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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Jim Schultz

#9004

Tombstoning | 12 October, 1999

I need your help! We have been doing double sided surface mount for 4 years now. Up until 6 months ago I have not had a tombstoning problem. Since then I have a fairly high incidence rate of tombstones on the 0603 size resistors (no other parts). I have run profile after profile and have not been able to elimnate the problem. I even went so far as to increase the exhaust pressure on the reflow ovens (per the vendors recommendation).

We use the Conceptronics HVN 155 ovens. I run in nitrogen mode with 2000ppm O2. Conceptronics and the paste vendor has verified that my profile is optimized for the board.

Does anyone else have any other clues as to why this might be happening?

Thanks in advance for your help.

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Brian

#9005

Re: Tombstoning | 12 October, 1999

| I need your help! We have been doing double sided surface mount for 4 years now. Up until 6 months ago I have not had a tombstoning problem. Since then I have a fairly high incidence rate of tombstones on the 0603 size resistors (no other parts). I have run profile after profile and have not been able to elimnate the problem. I even went so far as to increase the exhaust pressure on the reflow ovens (per the vendors recommendation). | | We use the Conceptronics HVN 155 ovens. I run in nitrogen mode with 2000ppm O2. Conceptronics and the paste vendor has verified that my profile is optimized for the board. | | Does anyone else have any other clues as to why this might be happening? | | Thanks in advance for your help. |

Have you changed any other parameters, such as paste height or volume? Did you change the shape of the paste blocks? How are the pad sizes and/or the between pad dimensions (does the part fit the pads correctly)? Does your oven conveyor run smoothly, or is there excessive vibration? Have you run a profile with a thermocouple on each side of the component? Is there a differential thermal mass from one side of the component to another?

I have had similar problems, and these are the things I had to check. In some cases, the pad design and spacing was incorrect. This casued the component have minimum contact with the solder on either side, causing tombstoning. Other times it was just poor placement. I also had boards that had different thermal masses on either side of the component, casuing one side to heat or cool different from the other. I had to adjust my profile to compensate.

Hope this helps. Brian

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#9006

Re: Tombstoning | 13 October, 1999

I am assuming the parts are good prior to reflow....

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Paul Gerits

#9007

Re: Tombstoning | 14 October, 1999

Hi Jim,

I'm a Technical consultant/Application specialist at Philips EMT.

Tombstoning is a well know problem with Nitrogen ovens. Proberly there has been a change in your Nitrogen flow or in the nitrogen it self. To see if there was a change run a couple of pcb's without nitrogen and then have a look. Normaly the tombstoning should be gone then. If not then I need all possible info about pcb, paste, component variation on pcb etc. to give an advise.

Regards,

Paul Gerits.

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