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Chip Components with big ground pads - Unsolder

Vinny

#26980

Chip Components with big ground pads - Unsolder | 26 January, 2004

Hi All,

We are producing a veriety of RF related PCBAs in which we see Unsolder/Mislaigned on a regular basis. In these baords usually one of the pad is connected to a signal trace and the other pad is connected to very huge ground plane. This ground pad usually is formed by opening up an area in the solder mask.

Due to the unequal thermal distribution of heat between the signal pad and the ground pad the chip component gets pulled towards the signal pad (as this pad gets heated up earlier) and results in the other terminal not forming the solder joint. We are using Heller 1800 ovens for the process. We do not see a complete tomb stone effect either as the component still remains on the pad but does not form the solder joint.

We have tried to check with the customer but they are not willing to change the pad design as it degrades the RF performance.

It would be greatly helpful if any of you guys can suggest a way to deal with this situation.

Thanks & Best Regards Vinny

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#26981

Chip Components with big ground pads - Unsolder | 26 January, 2004

First of all, did you profile this board with a mole, such as a KIC? Is the solder flowing, or just not wetting to the part or pad? I work with RF boards as well, but the main problem is tombstoning due to the ground plane side being larger because of the size of the masking opening.

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Vinny

#26998

Chip Components with big ground pads - Unsolder | 26 January, 2004

I have done acurate profiling which meets the solder paste specifications. The point I am trying to highlight here is also due to the ground plane side being much larger causing the tombstone defect. I have even tried doing a linear profile, it helps to reduce the defect but does not eliminate it.

Please advise how did you overcome this issue in your case.

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Vinny

#27034

Chip Components with big ground pads - Unsolder | 28 January, 2004

Hi Guys,

Really need your help to suggest how to tackle this situation.

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#27035

Chip Components with big ground pads - Unsolder | 28 January, 2004

Vinny: From a thermal standpoint, your choices are: * Get your customer to design thermal relief in the board. * Set your thermal recipe to assure that the grounded pad reaches reflow temperature. This may require a long soak at a single temperature.

Other thoughts are: * If the two solderable pads for this component are different, consider a temporary solder mask to make them equal. * Consider gluing this part in-place.

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Vinny

#27037

Chip Components with big ground pads - Unsolder | 29 January, 2004

Hi Dave,

Thanks for your reply.

I have tried getting the design changed from the customer but they cannot provide thermal relief as this changes the RF performance and they cannot afford to do that.

As for profile it does reach the reflow temperature for both the ground as well as the normal pads. I had been using a ramp-soak-ramp profile all this while but our internal discussions suggested that it would be better to have a Linear profile with no soak as it will prevent the last rapid ramp during the reflow stage whereby the ground plane may not be able to ramp upas fast. Seek your comments.

I am suggesting the pad sizes to be made similar to the customer but will this help! as the huge ground plane is still directly conected to the pad.

Gluing is definitely an option but it will hamper the productivity significantly... Any other suggestions please advise.

Thanks

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#27042

Chip Components with big ground pads - Unsolder | 29 January, 2004

Ramp / Soak: Tombstoning is occuring during reflow. You need to get more heat into the ground plane. Obviously a ramp aint gettin' it done. Further, without intending to disrespect your internal discussion, if a linear profile improved the situation, why not try some more of that stuff [soak longer]?

Consider putting the ground into the oven first.

Glue / rework / glue / rework / glue / rework: Choices, choices, choices [hopefully you can make other fixes]

Will similar pad sizes help? Yes, because different pad sizes cause the surface tension of the solder to be unequal between the two pads. [You have two problems: 1 Thermal sinking by the ground plane. 2 Unbalanced soler forces on the pads]

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#27056

Chip Components with big ground pads - Unsolder | 29 January, 2004

Rethinking to Ramp / Soak: Yano, staying with ramp might not be all that bad after all, just decrease the ramp / slow-down the conveyor.

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#27188

Chip Components with big ground pads - Unsolder | 10 February, 2004

Hey Vinny, Sorry I haven't looked at this for a while.

3 things to try: Tombstoning- The ground plane side is larger because the solder mask allows for a little slop at the board shop (clearance around a pad of a few mils). Since there is not a defined pad, the only limiter of the solder spreading is the solder mask on the ground side. The aperture files for the stencil are probably the same size for both sides of the component. So you have the same amount of paste spreading over a larger area, creating more surface tension, pulling on the part and resulting in a tombstone. To resolve this, either make the solder mask smaller (preferred) on the ground plane side to match the size of the pad on the other end, or make the apertures bigger for the ground plane side to even out the surface tension. Neither of these should hamper the rf function much, if at all. For the non-soldering ground plane, it may be that it is a combination of requiring so much heat and using too much ramp to get there. The profile recommended by the paste mfgr is a good starting point, but you may have to adjust it as needed, especially in this application. I have had to make the reflow zone hotter and longer in duration in many case. The problem with more ramp is that the flux in the paste may be de-activated or dried out, making it very difficult to reflow the paste. I hope you have already solved these problems, but if not, maybe this will help.

Blessings, Patrick

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