SMTA International Conference Proceedings

Factors Impacting Solder Extrusion In Reworkable Underfills

Authors: Neil Poole, Ph.D.
Company: Henkel Electronic Materials LLC
Date Published: 10/13/2013   Conference: SMTA International

Abstract: The phenomenon of solder extrusion during thermal cycling of underfilled flip chips is well known and studied [1,2] However, the same can not be said for its assembly level counterpart. The ability to rework assembly level underfills, to reduce scrap and maximise wafer thin margins is essential for both OEMs and subcontractors. The easier the underfill is to rework, the less time and cost is involved in the process. There are several factors to ease of underfill reworkability, which include: the ease of component removal, the level of site clean up and avoidance of board damage (e.g. solder mask removal, pad lifting etc.). With today’s high-density assemblies, a further parameter must be added: solder extrusion. Solder extrusion is a phenomenon that occurs when an underfilled part is exposed to temperatures above the melt point of the solder in the component joints during rework of an adjacent component or the removal of an RF shield. The molten solder is squeezed out from under the component to produce a peripheral solder ball, or a solder bridge, resulting in a failure of the component.

Anecdotal evidence suggests that if the underfill breaks cleanly from the PCB (i.e. an adhesive failure) during component removal, it is more prone to solder extrusion then those that leave a significant film of underfill (i.e. a cohesive failure). It has also been suggested that environmental conditions can impact the tendency for solder extrusion. This paper investigates the causes of this phenomenon by analysing the relationship between a range of physical properties of the underfill (CTE, Tg, flex modulus, elongation to break and die shear) and their impact on the tendency for solder extrusion during rework. The impact of the presence of solder flux residues and environmental exposure are also studied to give a broad view of the factors impacting solder extrusion.

Different underfills were observed to have varying responses to environmental conditions. The presence of flux (a potentially a weak boundary layer) was not observed to be of significance with the underfills evaluated in this study. Both Tg and die shear are determined to have a significant relationship with the tendency for solder balling. Hot adhesion was also determined to be a key parameter.

Key Words: 

Underfill Rework, Extrusion

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