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

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SMT electronics assembly manufacturing forum.


Double sided parts stuffing

Will Buehler

#8483

Double sided parts stuffing | 18 November, 1999

We currently stuff parts on one side and use a reflow oven. We have been told that many companies are stuffing both sides without high-temp solder or glue. I understand that glue is usually only needed for wave soldering processes. Is it true that high-temp solder is not needed? Also, our oven only has a belt and does not suspend the boards. Can anyone recommend a retrofit to a Quad oven or a fixture design? Thanks.

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#8484

Re: Double sided parts stuffing | 18 November, 1999

Hi Will, it�s possible to use second reflow for second side components without glueing or use of different alloy solders. Although the first soldered joints become liquid during the second pass the parts will not fall off as long as their weight does not exceed the sticking force provided through the molten solder. That works for all small devices up to at least SO16 proved by our daily experience. Later wavesoldering prohibited except selective soldering. For the second pass we use small standoff devices to gain clearance between parts and the transport. You should in your case determine a suitable profile for the second pass considering the different hight the PCB goes through and the thermal mass of the fixtures you use. That depends IMO on what kind of oven you use. Also check the archive for more information on this topic, there are more entries.

Good luck Wolfgang

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Deon Nungaray

#8485

Re: Double sided parts stuffing | 18 November, 1999

Deon Response: Hello Will,

Yes, you can reflow a double sided board using 63/37 solder paste on both sides. There are some limitations as far as the mass of the components to reflow the second time. If you have doubts or are concerned that a component will fall off during this secondary reflow, use the following formula: Take the weight of the suspect component in grams and divide it by the total lead to pad area mating surface. If this equals to less than 30gms/sq.in. the surface tension should keep this component in tact during reflow.

As far as your reflow oven is concerned, I would have some board supports made at a machine shop. I would make these out of aluminum or titanium. These should be light and as small as possible this way they will not act as a heat sink during reflow. I have had some made in the past. These were long rods with a long slot where the PCB can be inserted in or on. Nothing too complicated but functional.

Hope this helps.

Deon Nungaray SMT Mfg Process Engineer GMI USA

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Boca

#8486

Re: Double sided parts stuffing | 22 November, 1999

Will,

Wolfgang and Deon are exactly right on target. You just have to watch which side is built first. I began doing double sided reflow like this in the mid '80's, it works great. I used FR4/ GT material to hold the PCB up off of the conveyor, holds less heat, cheap, easy to machine. Have a great time! Boca

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