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Reflowing large aluminum electro. caps


Reflowing large aluminum electro. caps | 3 March, 2005

I just ran into a reflow profiling problem with a new assembly that we are processing in SMT.

There is a cluster of six or so electrolytic caps that measure approx. .710" wide by .660" tall. They are spaced .100" or so apart. When I run a profile, the area under these caps are 35 deg. C cooler than the rest of the board. The rest of the board measures 217 C.(My target temp.) If I try to increase the zone temperatures and lower belt speed etc, I am going to heat the rest of the board up too much. We are running 8 zone BTU TRS ovens. Does anyone have any experience reflowing these big caps ?

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Reflowing large aluminum electro. caps | 3 March, 2005

I wouldn't raise the temps just slow the conveyor, You need to get the PCB "soaked in". It seems as if you are applying more thermal energy than the PCB can accept in all areas. Does this oven have convection heating? Hopefully so. I have found that by actually lowering a temp and slowing the speed down you can sometime acheive more equilibrium acroos the board. I look at it like this.

If you place a dry sponge under a faucet and turn it on full blast the majority of the water will just splash off and run down the drain, If you turn the faucet on slowly you will find that the sponge can absorb all of the water and you can turn up the flow after the sponge is saturated.

Of course you need to be careful as to not soaking too long or too hot. With 8 zones I believe that you can get this accomplished.


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Reflowing large aluminum electro. caps | 3 March, 2005

This sounds like you are using a straight ramp profile. For this type of an assembly you may need to use a soak type profile. Most solder paste providers show you both types of profiles that work best with that paste. I would try using that profile.


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Reflowing large aluminum electro. caps | 3 March, 2005

Also try boosting the bottom zones up to get extra heat in.

Now try this at lead free temperatures & watch them go pop as the air inside expands & vents at the base. O to be in America & be free of our wonderful european decision makers.

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