Swabbing it clean: how clean are your cleaning materials?

Recently, an instrument manufacturer found corrosion on the metal interconnects at the edges of liquid crystal displays (LCDs) after undergoing environmental testing. He wanted to know where the corrosive residues originated--inside or outside the LCD?

Analysis

Several assemblies, components and processing materials were analyzed using ion chromatography per IPC-TM-650, method 2.3.28, to determine the cause of the corrosion.

Data Discussion

Samples from the interior of the LCD (Table 1) appeared to be ionically clean. Therefore, I concluded that the residues were originating outside the LCD. The exterior of the uncleaned and untested LCD samples showed low levels of chloride and sulfate. However, the failed LCD samples and the cleaned, but not yet tested, LCD samples showed high levels of chloride and sulfate, as did the cotton swab and the isopropyl alcohol (IPA) dispenser. Consequently, I concluded that the cleaning process was introducing the corrosive residues.

Root Cause

The LCDs were cleaned prior to the installation of a Zebra connection strip using a cotton swab, which was dipped in IPA. The cotton from these non-clean room quality cotton swabs contained chlorides and sulfates. These contaminates, which entered the cotton from tap water used during the cotton swab manufacturing process, were then absorbed into the IPA. Capillary action pulled the IPA into the corner joint of the two glass plates where it dried on top of the metal interconnects, leaving residues. Under electrical power, these invisible corrosive residues combined with moisture to form acids, such as hydrochloric and sulfuric acid, and caused corrosion and electromigration of the metal interconnects--resulting in the LCD's failure.

Corrective Actions

The cotton swabs and alcohol dispenser were replaced with prepackaged clean room purity alcohol wipes. Furthermore, regular ionic contamination testing of assemblies, components and materials were added as a means of process control.

Conclusion

LCD manufacturing is just as sensitive to corrosive residues as printed circuit board (PCB) manufacturing can often be. Any product that will eventually see electrical voltage is sensitive to corrosive residues and requires careful examination of all cleaning materials used in the process.

TABLE 1: Summarized ion chromatography results.

Sample Description          Chloride Levels          Sulfate Levels
(Levels represent        ([micro]g/[in.sup.2])    ([micro]g/[in.sup.2])
the mean of samples
tested)

Interior of LCD
Internal liquid from
  failed LCD                      0.57                    1.02
Fluid side of LCD top             0.32                    0.76
Fluid side of LCD
  bottom                          0.23                    0.67
Exterior of LCD
Un-cleaned and
  untested LCD                    0.78                    0.62
Failed LCD                       20.42                    9.81
Cleaned but not
  tested LCD                     13.22                    9.62

Terry Munson is with Contamination Studies Laboratory (CSL), Kokonto, IN; [phone]; e-mail: [e-mail]; Web page: www.residues.com.