mind that the bend radius is usually recommended for something that might actually flex on a regular basis, so the bend radius mitigates work-hardening and breakage. If this is not intended for those applications you could probably fold the flex tape. Alternately, you could possibly run the tape from the top of the outer to the bottom of the inner board, which would probably suck up the 0.1" pretty easily

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[RAndres]

It's the tight bend at the edge of the round board that worries me. From what I read, 10x thickness is the small end of good design for static bend. Actual dynamic flex applications call for a good bit more to avoid hardening. Maybe there's a way to have the flex exit before the edge of the board, that would give more room for the bend to the inner surface of the housing. I'm looking at the possibility of using a stiffener and keeping the entire round board on polyamide. I will probably have to put copper at the back in order to guide heat out (as if this wasn't tough enough) and couple it to the housing.