By Duane Benson, Screaming Circuits
Reflow-related problems are far more common today than black pad, especially on larger BGAs. This article examines one less-common, but persistent BGA assembly issue black pad.
While delivering a seminar on advanced packaging, one attendee pulled me aside to discuss a particularly difficult problem he was having with a large BGA on an electroless nickel immersion gold (ENIG) board. The corner balls would detach from the solder pads. Black pad didn't immediately come to mind, because there are a lot of different things that can go wrong with a BGA assembly, and it's not as common as it used to be. But after discussing the assembly conditions, such as the thermal ramp-up and cool-down during reflow, it started to look more like a marginal black pad situation.
Reflow-related problems are more common today than black pad, especially on larger BGAs. The greater impact of thermal inertia or board flexing on larger parts makes it more challenging. If you bring the heat up during reflow and start cooling too soon, you can have good soldering on the outside rows, but still have unsoldered balls in the middle. If you do the opposite cool too slowly you can overheat the balls in the middle, causing them to sag, dry out, or get brittle.
In a typical black pad scenario, the thermal profile will be acceptable, but you may find that a few solder balls on the edges or corners of the BGA are not mechanically connected to the PCB, or the entire part may pop off with just a minimal amount of stress or shock. A poor reflow profile will worsen black pad, but a good profile likely will not fix it.
On an ENIG-finished board, gold is more of a surface preservative. When properly soldered, gold will dissolve, migrating into the solder ball, and the solder will adhere to the nickel layer directly. However, the nickel layer can have small amounts of contaminates that prevent good adhesion. In such a case, gold dissolves, but the solder cannot mix and adhere to the tin. This is the cause of black pad.
Other things, such as an incorrect oven profile, poor solder mask registration, or mixing lead and lead-free can cause BGA problems. But if your BGA is experiencing these problems, and you are using an ENIG surface, talk to your board fabrication house. It likely is a process issue in their shop.
Black pad also occurs with QFN parts. It tends to be more difficult to detect visually because the parts are much closer to the board. Sometimes an X-ray inspection will not catch it, and you may not uncover the problem until its run through a few thermal cycles or has been exposed to mechanical stress or vibration.
What Can You Do?
Most BGA component vendors recommend the use of non solder mask defined (NSMD) pads for BGA lands, if possible. With an NSMD land, the mask opening is larger than the copper pad, so solder will overlap and adhere to the sides of the copper pad as well as the surface. This adds to the shear strength of the joint. Doing so can only help slightly, though. As shown in Figure 1, worst-case black pad can still cause the entire joint to fail.
The most important thing you can do is carefully select a PCB fab vendor. If you haven't used a particular vendor before, ask about black pad in their ENIG process, if they have had problems with it, and how they prevent it. Don't be hesitant to ask for details. They may say that it's not a problem, and it probably isn't. But you may be putting thousands of dollars of parts onto their hundreds of dollars in boards. It's worth it to press for details.
Immersion Silver and Micro Voids
We sometimes see a similar problem with immersion silver-finished boards, which are confused with black pad, or micro voiding. Like black pad, this is caused by contamination on the metal layer under the silver. It doesn't go by the same name, but in practice, it's effectively the same thing pads are not secured mechanically or electrically to the board. As with black pad, this is primarily a problem at the board house. Figure 1 shows a worst-case effect of either black pad or sliver micro voiding. In both cases, initial inspection likely will show a good connection, but thermal expansion or vibration will cause the solder ball to crack or separate at the line of interface between the solder and the nickel sub-surface. If you are using immersion silver, ask your board house some difficult questions about their reliability, board shelf life, and process control.
The other problem with immersion silver boards is tarnish and surface contamination. Poorly processed silver boards can tarnish when exposed to air, dirt, or finger grease, and can offer a short pre-assembly shelf life. Even the best silver boards can tarnish if exposed to too much light, heat, and airborne contaminants. With all PCBs, it is important to treat the surface as you would a camera lens. Most assembly processes can deal with some dust and finger grease, but the cleaner the board, the better results. The same should be upheld for exposure. Boards will keep much better if they are sealed and stored in a cool, dark place.
Despite assurances from PCB fab houses, black pad and its silver cousins micro-voids and tarnish still pop up and frustrate us from time to time. First, make sure your board fab house does have good process controls. Then, treat your boards kindly. Keep them clean, store them properly, and remember that they do have a shelf life. SMT
Duane Benson, marketing manager, Screaming Circuits, may be contacted at (866) 784-5887; firstname.lastname@example.org.