'Every tool has its place': the final part of our comparison of test methods looks at the C3.

High-performance liquid chromatography (or high pressure liquid chromatography, HPLC) is a form of column chromatography used to separate, identify, and quantify ionic and organic compounds. HPLC uses a column that holds chromatographic packing material (stationary phase), a pump that moves the mobile phase(s) through the column, and a detector that shows the retention times of the molecules. Retention time varies depending on the interactions between the stationary phase, the molecules being analyzed, and the eluent used with a conductivity or UV-Vis detector. (1)

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The ion-exchange chromatography (ion chromatography) process permits separation of ions and polar molecules based on the charge properties of the molecules. It can be used for almost any kind of charged molecule, including large proteins, small nucleotides and amino acids. The solution to be injected is usually called a sample, and the individually separated components are called analytes. It is often used in protein purification, water analysis and quality control. In electronics analysis, the critical step is to get the ionic residue into a solution for analysis. Traditional bag extractions are limited to extracting large areas of the board and components when failures and residue areas are typically found in specific locations.

C3 (Critical Cleanliness Control) is a localized extraction system that permits ionizable ionic and organic residues to be solubilized under normal humidity exposure and voltage conditions in a specific component location. (Disclosure: C3 is my invention.)

Using the localized C3 extraction of a specific area of the assembly reveals and quantifies certain ionic and organic residues (Figure 1). This permits understanding of what residues would naturally ionize in high humidity conditions, and determines which ions (anions and cations) and organic acids are present and at what level. Now, a comparison of residues from areas of the assembly that did not fail with areas that did fail will show key pieces of information. The limitation of understanding pockets of contamination is that you need to understand the areas that are more sensitive to test (such as high-impedance circuits, or components that entrap flux such as QFN and RF filters).

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Investigation of a Dendrite Short

This investigation will use all four investigative techniques to understand the conditions at the site of the short (Figure 2). Each tool will add to the information to draw a conclusion about the root cause of the failure.

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* FTIR analysis shows only the conformal coating signature below the dendrite.

* SEM/EDX analysis shows the elements in the dendrite are carbon, oxygen and copper.

* XRF analysis shows the elements in the dendrite are copper and bromide.

* C3 and IC analysis show the residue was very corrosive at 23 sec, and the chemical makeup of the residue from the via and top surface of the coated assembly was high in sulfate at 5.34 [micro]g/[in.sub.2] and 231.11 [micro]g/[in.sub.2] of WOA (succinic + malic acids). The opposite side of the assembly is next to a selective solder location; the amount of flux residue from overspray permitted a large amount of flux to build up on the other side and in the hole barrels, and the sulfate residue came from the etch process prior to OSP.

The root cause of this dendrite short over the top of the coating was very thick layer of flux and a deficit of proper heating to complex the flux residue.

This thick buildup of partially heat-activated flux absorbed moisture over three months in the field (office environment), creating the proper three conditions to cause a dendrite to form.

Conclusion. The tools used to determine the effect of a residue on a failed assembly can be varied and personalized, depending on the strengths of the failure analysis group. Each tool has it place, and some are better than others, but what's important is to understand the limitations. In the dendrite growth failure, we discussed how SEM/EDX analysis confirmed that the copper alone corroded, that the large level of carbon and oxygen came from something else, and there was not a large amount of chloride or sulfate residue to be detected. FTIR shows the dendrite was in and above the conformal coating. XRF confirms the copper alone is corroded, and the normal bromide signature revealed this was a brominated flame-retardant fabricated bare board. C3 and IC show the high WOA and marginal level of sulfate came from the process and not an external source of contamination (i.e., coffee, tap water or carbonated soda), and the residue is not used up but still corrosive on the C3 electrode in 23 sec. The corrective action is to better clean the bare boards before OSP and to heat-active the flux after selective soldering by exposing the flux to post-soldering heating (cure oven with a 150[degreess]C set point for 30 sec.) to complete flux activation.

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Each tool has an appropriate application. When determining material degradation of lot-to-lot variation, FTIR is my choice. To look at a metal surface for tin whisker or the grain boundaries and intermetallic formation of a solder joint, I would use SEM/EDX with FIB or even Auger. To determine a quick, nondestructive metal or plastic makeup, or to screen for RoHS elements and percentages in the solder mass, XRF works wonders. To understand the amount of residue present on a corrosion site (or even a location that shows no dendrite but is shorting due to stray voltage), we use the C3 and IC system to determine ions/organic residues and if the residues are corrosive, along with how much is in this spot compared to a different spot on the same board.

References

1. Wikipedia.

Terry Munson is with Foresite Inc. (residues.com); [e-mail]. This column appears monthly.