Is There a Test for Black Pad?

Phil
Welcome to the Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, who by day go as ITM Consulting. What have we got here today, Jim?

Jim
A question from P.C. who asks, "We have identified some black-pad problems and are concerned that they may extend to some PCBs that have already been assembled. The units test functionally okay, but we are concerned about premature failures in actual field installations."

The question - "Is there a nondestructive test that can be performed on these boards, which would identify problems without damaging any assemblies that do not have any black pads?" That's a really good question, isn't it, Phil?

Phil
Black pad is definitely one of the scourges of the industry.

Jim
We will not get into an in-depth description of black pad. Let's just say we're talking about ENIG finishes where they appear to be good, they appear to solder well, but after soldering it's found that the joints are weak because there has not been good intermetallic formation between the tin in the solder, and the nickel under the gold.

There's a whole rash of analysis about why this happens, but the question here is whether there a test that can tell us if the solder joints are okay that won't damage good boards.

Phil
In search of that silver bullet, sadly we have to report that at present, there is not a nondestructive method of determining whether black pad is present or not.

Jim
Short of actually removing BGA components and inspecting the pads and then resoldering the parts which is risky and expensive, but there is good news on the horizon.

Phil
We have learned of a consortium being formed in the EU, I believe based in England, of people involved with fabrication and assembly. One of the main objectives is to come up with a nondestructive method of determining the presence of black pad.

Connected with this, they're also going to be looking at developing alternative finishes such as more advanced versions of OSP as a possible alternative to ENIG where black pad occurs. They don't even have an acronym for their name, yet, so it's something to watch.

Jim
This highlights a significant problem. Boards with black pad, may look okay, the gold seems to be covering the surface and it looks good, they seem to solder okay. You could do a moderate drop test and see if parts fall off.

With any kind of accelerated life testing, you have to be concerned that you may be reducing the service life or affecting the product in some negative way.

Phil
P.C. having identified black pad, hopefully you've gone back to your board supplier and see what they've done to rectify the process to avoid the instance of black pad.

I want to do a shout-out to our favorite expert on black pad, among many other failure things - Mark McMeen down at STI in Madison, Alabama. And, on that note, I will say that this is Phil and Jim, the Assembly Brothers, bidding you a farewell, and, whatever you do ...

Jim
Don't solder like my brother.

Phil
And regardless of whatever surface finish you have, don't solder like my brother.

I agree with some of the other reader suggestions, but I don't think you need to strip the gold. Just examine under magnification any ENIG plated copper surface that has not been soldered. Look for large and or open grain boundaries between the nickel nodules. Compare it to known good boards. Some nodularity of the nickel plating is normal, but if phos content, plating rate, or corrosion in the gold bath is not normal, it should look different.

On a previous black pad experience we soldered a wire to a fiducial and did a pull test to failure. On the black pad board the wire and solder pulled of clean leaving the fiducial on the board. On good boards the copper came off with the wire. If you have local fiducials you can get a reading near your BGA. Ultimately everything in that board lot will have the issue despite wishful thinking that something can be done to fix things.

The European ENIG improvement project is known as ASPIS.

I would like to suggest that you can chemically strip the Gold from a non-soldered convenient pad and view the underlying Nickel surface with high magnification to check if any "mud-cracking" effects are present. This will give an indication if the Nickel surface has been affected by hyper-corrosion. I hope this helps.

ENIG boards "Look Better" than HASL but then HASL never experiences Black Pad.

So many folks swear by ENIG but why? I don't accept the "ENIG is Flatter" reason as once paste is on the board you can't tell what finish is being used.

Now I did hear that the Ni layer prevents copper being dissolved during multiple rework/reflow phases. So, why do so many folks use the ENIG?