Symposium Conference Proceedings


HnP Defects by Emulating The Gap Between BGA and PCB During Reflow

Authors: Ken Chiavone and HT Poh
Company: Akrometrix
Date Published: 4/12/2016   Conference: Symposium


Abstract: SMT assembly planning and failure analysis of surface mount assembly defects often include component warpage evaluation. Coplanarity values of Integrated Circuit packages have traditionally been used to establish pass/fail limits. As surface mount components become smaller, with denser interconnect arrays, and processes such package-on-package assembly become prevalent, advanced methods using dual surface full-field data become critical for effective Assembly Planning, Quality Assurance, and Failure Analysis. A more complete approach than just measuring the coplanarity of the package is needed. Analyzing the gap between two surfaces that are constantly changing during the reflow thermal cycle is required, to effectively address the challenges of modern SMT assembly.

To fully understand and characterize variation in the gap that is the main cause of solder joint defects such as head-in-pillow, shorts, and opens, analysis of the interface between the mating surfaces needs to be:

• Full-field: A high density of data represents surface warpage much more finely than an approximation such as coplanarity, allowing area-specific review and analysis

• Dual-surface: Making assumptions about how the “bottom” surface is shaped when mounting a component neglects the complex behavior the land area might exhibit during the thermal cycle, and is a deficiency when attempting thorough assembly planning or failure analysis

• Full-profile: Making assumptions about when the single temperature when warpage of surfaces is important during the thermal cycle can lead to missing critical parts of the reflow process, overlooking when defects such as head-in-pillow can be caused

• Statistical: A large enough quantity of samples should be measured to allow confident calculation of expected average, maximum, minimum, and extreme (3s) gaps between surfaces. After measuring warpage of multiple surface mount components and land areas separately, the collected data can be combined into statistical summaries for each temperature point.

Reviewing the combined data at the start of assembly planning can provide an overview of dual-surface warpage at each temperature, and for the entire thermal profile. The measurement and analysis results that follow include a package-to-board assembly interface case study, and calculations, graphs, and methodology highlighting the use of gap limits and pass/fail maps to visualize areas with potential assembly issues.

This analysis method provides new capabilities when planning and monitoring the assembly interface across a full reflow cycle, that can help predict and compensate for defects such as head-in-pillow.



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