Porosity in Good Plating

I have a CEM-1 PCB board with 35 microns copper. This is PCB is finished with SMOBC+HALS. We do solder SMT LEDs, but till date we do not have problems that you have listed. The LEDs are good & bright. It even passed high & low temp cycle tests (-40 to +105 deg.c). It also passed high humidity tests (90%). Both the tests were performed with the circuit in powered condition.

I suspect the chip resistors that supply current to the LEDs. Check the wattage of these chip resistors. Check the forward current that goes in to the LEDs. You might have already checked for shorts in the circuit. Check if your board is bending during assembly. To avoid moisture condensing & creating shot/reistance,You need to have a conformal coating on the soldered leads.

Clean the Boards throughly in an ultrasonic cleaner and see the sodlered pads under a high zoom microscope to see if it is surrounded with solder balls. Presence of solder balls, can lead to interrupted electrical conncetion resulting in loose contact. Parallelly, you can manual solder some LEDs in one sample board and check after 48 hours. If this manual sodlered LED is OK, then you wave soldering/reflow process has to be improved.

Check the board in the powered condition. Is it heated up too much with all the LEDs? If, so try to reposition them on teh circuit board (change the layout, move the LEDs away from each other). Are you passing high forward current to brighten up the LEDs? Try high wattage Chip resistors. In my experience, just increasing the forward current to 50%, the resulted brightness is not all that claimable.

If these are OK, then, you can also suspect the presence of hair line crack in copper resulted from etching conditions, deep scratches or poor handling.

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If the problem as you described as the gold plating contaminated, and the Nickel layer exposed to the solder pot. However, the purpose of the gold layer is to protect the nickel and copper layer not turn into corrosion and resist to other issue (humidity, temperature, etc..); if the gold layer is washed away, the tin itselft very active and can possibly eat up your nickle and copper layer just within a short period of time, make the circuit itself can't function properly. Before going to the ICT, I suggest you perform the ultrasonic wash as soon as the board come off the line, If this doesn't help, then call the bare board supplier to tackle this issue. I hope this will help you. good luck

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Hi Sam,

Great answers so far.

Let me take a stab at a few of your questions.

Q1. whether this is named Pososity?

I can't tell from your description. Several companies and the IPC have porosity tests to see if the Gold has good coverage of the underlying metallurgy. Pores (Holes) and such are found sometime with reactive chemicals and dyes.

Q4. Can such contamination be cleaned? Q5. If it can be cleaned, what chemical shall be used.

This, in general, is a job for the PCB shop with some agressive chemicals. For cost and difficulty reasons they will often chose to build new PWBs rather than cleaning and then re-plating. --------

Can you run the LEDs with hand held tools. I wonder if the LEDs might not be compatible with your process or maybe something else is draining the voltage?

YiEngr, MA/NY DDave

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Thanks for all the answers.

I should clarify that the problem was found to be on the SMT LED gold plating, instead of PCB gold plating. We had confirmed this problem by running some DOE experiments and some reliability testings.

We had decided to dis-continue the use of this problematic LEDs, but the failure sympthom was still a question that why the poor gold plating in LED could lead to a intermittent failure AFTER around 48 hours. The LEDs were 99.5% bright and gold immediately after assembly and functional testing. But failure rate were up to 30% after 48 hours and were still moving up.

Dear AlCapone, It seemed that the Tin-Nickel chemical result like what you stated might be the root cause of the problem. Can you tell me more about it? What will be the end-result of this chemical reaction? Is that leading to some insulated materials?

Sam Hung

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Sam: If your gold overcoat was pourous, the nickel undercoat would corrode prior to your attempting to solder and you would not be able to solder without an aggressive flux. Since you see solder flow and wetting, it indicates the gold did an adequate job of protecting the solderability of the nickel and was NOT pourous.

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