Collaboration between OEM and EMS to Combat Head-on-Pillowing Defects (Part 2)


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By Alex Chan, Paul Brown and Richard Coyle, Alcatel-Lucent; Lars Bruno, Anne-Kathrine Knoph, Ericsson; Thilo Sack, Celestica Inc.; David Geiger, David Mendez and Ron Kulterman, Flextronics International; Mulugeta Abtew, Iulia Muntele, Sanmina Corp; Kirk VanDreel, Plexus Corp.

There have been many publications, industry workshops, and symposia that describe process mitigation techniques for minimizing the occurrence of head-on-pillow (HoP) defects during surface mount assembly. These include inspection effectiveness as covered in Part 1 of this two-paper set. This second paper addresses the root cause of the HoP defect—specifically the link between HoP defects and component warpage. Multiple case studies contributed by the participating companies are presented to support the proposal that the current industry warpage standard needs to be revised and to demonstrate that further work is needed by the industry to understand the root cause of this defect better. Based on the data from the case studies, the authors propose revised acceptance criteria that set the maximum warpage at 0.090 mm (3.5 mil) for BGA of 0.8 mm pitch and above.

Introduction

Head-on-pillow (HoP), head-in-pillow (HiP), head-and-pillow (HnP); regardless of what you call it, it is a defect that is growing in prevalence. HoP is particularly problematic because the intimate contact between ball and paste can easily escape current x-ray inspection processes and, in many instances, adequate electrical continuity exists to pass initial electrical testing. Eventual separation of what was never a proper metallurgical bond can lead to late stage manufacturing defects and even early stage field return issues.

Significant effort has been invested in the study of HoP. As a result, the defect mechanism and its contributing factors have been substantially described in the literature. There is wide recognition that package warpage is a primary factor in the formation of HoP. In fact, progress to help characterize factors influencing package warpage behavior in an attempt to define mitigation measures. These include the iNEMI Package Warpage Qualification Criteria and the HDPUG FCBGA Package Warpage projects.

As discussed in Part 1 of this paper, the authors worked on a number of mitigation practices to minimize HoP defects on product. However, it also sparked interest from the different participating OEM and EMS companies to discuss the need to address the root cause of HoP in the industry. It is the belief of the authors that there are a number of factors that lead to the formation of the HoP defect, but package warpage during reflow is viewed as the most significant contributor.

In this paper, a number of real product examples collected by different participating members will be presented. Current industry specifications governing package warpage will be reviewed and recommendations for revised process warpage criteria for BGA packages with a pitch of 0.8 mm and above will be made.

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Editor's Note: This article originally appeared in the August 2015 issue of SMT Magazine.

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