Hardware Firmware Tools Download Community

 

Black Pad Defect

Producing printed circuit boards is a well known procedure, even used by hobbyists at home for several decades. With finer structures of todays micro electronic parts, plane surfaces are required to guarantee smooth automatic stuffing and reflow soldering and gold plating had been used for many years to meet this requirement. However, that technique bears a risk, which you should be aware of.

From Production to Shipping

In July 2004 our company ordered several hundred Ethernut 2.1 Rev-B. PCBs after the previously ordered prototypes worked as expected. The prototypes had been manufactured with tin finish (HAL), which is fine for manual soldering. For automatic mounting of fine pitch parts a plane surface is required. Thus, the production quantity had been ordered with nickel/gold (Ni/Au) finish. In contrast to the prototypes, these boards had been electrically tested against the Gerber Data.

The boards were then delivered to another company for stuffing the parts. Automatic SMD stuffing and reflow soldering worked without problems. Afterwards a major part of the boards had been wave soldered by this company after manually mounting the through hole parts. Finally, all boards were visually inspected by the stuffer. As usual, there had been a few mounting problems, which are typically corrected manually by the stuffer. In general the whole production worked very well.

When the assembled boards arrived at our company, they are visually inspected a second time. We normally mark boards with minor scratches or stains and offer them as second quality for reduced prices if they passed our automatic test procedure. During this step several special applications are loaded on the target to perform various checks. When the board passes all tests, it is preloaded and ready for packing and shipping. Again, nothing special happened during the test of our Ethernut 2.1 production. Boards were packed and shipped world wide.

Of course, not all boards reached this status. One of the boards failed during network test. We tried to analyze the cause and discovered, that one solder joint at the fine pitch Ethernet controller seemed to provide contact only after touching it. Somehow alarming, but for individual cases it could be safely ignored. Actually, this was the first sign of an upcoming disaster.

Black Pads

Trying to re-solder the pin failed miserably. The pad rejected to accept solder and, hard to believe, appeared totally black. We talked to our PCB manuafacturer about this problem, but he wasn't able to explain this defect. Being one of the smaller companies, they don't do gold plating in house, but pass the boards for this step to a partner company. We started to inspect some boards again and specifically tried to find more bad solder joints under the microscope, without much success first. But then we noticed, that some of the larger aluminum capacitors were obviously loose and could be easily removed by applying very little force. Again, the pads were almost complete black. Only tiny areas at the edges were covered with solder tin.

After some research in the Internet I found out, that this defect is named Black Pad Problem. Although labeled well known, it was totally new to me.

The Problem's Cause

What happened to our boards? A few words about the gold plating of PCBs first. The process is called Electroless Nickel / Immersion Gold (ENIG). In opposite to electrolytic gold plating, it is not required to electrically connect all areas to be plated. However, there is no way to put gold directly onto the copper. In a first chemical bath, the copper is covered with nickel, which in turn attracts gold atoms in a second bath. During this stage, a higher concentration of phosphor may appear at the nickel surface. Unfortunately the gold surface of the bare board doesn't show any abnormalities. During soldering, the gold atoms move into the solder tin and the nickel surface immediately turns into something hyperactive corrosive.

Up to day, it is not fully sure, what exactly causes the higher phosphor concentration. There are methods of monitoring the concentration of phosphor, but they require to destroy the test target and it is still unknown, which concentration is acceptable or will result in black pad defects.

Problem Avoidance

It is simply not possible to fully avoid this defect, except using a different plating method. A few large companies offer alternatives, but they are not as widely used as ENIG and may introduce other problems, which are not yet known.

Researchers are discordant in naming the root cause, but for sure the risk can be limited by properly maintaining the chemical process. Probably chemical liquids staying idle for some period tend to turn into a critical state.

Legal Issues

The failure of a PCB production on its own can bring involved companies under financial pressure. In the case of black pad problems, the situation may be fatal, because the defects are detected earliest after assembly. As solder junctions do not show any anomalies and the boards pass electrical tests, they may fail months or years later due to thermal or mechanical stress.

For our company as well as for the PCB manufacturer this is a new experience. We are curiously waiting for the response of the insurance companies.

Although the boards had been tested and are working fine, they may fail at a later time. We offered our customers return them and get them refunded or replaced. We highly recommend not to use them for production.

Links

[1] Mike Walsh
Galvanotechnik, Sep, 2002.
Electroless Nickel Immersion Gold and Black Pad
A description of the black pad problem and its possible elimination.
Also available in PDF format at www.omg-galvano.com

[2] BabHui Lee
Nov. 2003
Implementing a Simple Corrosion Test Method to Detect "Black Pad" Phenomenon in Electroless Nickel/Immersion Gold Plating

[3] Robert F. Champaign, Jodi A. Roepsch and Marlin R. Downey
Circuits Assembly, Jan 2003
Afraid of the Dark?

[4] Nicholas Biunno
March 1999
A Root Cause Failure Mechanism for Solder Joint Integrity of Electroless Nickel / Immersion Gold Surface Finishes

[5] Wade E. McFaddin
Sept. 2003
Non-Destructive Analysis Method for Detection of the "Black Pap Defect" on PCB Surfaces
This analysis method uses a low-vacuum (partial pressure) scanning electron microscope (LVSEM) in the backscattered mode at very low electron voltages to detect the presence of black pads on unpopulated PCBs. Full article for members of the SMTA only.