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Ways to control SMT gap to PCB

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#46058

Ways to control SMT gap to PCB | 7 December, 2006

I need advices as to how a gap between the bottom of an LED (1209-package) and the PCB can be controlled on a small rigid-flex PCBA? The problem that we experience is due to tight tolerance in the design of our molded housing in which this PCBA is assembled. The alignment of the LED (used as a mouse tracking device) thru an optical window is often misaligned due to slight gap beneath the LED and PCB, typically on one side and the other side is flush (no gap). The gap typically ranges between 0.005 to max of 0.015". Often times, the gap is on the opposite side as well. I don't think there is a workmanship standard about this in IPC-610. Controlling this gap to a minimum of zero: Is this possible? Is it practical and economical? I feel that creative stencil design may offer some control but not totally. Any feedback is greatly appreciated.

Our company uses a CM to manufacture our PCB assemblies.

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RDR

#46064

Ways to control SMT gap to PCB | 7 December, 2006

Bad design if you have to have a perfect 1206 part. you will always have a lift of some sort when reflow soldering. Solder is 3 dimensional, therefore you cannot get any underneath component, thus eliminating the automated SMT process. Cannot glue down either

probably would have to handsolder to ensure total flatnees to board as you cannot get any solder underneath component. you would have to place part then add solder while holding down the component.

Tough spec to hit my friend!!!

Russ

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#46067

Ways to control SMT gap to PCB | 7 December, 2006

Thanks Russ for the feedback.

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RDR

#46070

Ways to control SMT gap to PCB | 7 December, 2006

Just had anopther thought, what type of parts are you using on this board? You may want to try a thin stencil like 5 mil or so this may help you. also can this "tilt" be from a movement during reflow?

Russ

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#46073

Ways to control SMT gap to PCB | 7 December, 2006

Most likely there is movement during reflow. How about conduction oven over convection oven for reflow? This is a small PCB with the LED sitting on a Flex-Rigid extension from the PCB. I was thinking if the stencil aperature for the LED was to be cut with triangular shape reducing the paste under the LED by about 1/3 (surface contact area). That would reduce the chance of solder paste raising part of the LED. Then that would also reduce the contact surface area of the LED with the paste, making it more prone to movement and misalignment. I am going ask our CM to try a few things and see what we'd get. Other engineers in our company want to establish specification for this minimal gap. I tell them it is beyond current standards, not practical nor cost effective. We produce thousands of these things/ My only sure hope is to have our ME re-design the mating surface to allow for manufacturing variations like this LED gap. So far, it's been tough to get this thru them.

I appreciate your opinions very much.

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Loco

#46087

Ways to control SMT gap to PCB | 8 December, 2006

Are you able to glue them on using SMT glue before reflow?

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#46097

Ways to control SMT gap to PCB | 8 December, 2006

We discussed about applying 2 dots of glue on either sides of the LED body, but have yet to try it out. Other discussions include conductive adhesive, using fixturing with weight, different stencil aperature design, hand soldering.

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RDR

#46099

Ways to control SMT gap to PCB | 8 December, 2006

Wouldn;t the glue hold the part up?, usually a glued part has much higher standoff than a soldered one.

Russ

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#46101

Ways to control SMT gap to PCB | 8 December, 2006

Sure would. We discussed applying the glue on the sides rather bottom of the part. Hopefully the seating of the part during P&P helps push it down. Certainly there will be still some gap as some of the glue will be under the part. Each experiment offers different challenges./

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#46106

Ways to control SMT gap to PCB | 8 December, 2006

My experience with SMT LED packages is that certain LED package styles tend to float. This floating is caused because the LED is often only terminated on the bottom side of the LED package. Sometimes the LEDs are terminated like a SMT resistor with solderable surfaces on three sides of the packaged LED. I had to place 50 0402 LEDs to make a display board. The LEDs floated all over the place. I had to go to a 0.0025" thick stencil and reduce the apertures way down to stop the floating. Just way too much solder for an LED only terminated on the bottom of the component. I ultimately found a 3 sided terminated 0402 LED (like SMT resistor)and bumped the stencil thickness back up to 0.004" thickness. The parts now sit flat and are centered well.

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#46107

Ways to control SMT gap to PCB | 8 December, 2006

Thanks Chris for the valuable experience! We'll try the stencil and see what the result will be.

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