Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Double sided smt

Pat Copeland

#8733

Double sided smt | 31 October, 1999

Parts are falling off on first side when reflowing second side, have tried changing temp profiles, and ended up using tin foil. any recomendations.

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John A.

#8734

Re: Double sided smt | 31 October, 1999

Do both sides of that board contain larger IC's and other large components or just one side?? If just one side contains large components, try running the side with the smaller components first, and then run the other side last. If that doesn't work, you can always try using Epoxy(chip bonder). Hope it works out ok for you.

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Michael Fogel

#8735

Re: Double sided smt | 1 November, 1999

I suggest tow more ways: 1. try to differ the temperaturte between top and bottom side duering the reflow process. 2. try to add some glue dots under the falling parts befor the placement/reflow process

Good luck Michael

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Michael Fogel

#8736

Re: Double sided smt | 1 November, 1999

I suggest tow more ways: 1. try to differ the temperaturte between top and bottom side duering the reflow process. 2. try to add some glue dots under the falling parts befor the placement/reflow process

Good luck Michael

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Dean

#8737

Re: Double sided smt | 1 November, 1999

Without a detailed description of the suspect parts It is difficult to pin-point the exact solution to your process problem. However, whenever possible avoid non-value added additions to your current process.

Here is the reality of the situation: 1. With convection reflow technology, there is so much heat energy available attempting to run only top heaters to proper reflow tempeatures will prove difficult and unrreliable.

2. Some component geometries lend themselves to difficulty. Follow sound DFM practices or capture these difficulties during re-quoting (now that you know).

3. Epoxy is only value-added if it prevents rework of your original process. Experiment with switching first pass and second pass sides (if permitted. If possible, change geometries of stencil to accomodate problematic parts. Run DOE on aperture dimensions and geometries. This has pulled my but outa the sling several times!

4. Many designers are not aware of the process problems faced by us guys in the trenches. Feedback process yields to customer. Especially deviations by part type. Sometimes its an eye-opener!

Good luck.

Dean

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Gary Moravchik

#8738

Re: Double sided smt | 6 November, 1999

Assuming you are using Sn63/Pb37 solder paste, have you tried using a lower temp melting point solder paste for the second pass? For example, use 63/37 for first pass and a Bi58 compound solder paste for the second pass. If I recall, Bi58's melting point is 131 degrees C. With this two pass process, bottomside components will never reach liquidus during the second pass reflow. Most solder paste suppliers can provide you with a bismuth solder paste sample to try out. Good luck.

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#8739

Re: Double sided smt | 9 November, 1999

Gary: This could be messy is some applications, because Bi will form an alloy with lead with a melting point of 93�C.

My2�

Dave F

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