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Reliability of ENEPIG by Sequential Thermal Cycling and Aging
Reliability of ENEPIG by Sequential Thermal Cycling and Aging
Paper presents the reliability of LGA components assembled with tin-lead solder onto PCBs with an ENEPIG finish and subjected to thermal cycling and aging.
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Authored By:
Reza Ghaffarian, Ph.D.
Jet Propulsion Laboratory
California Institute of Technology

Summary
Electroless nickel electroless palladium immersion gold (ENEPIG) surface finish for printed circuit board (PCB) has now become a key surface finish that is used for both tin-lead and lead-free solder assemblies. This paper presents the reliability of land grid array (LGA) component packages with 1156 pads assembled with tin-lead solder onto PCBs with an ENEPIG finish and then subjected to thermal cycling and then isothermal aging. To determine thermal cycle reliability, daisy-chain LGA1156 packages were used to enable the monitoring of solder joint failures.

The assemblies, were built with a vapor phase reflow machine or using a rework station. Then, they were subjected to thermal cycling ranging from -55 Celsius to 125 Celsius. Subsequent to the completion of two hundred thermal cycles, the assemblies were isothermally aged for 324 hours at 125 Celsius to determine the effect of isothermal aging on intermetallic formation and growth, which is one of the concerns for tin-lead solder assemblies. To determine the effect of exposure at temperatures higher than 125 Celsius, the aged samples were subjected to 100 thermal shock cycles between -65 Celsius and 150 Celsius.

A number of characterization methods were used to ensure the integrity of solder joints. These included nondestructive evaluation by X-ray, daisy-chain monitoring at thermal cycle/aging intervals, and destructive characterization by cross-sectioning. The cycled/aged samples were cross-sectioned and characterized by optical and scanning electron microscopy (SEM). Assembly processes and SEM photomicrographs showing damage progression and IMC/microstructural changes, as well as elemental analyses by x-ray energy dispersive spectroscopy (EDS), were also presented.

Conclusions
ENEPIG PCB surface finish reliability characteristics for RoHS solder joint assemblies have been the subject of numerous papers showing favorable results. However, reliability data for tin-lead solder is scarce - sometimes showing negative effects on reliability. This paper addressed the ENEPIG with tin-lead for high reliability electronics hardware to see if there were any apparent issues. For this reason, a number of accelerated thermal cycles and shocks along with isothermal aging were performed using the LGA1156 assemblies to determine the integrity of the solder/ENEPIG interface after each environmental exposure. A summary of findings is listed below:

1. After 200 thermal cycles (TC, -55 Celsius to +125 Celsius), no failures were detected by daisy-chain monitoring and no microstructural anomalies occurred at the ENEPIG/ solder interfaces

2. After 200 TC plus 324 hr of aging at 125 Celsius, the ENEPIG/solder microstructural changes at the interfaces were within normal expectations

3. fter 200TC plus 100 thermal shock cycles (TS, -65 Celsius to +150 Celsius), the LGA1156 assemblies failed in the daisy-chains showing resistance opens, but no significant degradations were detected at the ENEPIG/solder interfaces by cross-sectioning and SEM elemental evaluation

The preliminary test results showed the acceptance of the ENEPIG PCB finish with tin-lead solder for short-term duration in high-reliability applications. It also revealed the short-term thermal cycle reliability acceptability of LGA1156 assemblies under standard harsh TC (-55 Celsius to +125 Celsius), but not under a more severe thermal shock cycle (TS, -65 Celsius to +150 Celsius).

Initially Published in the IPC Proceedings

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