PCB Assembly Inconsisentcy - BSC067N06LS3GATMA1 and BSC100N06LS3GATMA1

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kparrent
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We have been successfully using these two MOSFETs for three years on our low volume custom boards. We have roughly 65 boards in existence with 9 instances of BSC100N06LS3GATMA1 and 3 instances of BSC067N06LS3GATMA1 on each board. This circuit has never had a single problem until our last run of 14 boards. These 14 boards had very minor part changes from the previous iteration, but none of which would have had any affect on this circuit as these are all in an isolated design. They do run flying probe testing after PCBA, and they reported that it passed that test.

On these 14 boards, we had 3 boards with a failure with at least one of these parts which we could narrow down to four MOSFETs. Based on electrical waveform analysis it appeared to be an issue in the power stage of the design so I requested X-Rays on these MOSFETs and the controller IC. After reviewing the X-Rays it looked to me like there was possibly excess solder voiding which was maybe limiting current transmission. The application is a 5A/48VDC output buck DC/DC which is only dropping the input voltage by roughly 5V. 250W of output is required. I am not an expert in solder void analysis and would like additional opinions from the community on that. Those X-Rays are attached here- note that the vias under the pad are filled with resin.

I asked the assembler to remove those parts, clean them, and replace them to see if it fixed the issue, and it did in fact correct the issue on the first board. We have two more that I also suspect it will correct it, but I have asked him to hold off on reworking until we can come up with an isolation procedure to verify the root cause. 

I asked the assembler to supply more information about the source of the parts and the storage. The BSC067N06LS3GATMA1 parts were received from a broker due to supply shortages, and they were not received or stored in moisture sealed packaging. Upon observation under a microscope they believe they saw oxidation on the leads of one of the unused parts. That picture is included as an attachment. The BSC100N06LS3GATMA1 were received from Mouser and were stored appropriately.

Which is more likely to be the issue? The first experiment of removing the issue and manually replacing would have also cleaned the oxidation off of the parts. I also don't generally feel like for just the small current we are talking about on these that the solder voiding is extreme enough to cause the issue. This is not a finger pointing exercise- it is simply trying to understand and prevent issues on upcoming builds.

Thanks in advance!

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vigneshkumar
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25 likes received 250 sign-ins 50 solutions authored

Hi kris,

Solder joint voiding Voids in solder joints are primarily caused by flux gases from solder paste compounds that are entrapped in the molten solder during reflow. They may be trapped simply by the balanced inner pressure levels, by geometries

  • Solder paste flux chemistry
  • Cleanliness of plated surfaces
  • Solder reflow profile
  • Paste type (grain size)
  • · Stencil layout
  • Via layout

Detailed explanation given in the link.

General assembly guidelines :

https://www.infineon.com/dgdl/Infineon-Application%20Note_PowerMOSFETs_RecommendationsforBoardAssemb...

Assembly guidelines for PG-TDSON pacakge

https://www.infineon.com/dgdl/Infineon-General_Recommendations_for_Assembly_of_Infineon_Packages-P-v...

For further information including typical acceptance levels of voiding, please refer to documents like IPC-7095 or IEC/TR 61191-8

Oxidation may create solderability issue. moisture absorption leads to moisture concentrations in the component that are high enough to damage the package during the reflow process.

We just want to understand more about the failure of MOSFET. Could you please describe what kind of failure happened in MOSFET(ex. Short or burst)?

Regards,

Vignesh kumar.

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vigneshkumar
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25 likes received 250 sign-ins 50 solutions authored

Hi ,

Thanks for Posting on the Infineon community,

May I know the type of soldering process ?

Regards,

Vignesh kumar

 

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Hi Vignesh, thank you for your reply. The assembly shop is closed for the USA Thanksgiving holiday, but I will get back to you next week.

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Faster reply than expected!

  1. Printing process ( DEK printer)
  1. Solder type - Indium Type 4 Non water washable paste
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vigneshkumar
Moderator
Moderator
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25 likes received 250 sign-ins 50 solutions authored

Hi kris,

Solder joint voiding Voids in solder joints are primarily caused by flux gases from solder paste compounds that are entrapped in the molten solder during reflow. They may be trapped simply by the balanced inner pressure levels, by geometries

  • Solder paste flux chemistry
  • Cleanliness of plated surfaces
  • Solder reflow profile
  • Paste type (grain size)
  • · Stencil layout
  • Via layout

Detailed explanation given in the link.

General assembly guidelines :

https://www.infineon.com/dgdl/Infineon-Application%20Note_PowerMOSFETs_RecommendationsforBoardAssemb...

Assembly guidelines for PG-TDSON pacakge

https://www.infineon.com/dgdl/Infineon-General_Recommendations_for_Assembly_of_Infineon_Packages-P-v...

For further information including typical acceptance levels of voiding, please refer to documents like IPC-7095 or IEC/TR 61191-8

Oxidation may create solderability issue. moisture absorption leads to moisture concentrations in the component that are high enough to damage the package during the reflow process.

We just want to understand more about the failure of MOSFET. Could you please describe what kind of failure happened in MOSFET(ex. Short or burst)?

Regards,

Vignesh kumar.

Hi Vignesh,

Thank you for the detailed response. We will review the information and ensure our design aligns with the recommendations. On the surface of the documents it appears we are in alignment. The failure seems to simply be poor conductivity.

The assembly shop did an experiment with the other two boards with the same issue. One board they removed the MOSFETs, cleaned/tinned, and manually replaced. That board worked.

Another board they tried to just add solder and run through reflow. That board did not work.

The shop is wanting to called this oxidation since the second set which was not thoroughly cleaned did not work afterwards. Is that a reasonable conclusion?

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vigneshkumar
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25 likes received 250 sign-ins 50 solutions authored

Hi kris,

My conclusion aligned with yours.

solderability issues due to oxidation could be the cause.

Thanks for the interest shown.

Regards,

Vignesh kumar

 

 

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