Assembly Challenges of Bottom Terminated Components



Assembly Challenges of Bottom Terminated Components
This paper discusses the design, assembly and inspection challenges of bottom terminated components.
Production Floor

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Authored By:


Jennifer Nguyen, David Geiger, and Dongkai Shangguan, Ph.D.
Flextronics International
Milpitas, CA, USA

Transcript


Bottom terminated component assembly has rapidly increased in recent years.  

This type of package is attractive due to its low cost and good functional performance including improved signal speed and good thermal properties.  

However, it creates many challenges to the assembly process and post assembly inspection.  

This paper discusses the design, assembly process and inspection challenges of bottom terminated components.  

The study considers many factors. Design variables includes solder mask defined pads, non solder mask defined pads, mixed pad designs, differing via designs, thermal connections, orientation and more.

Process variables include stencil design, reflow profiles, and reflow atmosphere. Plus board surface finish variables and fabrication variables.  

There are many challenges with the assembly of bottom terminated components. Solder joint voiding is the most common defect. It usually happens at the thermal pads of the device. The voiding behavior varies from component to component, and varies among the pads within the same component.  

This makes the quality assembly of bottom terminated components more difficult. Many common process variables such as reflow temperature and reflow atmosphere do not affect voiding significantly; neither do the design variables.

Summary


Bottom terminated component (BTC) assembly has rapidly increased in recent years. This type of package is attractive due to its low cost and good functional performance (improved signal speeds and good thermal performance). However, it creates many challenges to the assembly process and post assembly inspection.

This paper discusses the design, assembly process and inspection challenges of bottom terminated components. The study considers many factors, including design variables (solder mask defined pad, non solder mask defined pad, mixed pad design, different via design, thermal connection, orientation, etc.), process variables (stencil design, reflow profile, reflow atmosphere, etc.), board surface finish variables (OSP, I-Ag, ENIG) and fabrication variables (solder mask thickness and type).

Conclusions


There are many challenges with the assembly of BTC components. Solder joint voiding is the most common defect seen on all BTC components. It usually happens at the thermal pads of the device. The voiding behavior varies from BTC components to BTC components, and varies among the pads within the same component. This makes the quality assembly of BTC device more difficult. Many common process variables such as reflow temperature and reflow atmosphere do not affect voiding significantly; neither do the design variables (such as SMD or NSMD pad design). There is lack of standardization for the component pad design, and there are no industry criteria for voiding acceptance for BTC components. Further study
on mitigating the common BTC quality concerns such as voiding and solder balling will be published in the future.

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