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Trouble in Your Tank: Understanding Interconnect Defects, Part 2

Part 1 of this two-part series presented a more detailed look at the underlying causes of interconnect defects (sometimes known as interplane separation), with these three modes of interplane separation:

• Type 1: Separation of the electroless copper deposit from the interconnect
• Type 2: Separation of the electrolytic copper deposit from the electroless copper deposit, but the electroless remains on the post
• Type 3: Cohesive failure of the electroless, whereby the electroless copper deposit actually separates from itself

Type 2 ICDs

It is important to stress that a Type 2 ICD is not an electroless copper/direct metallization/desmear problem. The electroless deposit in Figure 2 is firmly on the interconnect. The electrolytic copper has separated from the electroless copper. The root cause of such a defect is lack of adhesion of the electrolytic copper to the electroless copper. When encountering this problem, we must first determine whether the printed wiring board was panel- or pattern-plated. If pattern-plated, did developer or resist residues remain on the interconnect? Was the developer solution at the proper concentration, temperature, and pH? Is there resist lock-in? These problems will prevent the electrolytic copper from adhering to the electroless.

Other potential sources of Type 2 defects:

• Oxidation at the interconnect
• Poor adhesion of electroplated copper to the electroless 
• Dry film residues remaining after incomplete development

Long hold times or rinsing with excessively hot water cause oxidation between operations. To improve adhesion, a sufficient amount of copper must be micro-etched. Typically, a 15–20 microinch etch is sufficient. Concern arises when 10–15 or fewer microinches of copper are removed. This is generally an insufficient amount of copper removal to provide enough anchoring sites for the copper plate.

Since many printed wiring board fabricators have reduced electroless copper plating thickness to reduce costs, they are reluctant to be aggressive on the micro-etch. The result is poor or insufficient anchoring of the electrolytic copper to the electroless. The solution to this problem is simple: If your electroless copper process cannot stand up to a micro-etch that removes 15–20 microinches, then find one that does. One suggestion is to understand the grain structure orientation of the electroless copper. A tighter, more angular grain structure with well-defined grain boundaries helps mitigate excess electroless copper removal.

Another cause often not considered is the incomplete removal of unexposed dry film. A lower pH in the developer solution usually causes this, or excessive loading of dry film resist in the developing chamber. Use an online real-time controller for the developer to ensure optimum performance and resist residue removal. Also, look for any possibility of overexposure or defects in the phototool that could lead to partial polymerization of the resist in the via.

Type 3 ICDs
Type 3 ICDs are a horse of a different color. Here, the electroless copper deposit actually separates from itself, known as cohesive failure (Figure 2). Copper is deposited in a disorderly fashion. Excessive deposition rates, over-catalyzation, and too high an operating temperature are all known causes of this type of ICD. Various conditions of the plating operation prevented the copper deposit from achieving epitaxial growth. Epitaxial growth is a crystallization process that deposits a new crystalline layer on a substrate with a specific orientation relative to the substrate’s crystal structure, essentially creating an ordered, single-crystal film. This technique produces high-quality, atomically flat layers.

If the electroless copper deposit is plated at excessive deposition rates or has been over-catalyzed, there is a good chance the deposit will be disoriented, with the copper looking like a sponge. When this happens, the copper will separate from itself, resulting in cohesive failure. 

Constantly check the deposition rates with coupons. If you detect copper deposit plating at excessive thicknesses, this should be an alarm to check the entire process for the root cause.

Summary
No matter the defect, brainstorm the various causes with your team so as not to go down a rabbit hole with a misdiagnosed defect.

This column originally appeared in the November 2025 issue of PCB007 Magazine.