SMTA International Conference Proceedings

Impact of Test Vehicle on Process Development for Fine Pitch WLP-CSP Assembly

Authors: Domingo Vazquez, Alfredo Garcia, Ricardo Macias, Surasak Sripanyavich, Joe Smetana, Richard Coyle Humberto Ramirez, Cristina Amador, Mulugeta Abtew, Iulia Muntele, Shane Lewis
Company: Sanmina, Nokia
Date Published: 9/17/2017   Conference: SMTA International

Abstract: Data driven process development for 0.3 mm pitch for wafer –scale package or wafer level package (WLP) test vehicles can be obtained either by assembling an appropriate number of components using an off the shelf test board and components, or by generating a new test board that may impose additional challenges to assembly process. Both methods were investigated, and while the off the shelf test board presented obvious advantages to expediting the qualification process, the custom test board provided additional insights both for the experiment design and assembly. The custom test board was developed around commercially available daisy-chain dummy components. The custom test board differs from the commercially available test board in thickness, number of components and components sizes, has three different size pitches, and the daisy-chains were completed through the internal layers of the board using stacked via in pad. The custom test board was needed because a commercially available test board with a similar level of complexity was not identified at the time the experiment was run. The impetus for development of custom test boards is a reflection of the increased complexity of products in electronics manufacturing, as well as by the difficulty to extrapolate characteristic life of solders joints from simpler designs and test vehicles to the more complex ones.

The usage of fine pitch components on mixed technology assemblies – containing both coarse and fine pitch components - has risen in the recent years. One of the main concerns is maintaining a high SMT yield while not extending significantly the cycle time, or upsetting the line balance. Two series of experiments were conducted, one basic assembly experiment using an off the shelf kit containing one type of 0.3 mm pitch CSP component, and one experiment based on a custom test vehicle containing several sizes and ball arrays of 0.3, 0.4 and 0.5 mm pitch WLP components. For the custom test vehicle, additional processes were investigated: cleaning off no-clean assemblies, followed by underfill application and thermal cycling evaluation of solder joints reliability. A couple of reworkable and non-reworkable underfill material options were chosen based on supplier recommendations. Assembly of the components using flux dipping or solder paste printing methods was demonstrated successfully for a reasonably wide lead free process window (reflow profile length between 3 and 5 minutes) with low DPMO and low voiding level. Post assembly inspection included visual, Xray and electrical continuity measurements, followed by initial cross-section characterization of the solder joints, and parallel sectioning for underfill characterization.

Key Words: 

WLCSP, fine pitch, underfill, lead free

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