BGA Soldering Problems
Problem:
The client encountered BGA soldering problems and suspected the ‘black pad’ phenomenon. The ‘black pad’ phenomenon occurs when high levels of phosphorus are present in the nickel-phosphorus pad surface, and results in non-wetting of the joint when solder is applied. Pads that exhibited good wetting of the solder joint will be referred to as ‘good pads’, while those that exhibited non-wetting will be called ‘bad pads’.
Approach:
ITL used a combination of optical microscopy, to determine which contact parts exhibited gold and solder, and Scanning Electron Microscopy (SEM)/Energy Dispersive Spectroscopy (EDS), to obtain high magnification visuals and to identify the chemical/elemental composition. Cross-sectional and planar SEM were used to characterize the surface and sub-surface composition. Planar SEM revealed nickel and phosphorus as the primary components on the surface of the ‘bad pad’, while lead and tin were found on the surface of the ‘good pad’. Cross-sectional SEM was used to analyze the nickel-phosphorus layer of the ‘good pad’ to compare it with the surface of the ‘bad pad’. It was found the the two EDS spectra were identical, except for a bromine peak that was found only on the ‘bad pad’.
Conclusion:
Using the above approach, it was determined that the ratio of nickel to phosphorus on the surface of the ‘bad pad’, and the sub surface of the ‘good pad’ were the same, therefore indicating that ‘black pad’ was not the problem. Instead, bromine contamination was the source of the non-wetting of the ‘bad pads’.
Benefits to Customer:
The client was able to eliminate the bromine contamination, and therefore experience solid and electrically sound joints.
