Plating voids, Part 1: the objective of metallization is deposition of a continuous copper layer.

THE GERMAN AUTHOR Kurt Tucholski defined a hole as the following:

1. "A hole is where nothing exists."

2. " A hole is a permanent companion of the non-hole."

3. "If man sees a hole, he intends to fill it. While doing so, he often falls into it."

Certainly, the objective of the metalization operation is to put something into the holes, namely a continuous, void-free and tightly adherent deposit of copper. However, despite good intentions, one does not always accomplish this. The result is voiding: those areas on the resin or glass or at times both that do not receive the copper deposit.

Mention the word "voids" in a PCB facility, and watch everyone scramble. The problem with voids in plated through holes (PTH) is that there are so many potential causes that it is very difficult to quickly identify the source or sources of the problem. It is the purpose of this and the next few columns to discuss voiding and its causes in detail.

Generally, voiding is most often blamed on the metalization process itself. Certainly, this process is not without blame. However, since PTH metalization and the desmear process are so interdependent on each other, it is easy to see that the upstream processes can influence a defect that manifests itself later in the sequence. We will discuss this in detail later.

The IPC-A-600 standard provides an excellent guideline with respect to voids. Basically, if your company is building PCBs for Class 3, any voiding is considered a nonconforming defect. This includes glass voids, resin voids, circumferential voids ... any voids. Voids come in all shapes and sizes. Some are very evident, such as the one seen in FIGURE 1. Other voids are much more subtle, as shown in FIGURE 2. Basically, we strive for a continuous void-free deposit as shown in FIGURE 3.

[FIGURE 1 OMITTED]

Assume voiding has been discovered. An analysis of the problem tells you that the voiding is confined to the glass only. The resin coverage looks fine. Backlight testing confirms voiding through the electroless copper process. The voids are on multilayer PCBs made with a tetrafunctional resin. This resin system is used regularly in the PCB facility, so there are no new wrinkles here. Where would the troubleshooting effort start? Let's first discuss the possible causes of glass voiding.

Numerous possible causes for glass voiding are listed below:

* Insufficient cleaning and conditioning of the holes

* Drill debris in holes

* Overall poor hole wall quality

* Insufficient or ineffective desmear

* Permanganate or manganate residues remaining on resin

* Insufficient catalyzation (in electroless copper preplate)

* Insufficient acceleration (leaving excess tin and not allowing palladium to initiate the deposition reaction)

* Air/gas bubbles lodging in small diameter vias

* Inadequate rinsing

* Panels racked too close together

* Rack agitation.

[FIGURE 2 OMITTED]

These are just a few of the causes of voids. One of the first places to look is the cleaner/conditioner step. This step is critical to the success of the electroless copper process in that the conditioner enhances the adsorption of the activator species (palladium) to both the resin and glass.

Inadequate conditioning will reduce the charge density on the glass and resin, resulting in less than optimal palladium (activator adsorption). This condition will lead to lack of a continuous void-free copper deposit in the hole. Sufficient activator is necessary to catalyze the electroless copper reaction.

Essentially, the cleaner/conditioner can be described as an activator promoter. After all the examinations are complete and the problem is minor glass voiding, (no evidence of resin voids), then the first place to look is the cleaner/conditioner. Generally, the concentration of the cleaner/conditioner or the operating temperature is less than recommended by the process supplier.

Another cause is that the useful life of the made-up solution has been exceeded. In this case, the "spent" chemistry must be sent to waste treatment, and a new solution made ready. If this doesn't solve the problem, there are other areas to look at.

[FIGURE 3 OMITTED]

MICHAEL CARANO is vice president, marketing and business development at Electrochemicals Inc. He can be reached at [e-mail].