An analysis of copper shorting caused by moisture and various residues.

Moisture reacts with high levels of weak organic acids, chloride and sulfate residues, leading to copper shorting. Poor adhesion from a low-pressure overmold exacerbates the issue.

The corrosive residues then dissolve the copper at the edges of the solder pads, and voltage bias drives the formation of dendrites. This creates a small gap at the board surface interface that permits moisture to ingress. The resulting colorful metal short exhibits is a result of the different states of the copper, voltage influences and organic materials from the substrate surface, which has titanium and silica in the white solder mask.

Figure 1. Electrochemical migration (dendrite) short of copper on printed circuit board.

LIBS analysis (laser-induced breakdown spectroscopy) is a type of atomic emission spectroscopy that uses a laser to turn the target surface into plasma. The Keyence EA Series elemental analyzer (integrated with a Keyence VHX-7000 digital microscope) uses a broadband (deep UV to near-infrared) high-resolution spectrometer to detect the emitted light color.

LIBS analysis of the dendrite reveals high copper content, while the blue/purples indicate higher sulfur, titanium and chlorine levels (Figure 2). In contrast, the copper-colored dendrite displays elevated silicon and lower chlorine levels, with no detectable titanium (Figure 2b). The reference area of the solder mask shows that the white coloring has high titanium content and no copper (Figure 2c).

Figure 2. LIBS analysis of the dendrite (a and b) and reference white solder mask (c).

Energy dispersive x-ray spectroscopy (EDX) analysis shows copper, silica, titanium, chlorine and sulfur residue (Figure 3a). While this analysis identifies specific elements, it provides limited information about the organic results. The levels of chloride and sulfur are higher, but don’t identify the source – whether they come from the board, assembly residues or outside contaminants (Table 1).

Figure 3. SEM/EDX analysis of the dendrite short using a SEMTech SEM.

Table 1. Elements Present in the Extraction

Localized steam extraction of various areas with dendrites and areas around the failure site show high contamination levels for chloride, sulfate and sodium (Figure 4). IC analysis reveals very high weak organic acid (WOA) levels by IC analysis (Table 2). IC-MS analysis shows residues in the reference area of the paste contain high levels of adipate (adipic acid), which matches those in the dendrite area. The dendrite area, however, shows increased levels of chloride, sulfate and sodium from an outside source.

Figure 4. A dendrite short functions as a linear plating cell, and different analytical methods reveal various aspects of the conditions that caused the short.

Table 2. Localized Extraction of Dendrite area and Ion Chromatography Mass Spec using C3 and Thermo Fisher Inuvion (IC) and ISQ EC (Mass Spec)

Terry Munson is CEO and owner of Foresite, Inc. (foresiteinc.com), a consultant and analytical test laboratory dedicated to solving product reliability; terrym@foresiteinc.com.

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