Journal of SMT Article

A Novel Test Methodology to Evaluate Solder Pastes for Mitigating Head-in-Pillow Defects

Authors: Guhan Subbarayan Ph.D., Scott Priore, and Sundar Sethuraman
Company: Cisco Systems, Inc. and Jabil Circuits
Date Published: 7/29/2010   Volume: 23-3

Abstract: One of the solder joint failures that is encountered universally in the electronics industry, during the assembly of Printed Circuit Board Assembly (PCBA) is Head-in-Pillow (HiP) defects. HiP defect causes a nightmare scenario to Original Equipment Manufacturers (OEMs) and Electronics Manufacturing Suppliers (EMS), because products with this kind of defects can escape initial inspection and fail after been shipped to the end customers. The primary cause for HiP defect is attributed to the warpage characteristics of the component during the reflow process. The timing and extent of the warpage, both influences the formation of HiP defects. The ultimate solution for solving HiP is to address and control component warpage, however that is very difficult to accomplish in all packages due to various material and construction constraints. Hence, there is a need to find other approaches to solve this problem. One effective solution would be to investigate for a solder paste to mitigate HiP defects.

The theory investigated here presumes that had the BGA spheres maintain contact with the main card solder paste the HiP defect would not occur. Therefore it is during SMT reflow that package warpage raises the BGA sphere(s) up off the applied solder during flux activation and reflow. The BGA sphere only returns to contact the melted/coalesces solder paste during cooling when the package has begun to return to its initial flatness. At this point, either the flux has exhausted and is unable to form the joint or, the flux itself has created barrier between the two solder features, BGA sphere and PCB solder bump created from the reflow paste on pad.

The Head-in-Pillow defects parts per million (DPPM) level would require a DOE sample size in the thousands therefore this study devised a method to create Head-in-Pillow defect in a controlled lab environment. This method eliminates the use of expensive problematic BGA components and instead applies control over reflow conditions and timing of the contact between the solder ball and the melted solder paste. The SRT BGA rework equipment was used to effect programmable control of the time and temperature profile and sphere contact timing.

A baseline SRT process was established using a solder paste common to multiply production line exhibiting HiP defects. The baseline profile was modified until the baseline solder paste consistently created HiP defects. Using these same programmed SRT parameters, eight other no-clean solder pastes from different vendors were evaluated. A high resolution video camera was used to record the entire reflow process and track the occurrence of the HiP joint. The performances of all the pastes were analyzed to determine the best solder paste to mitigate HiP defects. The selected solder pastes were further validated on a test vehicle with components that has a known history of HiP defects. After the study, the solder paste has been implanted at production and has successfully mitigated HiP defects in problematic components.

Key words: Head-in-Pillow defects, no-clean solder pastes.

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