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 assembly

Vicki J

#2121

Double sided assembly | 22 December, 2000

Hi! Has anyone out there ever tried to use a high-temp and a low-temp solder paste on a double-sided board without using an adhesive? (Populate the bottom side with the high temp paste, reflow, turn it over, then run the top side). I looked in the archives and found info on high-temp pastes, but nothing on this. I talked to our paste supplier, and was told it can be done, but I'm going to be trying it on a very small number of boards (10-20)and would like to have it go well the first time (so would my boss, these are very pricey parts!) Any tips would be greatly appreciated.

Thanks! Vicki

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#2122

Re: Double sided assembly | 22 December, 2000

I'd bet money our operators could use the wrong paste at the wrong time with out the slightest hesitance. This seems to be a fairly novel approach to making things complicated. I will admit I've heard of people talking about doing this, but I must have blacked-out the reason for doing it.

So, what's the purpose?

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acahill

#2123

Re: Double sided assembly | 26 December, 2000

High temp paste or not, I feel it's all up to the oven,and/or the stencil design. are there sufficient solder joints, before running the opposite side (are the apertures large enough to achieve a hardy fillet or to small, allowing enough paste to visually see a solder joint?. What Style of oven are you reflowing with, IR or convection ? Have you done a thermal profile with couplers attached to the top and botton? I run alot of double sided bds, with QFP's PLCC's hanging upside down, and I do not have any problem with components falling off, or cold solder.

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Hussman

#2124

Re: Double sided assembly | 26 December, 2000

Vickij,

Why not try using regular temp. paste for both sides? Unless you have some real heavy parts on both sides, you can run a double-sided reflow process with most boards. I myself like to place R's and C's first (or bottom side) and then do any actives 2nd. It does have a lot to do with your oven, but it is a lot easier than you think. If your bottom side is all R's and C's, it a cake walk!

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#2125

Re: Double sided assembly | 2 January, 2001

Dear Vicki,

If you decide to use high temp paste, I suggest that you verify the rating of the components, especially for moisture-sensitive devices which are normally qualified at peak reflow temperatures of 220-235C. The specified floor life will be substantially reduced if you use higher reflow temperature.

In addition, if you have moisture-sensitive components on both sides, be sure to account for the time between the first and second reflow when you track the maximum floor life. (Ref: J-STD-033, section 8.4.4. "If more than one reflow pass is used, care must be taken to ensure that no moisture sensitive components, mounted or unmounted, have exceeded their floor life prior to the final pass".

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