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# Calculate SMD movement during reflow?

twinston

Calculate SMD movement during reflow? | 2 November, 1999

Does the possibility exist where one can calculate the movement of a SMT component during reflow given component and pad geometries?

Re: Calculate SMD movement during reflow? | 3 November, 1999

Hi Anthony I'm not sure I understand your question. Do you mean move to a better or more precise placement or to an error or bad position? I'm not aware of any calculation that can predict movement since it is based on so many factors like component type, pad geometry, component geometry, component solderability, initial placement, paste registration, paste volume, oven profile, oven atmosphere, oven air flow, belt vibration, etc. Getting your process right will normally result in anything from your component staying right where you placed it to exhibiting a strong self correcting (centering) action. If your properly placed components are moving to a defect causing position something is wrong in your process. Tell us more about what is happening, what you are doing and what you have tried. John Thorup

twinston

Re: Calculate SMD movement during reflow? | 3 November, 1999

Well John here is the situation:

I need to place an infrared transistor (optical element) with a tolerance of +/-.003", this is not a problem. A lens is positioned over the optical element and snapped to the PCB via three holes that have a tolerance of +/-.003� referenced to the fiducial used to place the optical element. The lens has a sweet spot (optical focal region) of .008�.

What I would like to achieve is the following:

1, Calculate optimum pad geometry (none provided by the manufacturer) for the optical element.

2. Use the self correcting (centering) action of the optical element to insure that, if any movement occurs during reflow, the optical element will remain within the sweet spot.

3, Place the optical element without using adhesive.

Thanks again!

Re: Calculate SMD movement during reflow? | 3 November, 1999

Anthony: John's points are well taken. All SMT components float to some degree. Fly shit floats more than multi-lead. Well duh!!!

I know of no formula.

The most cogent discussion is in THE BOOK (Klein Wassink's "Soldering In Electronics), but it presents fuzzy, apparently experimental data.

Somewhere in the dark recesses of the archives, you may be able to find a similar situation as yours. As I recall, the answer was a removable fixture, used during reflow.

Good luck

Dave F