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Boards getting

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Boards getting | 22 July, 2005

hello all,

I�m experiencing problems with "warped" boards after first reflow, some of them i just can�t print side two due to this problem,,is it related only to reflow? i�m using a linear profile (225� peak / 70s time above) with a tin lead paste,the board sizes about 20x20 cm..and there isn�t a center rail in the oven...

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Boards getting | 24 July, 2005

Warpage problem causes * MLB constructions (not homogeneous or unbalanced - too resin rich or poor) * Supplier process capabilities (lamination cycles) * Material selections (usually cheap and dirty), etc.

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Boards getting | 25 July, 2005

In addition to Daves comments. You might want to look at the layer construction to see if the copper is balanced throughout the board.

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Boards getting | 2 August, 2005

Assuming it is not PCB supplier related, here are my thoughts;

1. CHK Component density top / bottom, I am assuming you do bottom reflow first then top. If have alot of heavy comps on bottom it might cause the board to sag in the middle.

2. You have a square board dsign, we always try to have a rectangular board design, so as it is travelling in the oven, the short width is between the chains.

3. Check you ramp up and cool down zones, maybe you are ramping up and cooling down too quickly.

4. Check you oven profile and see if you have blown a blower, causing one zone to overwork than the others.

All the best


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Boards getting | 3 August, 2005

if you have enough free space at the edge of the board you could try some titanium supports, I have to use these on some products to prevent warping.

Also, in relation to profile and I know there are a lot of variables to consider but is 225 not a bit high for a leaded paste. You could try and profile for peak 210-215 at between 50-60 secs and see how you get on.


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Boards getting | 3 August, 2005

We do pass board on mesh instead of chain for first side. For second side pass on chain.

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Boards getting | 3 August, 2005

What about PCB construction?: - CEM (2-3....)? FR4 ?....? - how many layers - what about TG ?


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Boards getting | 3 August, 2005

CTE mismatch.

X, Y, Z all expand at different rates.

Cooling or heating rates will not change this CTE mismatch.

Layer counts, power plane ballance, equal run lengths in x-y and copper balance all contribute.

Have your supplier evaluate your situation. They should make some suggestions to alleviate your issue. If no help, you have the wrong supplier.

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Boards getting | 12 August, 2005

Run on the "mesh" for the first side. Then on the chain of the second. It's a quick fix for a pain in the ass problem.

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Boards getting | 12 August, 2005

I agree with all the prior postings! But I have some additional thoughts about this. If there is a build up of moistures (% humidity) inside the pcb; before you reflow the pcba; then you will have the result out of the oven, just as you described. My english gramma is not that good (even worse), but maybe you will get it? Concider baking your pcb:s before making them pcba:s If this is the root cause of your problem; investigate how the pcb:s got moisture inside them. There could very well be a number of different resons for that. Possibilities: First: you store your pcb:s in an enviroment that holds more than 10% RH humitity - for how long period of time (depends also of how your pcb:s are made)?

Second: The pcb supplier does not ship the pcb:s in a proper sealed MBB (Moisture Barrier Bag) with a moisture sensitive indicator card in it (they never do, and We All Should Complain About This), just like the way you recieve your MS-level sensitive components. I had to admit, that is very seldom that we recivie pcb:s in that type of bags.

We had some success in baking pcb:s prior to assembly & reflow, beacause of board warp. But, This Is really Not an option for production! How about RoHS processes, moisture in the pcb:s will then be a factor to count with... Regards,

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