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I have a project I'm working on that is so tightly packed that I'm totally out of room. I could save the whole thing by slapping a bunch of surface mount LEDs (30 of them) on the back of a (non-surface mount) 28-pin ceramic package in my circuit and wiring them to the device's pins.

I've tried a few things without success, like gluing the LEDs in place and running very fine wire to the solder points on the surface mount LEDs and soldering them in place. The solder breaks down the glue (crazy glue almost works, but one way or another, the solder won't bond to the top of the contacts on the LEDs.)

I have copper tape that I've tried X-Acto-ing traces out of, but the soldering iron breaks down the adhesive and the whole assembly disconnects itself.

I've even tried conductive glue and conductive pens. They have such hit-or-miss impedance properties that the LEDs don't light evenly. One will be many times the brightness of another.

If I could afford the space, I'd just slap a PCB on there and call it good, but I really need the LEDs right on the back of the IC. I can fill in with epoxy after the fact to keep everything protected and in place.

Thanks...

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  • \$\begingroup\$ Did you try to bend the IC's pins and solder them to the leds? \$\endgroup\$ – Vladimir Cravero May 12 '14 at 21:08
  • \$\begingroup\$ The connection traces aren't neatly arranged. It's not quite what I'm doing, but imagine that I was trying to pack a bunch of manually arranged 7-segment displays made out of surface mount LEDs on there. Some pins connect to only one LED, some connect to 3 or 4. \$\endgroup\$ – user30997 May 12 '14 at 21:10
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    \$\begingroup\$ Now I get it... You can't use a PCB on top of that for thickness problems or what? \$\endgroup\$ – Vladimir Cravero May 12 '14 at 21:11
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    \$\begingroup\$ Why not glue a polyimide board to it? \$\endgroup\$ – Ignacio Vazquez-Abrams May 12 '14 at 21:15
  • \$\begingroup\$ Is this for mass manufacture or one-shot? Ignatio's polyimide PCB sounds like the best solution, if you really need the tenth of a milimeter thickness you could take it off the top of the IC package (microtome? careful sanding?) \$\endgroup\$ – pjc50 May 12 '14 at 21:19
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These will print traces onto injection-moulded plastic... I've never priced them, but I doubt they qualify as "inexpensive".

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The ceramic is too smooth for glue to adhear to properly. You need to scratch it up some with a coarse grade sandpaper or dremel bit. Then a thin line of 5 minute epoxy with the leds on top. Just enough that the contacs are not covered. Let it cure over night or two. Then you can solder the wires to the leds an pins.

Other than that, a quick pcb order from seeed or itead or any other cheapo pcb shop with the thinest board will only cost 10 bucks for 5~10 boards (5cmx5cm) which is enough for 20 or 30 of your project. Solder or reflow the leds, and use some standard 0.1" headers so the board can stack "or float" on to top of the ceramic chip with some breathing room around it. Since the pcb can be double sided with vias, it takes some of the routing and wiring issues away.

but if you are still in the pcb planning stages and have vertical room you could create the same board above, but use SIP instead of dip headers. Basically a single row of pins, which you then connect to the led board with a header or a ribbon cable. Also think about using 0.05' headers, which are half the width of 0.1" standard headers. IDE cables make for a great source of easy to access ribbon cables for you too.

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I don't know about printing, but various folk make adhesive copper PCB lands, usually used for board repair. As an example, http://www.intertronics.co.uk/products/crc1152600.htm Since you're modifying a ceramic package, I'd think it would work. This technique would be expensive, but not like a PC board. How steady is your hand?

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