Journal of SMT Article

An Investigation Into The Development Of Lead-Free Solder Paste For Package On Package (PoP) Component Manufacturing Applications

Authors: Jasbir Bath, Manabu Itoh, Gordon Clark, Masatoshi
Company: Koki Company Limited and Christopher Associates Inc.
Date Published: 10/31/2010   Volume: 23-4

Abstract: As mobile and other electronic devices become smaller with more sophisticated functions the assembly technologies for compact and high-density packages have advanced dramatically. Recently Package on Package (PoP) technology has increased in use in the industry.

In the conventional PoP manufacturing process, the solder bumps of the top package are immersed into a layer of tacky flux. Once the flux immersion is completed, the top package is placed on to the bottom package and soldered together. One of the challenges of PoP manufacturing is component warpage during the reflow process which had led to the incomplete merger of the solder bump from the top package with the solder land on the bottom package. Once the solder bumps of the top PoP component separate from the land of the bottom component, even if they come into contact again, the tacky flux activity is reduced leading to incomplete merger.

The work focused on the development of solder paste rather than tacky flux for PoP manufacturing to reduce the incomplete merger occurring during soldering. The solder paste would have the benefit of reducing the gap between the land and solder bumps caused by component warpage, and would hold the two together more when molten to prevent incomplete merger.

Tests were conducted on lead-free PoP solder pastes with adjustments in the flux thixotropic agents, activators and solvents in addition to the actual flux content used in the solder paste. The results of the investigations are presented.

Key words: Lead-free, PoP, Printing, Reflow

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