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Trouble in Your Tank: Pre-electroless Causes of Voids

To support advanced 2.5D and 3D heterogeneous integration, product and motherboard designs are now using higher layer counts. As layer counts increase, plated through-holes (PTHs) become deeper and higher in aspect ratio, making it more difficult to achieve a continuous, void-free copper deposit. Drilling these thicker panels and higher aspect ratio vias adds another layer of complexity. Since there are many potential causes of PTH voiding, I’ll focus on pre-electroless copper causes, including drilling.

Where Are They Coming From?
You’ll often hear seasoned engineers say that there are so many process steps and variables that influence the success or failure of achieving a continuous, void-free copper deposit in the hole. One of the obvious causes of voids, or at least poor copper coverage, is poor drilling. Figure 1 shows voids on the glass bundle fibers. A closer look at this section shows a very poorly drilled hole wall, with glass bundles protruding from the resin.

Many assume the plating process should compensate for damage like this, but surfaces of this kind are very difficult to activate with the palladium catalyst used in the electroless copper process. The drilling operation should be investigated immediately, with attention to the following:

• Drill bit condition: Review bit wear, number of hits, and the overall condition of the tool
• Spindle feeds and speeds: Confirm that the drill is cutting cleanly through the stack rather than punching through it
• Up-feed rate: Check whether a slow up-feed is contributing to torn glass bundles; it should typically be about twice the in-feed rate
• Chip load: Evaluate chip load settings and test adjustments to improve hole quality
• Stack height: Review the number of boards in the drill stack and consider reducing it to improve drilling quality

Another example of a rough hole wall is shown in Figure 2.

Figure 3 shows a section of a plated through-hole. Note the poor quality in the hole wall and its effect on the plating process again.

Even with all the issues shown in Figure 3, this board somehow passed in-circuit testing. Note the severe nailheading and some evidence of wicking along the torn-out glass fibers.

The desmear operation is another potential cause of PTH voids. The most effective desmear process available today is based on alkaline permanganate. In fact, the permanganate system is considered an industry standard. The use of alkaline permanganate for multilayer PCBs has had a dramatic effect on copper coverage and adhesion in the PTH. However, if not controlled, this process can be a major cause of voids. First, the process consists of three main chemical process steps:

1. A solvent conditioner designed to penetrate the polymer matrix of the resin system and weaken the polymer-polymer bonds of the cross-linked chain.
2. The alkaline permanganate solution consists of sodium or potassium permanganate and its corresponding hydroxide salt.
3. A neutralizer (possibly in combination with glass etch) for removing manganese residues.
4. Glass etch as a separate step to lightly roughen (frost) or more aggressively remove glass fibers that may be protruding into the hole due to etchback.

If not controlled, any of these steps can lead to voids. If this is the case, even the most robust of electroless copper processes will not be able to compensate.

In Figure 4, the microroughened epoxy resin has been properly prepped by the alkaline permanganate process.

However, today’s resin systems are far more complex than standard 140°C Tg epoxy, which remains one of the easiest materials to desmear. Modern resin systems require much tighter process control. These high-performance materials are highly cross-linked polymers that display very high Tgs and are more resistant to chemical treatments, including desmear. An effective alkaline permanganate process not only removes drill smear, but also micro-roughens the resin. This surface texture, often called a honeycombed appearance (Figure 4), is desirable for two reasons: The textured surface improves palladium catalyst adsorption, which supports electroless copper deposition and coverage It also improves copper adhesion

This improves solderability by reducing or eliminating blow holes during wave soldering. Inadequate texturing can increase the risk of voiding because reduced surface texture lowers catalyst adsorption. Electroless copper deposition requires sufficient catalyst on the hole wall to initiate and maintain uniform copper coverage. During troubleshooting, consider the following:

• Is the solvent conditioner, or swellant, penetrating the resin matrix effectively, and is the solvent system compatible with the resin? Some higher-Tg materials do not respond as standard FR-4 does. Because these resins are more highly cross-linked, the solvent may have difficulty penetrating and weakening the polymer bonds, reducing the permanganate solution's ability to remove resin and create the desired texture.
• Verify the operating temperature of the alkaline permanganate solution, along with the hydroxide and permanganate concentrations. Hydroxide increases the aggressiveness of permanganate's attack on the resin, while permanganate in the +7 oxidation state oxidizes and breaks down the resin. Manganate (+4), a reaction by-product, does not contribute to resin oxidation and should be controlled below 20–25 grams per liter. If manganate builds up, resin removal and texturing will slow. If solvent compatibility and penetration are adequate, consider increasing the permanganate concentration. Sodium permanganate can provide more available permanganate in solution than the corresponding potassium salt.
• Review the neutralizer and glass etch. A major source of voiding is manganate residue left on the resin and glass within the hole wall. Renew the solution regularly to maintain active chemistry, and verify the acid concentration because low acid reduces neutralization effectiveness. To reduce process steps, some fabricators combine the neutralizer and glass etch in one tank; however, separate steps are generally more effective. As the glass etch weakens with use, its ability to frost or roughen glass fibers decreases, reducing catalyst adhesion to glass and increasing the likelihood of voids.

This about covers the pre-electroless copper causes of voiding. In another column, I will explore other causes of voiding.

This column originally appeared in the August 2026 issue of I-Connect007 Magazine.