A customer called into the EMPF Helpline requesting a bare board solderability evaluation...

Recently, an EMPF customer requested a bare board solderability evaluation after experiencing problematic wetting with a lot of FR4 substrates. The substrates were single sided PCB panels with a traditional tin-lead hot air solder level (HASL) finish, similar to the surface finishes observed on many legacy designs. The HASL finish did not appear to be processed correctly and needed testing to determine if the boards were solderable. The EMPF performed Solder Float Testing to establish the solderability of pads on samples of the PCB; X-ray Fluorescence testing to establish the finish thickness and composition; Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS) to evaluate the surface of various pads and through-holes and cross-sectional analysis with SEM/EDS of a pad for solder characteristics (i.e. grain structure, intermetallic compound formation, etc); Sequential Electrochemical Reduction Analysis (SERA) to determine what metal oxides were present.

Solder Float Test and XRF
The samples were tested to J-STD-003A, 4.2.3, Test C. A board section was placed on to the molten solder surface (pot temperature 235°C) and allowed to float on the surface for five seconds maximum. The sections were removed, cooled and examined. The testing indicated incomplete wetting (Figure 3-1). Composition and thickness measurements were taken at various surface features on a number of bare PCBs from the panels supplied. XRF analysis indicated that the HASL thickness varied from 3.7 to 711 microinches. The composition of the HASL was also inconsistent.

SEM/EDS analysis
SEM images of the pads and other surface features on the PCB were captured before and after solder float testing.  SEM images of the surface prior to testing showed further evidence of the wetting problem. There were distinct areas where the solder pulled away from the surface. Micro-sections and subsequent SEM/EDS analysis of a pad after Solder Float testing indicated good grain structure with lead (Pb) regions and tin (Sn) regions. Elemental analyses of the bulk solder and interface area were done through EDS and showed no evidence of metallic contamination.

Sequential Electrochemical Reduction Analysis (SERA)
SERA is a technique that uses electrochemistry to identify tin oxides, silver sulfides, and thin or porous gold finishes. SERA analysis of three pads indicated varied amounts of tin II (SnO) and tin IV (SnO2) oxides were present. SERA also verified that there was no copper-tin intermetallic oxide present. The presence of intermetallic oxide would indicate that the HASL was thin and failed to protect the underlying copper pad.

Testing of the panel of PCBs confirmed some solderability issues were present. This reaffirmed the customer’s concerns that the boards were not suitable for assembly. There were many contributing causes for the solderability that were identified. However, the observations gained from testing indicated a questionable HASL finish was the main reason for the poor wetting.  The pads and annular rings which made up the top layer circuitry on the board displayed an inconsistent appearance, with mottled textural differences in various places within the metal area. X-Ray Fluorescence confirmed large variations in HASL finish thickness.  This inconsistent appearance was also revealed as de-wetting during solder float test in some instances. Both the appearance and large variation in thickness of the HASL finish of these boards indicated a significant concern with the HASL surface finish applied to the boards.

Boards showing such inconsistency in HASL finishes would not be appropriate for use in a sustainment application. The boards must establish excellent solderability at the time of manufacture to ensure that as time progresses, the storage conditions will not reduce the wetting performance to the point where the boards become unsolderable.