PCB bubbling durning reflow soldering

[shooter431]

The company I work for has been experiencing problems with bubbles forming between layers in our Printed Circut Boards. The bubbles have caused traces to break and pull away from pads and vias. The problem only occures after or during the population of the surface mount components. I would greatly appreciate any ideas about what may be causing this and possible ways to prevent it. Thank you all in advance.

Shooter

 

[VEBill]

Wrong spec or poor quality PCB substrate?
Overtemp or too long in the reflow oven?

 

[sreid]

The problem you describe is caused by air or water being captured between layers of multilayer PCB when the layers are laminated. The heat of soldering causes the air or water to expand and delaminate the layers.

I've had the problem twice. Both times the PCB manufacturer would not admit their problem and I had to switch suppliers.

This is not caused by anything you have done. FR-4 is epoxy-glass and it does not absorbe air or water. Epoxy-glass is used to make surfboards and boats.

 

[shooter431]

Thank you for the feed back it is greatly appreciated. I am still waiting to get the findings back from the board house. I was a little concerned that this may be caused by the increased temperatures used because of lead free solder paste. The boards are specd to be ROHS compliant, but does that mean they can withstand the temperatures required to use ROHS compliant solder? Once again thank you and any additional information would be great.

 

[sreid]

If the temperature gets high enough the boards will char but you will not see bubbling. If you have any two layer boards, try reflowing them. Two layer boards have no laminations (from the PC board house) and you will not see any bubbling. Or you could buy a piece of G-10 from McMasters for a test.

 

[VEBill]

Pop a sample in an oven set to something just above 100C. Leave it for hours. If there is moisture, then perhaps this test would reveal it. (?)

 

[Comcokid]

The new RoHs compliant subsititutes/replacements for FR4 have a variety of temperature ratings and inter-layer adhesion ratings. Combine one with a lower temperature rating with some entrapped moisture or air, and no doubt you can get layer seperations.

You need to investigate exactly what laminate your PCB house is using, and check the specifications. Go to the actual laminate web sites for data sheets. I listed the RoHs laminate manufacturers in thread240-151146 . If your PCB house does not have an actual process problem, you may have to carefully specify the laminate.

In the pre-RoHS era, for most board designs you might-of just specified "FR-4" and not paid any more attention. With the higher process temperatures required for RoHs solders, and possible longer dwell times (for solder wetting), the laminate material frequently needs additional closer specification. This situation will probably continue for a few years until the various new materials shake-out and a particular new material formulation becomes popular and generally accepted. A similar situation happened in the early FR-4 era.

 

[shooter431]

Once again thank you for the help. Comcokid the link helped alot, and right now I have found several possibilities for the bubbles, and I will begin eliminating them one by one. Thanks agan.

 

[shooter431]

Upon further discussion with our board house, they have not been baking the boards prior to soldering. I have baking temps and times, but my concern is how to place the boards in the oven. For example can they be stacked on ontop of eachother and set in there, or do they need to have spacers between them. Any more information on this would be great. Thank you in advance.

 

[MacGyverS2000]

There should be racks to place the boards in, standing them near vertical. Trying to place them all in a stack may prevent the inner boards from cooking properly. this is low temp and doesn't need to be anything special, so you could even go to the local home good store and find something.

Dan - Owner

http://www.Hi-TecDesigns.com