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Help needed to solder poorly designed SMT LED's

Gregg Temkin


Help needed to solder poorly designed SMT LED's | 24 September, 2002

I�d appreciate assistance in trying to resolve a placement/soldering problem. My company has designed a board which uses a large number of 0402 LED�s that are .028� high. The LED�s need to be soldered so that they point straight up and are not tilted after soldering . The board is being assembled for us by a contract manuafacturer. They�ve started out with a .005� thick stencil. On the first run of boards the LED�s were tilted all over the place. On closer examination it turned out the LED�s were designed with ONLY bottom solderable surfaces � no end, top or side metallization. I think they are poorly designed SMT parts.

Basically these small, tall LED�s are floating and tilting on the solder because there are no 3 dimensional surfaces on the part for the solder to wet and form a stabilizing surrounding meniscus. Our contract manufacturer does not have the capability to use surface mount adhesives on 0402 parts so we are persuing reducing the amount of applied solder � targeting somewhere around a .002� thick stencil.

Are we going the right direction? What issues do we have to concern ourselves with when using such thinly applied solder paste. Is it preferred to reduce the aperture size of a thicker (say .005�) stencil?

Any assistance would be appreciated.

Gregg Temkin 206 286-1814

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Help needed to solder poorly designed SMT LED's | 24 September, 2002

I'm just shooting ideas here, but maybe a via hole in the middle of the solder pad would drain the excess solder by capillary force?

Or using a tin plated PCB and printing flux only, then place and reflow. depending on the thickness of the tin plating, there should not be any excess solder for the LED to float on.

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Help needed to solder poorly designed SMT LED's | 24 September, 2002

Aperture reduction @ 5 mils thickness is your best bet. It may take several attempts to get just the right paste volume.

What aperture reduction was specified to the stencil house for these specific LEDS? or was the stencil blindly produced using a 1:1 ratio, straight from your gerber?

I wonder if your CM has the capability to measure paste volume? paste height only?

consider alternate aperture dimensions, i.e. home base, triangular, dog bone. At the 0402 package size I am concerned there is not a lot of room to play with paste deposit volume.

I don't think it is fair to blame the LEDs and call it a "poor design". If anything, blame your design engineer for not having the discipline of conducting a DFM prior to releasing the assembly. I would expect a DFM, properly conducted, would have revealed the potential for the problem you describe.

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Help needed to solder poorly designed SMT LED's | 24 September, 2002

First, I don't have a clear fix on the problem. Please drill-down to more detail. We appreciate that you probably don't know the parlance. Just describe what you have and then what you'd like to have.

Second, most 0402 LED are fabricated just like the device you describe. [Prove this by searching on "0402 LED" on the net and checking-out the packaging that comes-up from Fairchild, King Bright, Ever Bright, etc.]

Control of the post reflow postion of 0402 devices is largely a function of bare board pad design.

If you aren't careful with the bare board design and assembly processing 0402 devices will try to tomb-stone. We talked about this before on SMTnet. Search the fine Archives for background.

We don't know why you'd want to use a 2 thou stencil. Now if you're talking reducing the amount of solder to reduce variation in the height of components from the board surface that are soldered properly, you are making a BIG mistake. The amount of solder has a large effect on the long-term reliability of solder connections.

We don't like to talk about aperture reduction on stencils without knowing: * Pad design * Stencil thickness * Stencil fabrication

Closing on stencil fabrication points, consider following the guidelines of IPC-7525 - Stencil Design Guidelines and other references listed in the fine SMTnet Archives.

Next, there's no reason why your supplier couldn't print conductive adhesives in a manner identical to the method they used to print solder paste. So, don't buy that line of hooey.

Finally, someone appears to have bolted a smoke machine to the back of your pants. Don't buy any of that goofy garbage without checking with us first. In fact, why don't you get your supplier on-line here so that he can explain what's going on and we can get to the bottom of this stuff.

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Help needed to solder poorly designed SMT LED's | 25 September, 2002

Gregg, We had a similar problem with our 0603 leds. They were being placed bang on, however, during the reflow, they twisted, fell sideways etc. We played with our profile, played with the paste volume & land pattern design, and only had a small improvement. After a period of time, I had enough, and got the surface finsh of the led itself tested( should have thought of it in the first place). It turns out that this was causing the problems, not the process. The supplier was having batch variations in the surface finish, but the samples we trialled during development were fine. Admitadly, these were the z bend type, but maybe it is an avenue that you may not have looked at.

Basically in a nut shell, have you tried other suppiers, as when we qualified a new supplier, the problem disappeared. Now I cannot mention the brand name here, but give me a call if you want to know further. +64 9 481 0555, will get me direct. Good luck and may the force be with you CYA

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