1. High lead attach of the capacitors and chips;
2. Eutectic attach of capacitors, high lead attach of chips;
3. Eutectic attach of the capacitors and chips. Assembly process flow 1 required the least number of steps. Therefore, it was initially selected as the process flow. However, this process flow was found to be unsatisfactory due to problems with flux charring around the capacitors and a high incidence of chip to substrate opens. Assembly process flow 2 was then evaluated. Assembly process flow 3 was considered the least desirable because eutectic solder paste had to be coated on the substrate die pads and Mayo did not have capability to do this. In order to develop the capacitor and die attach processes and to set up the interposer attach process, two assembly test vehicles were built on glass substrates. A capacitor attach process was devised that involved stenciling the eutectic solder paste, placing the capacitors, and then reflowing the assembly. The flip-chip attach process development required greater effort. Experimental matrices were run that allowed an assessment of bond strength as a function of flux quantity, bond force, and reflow temperature and time. Water-soluble flux was used for the chip attach process, so a de-fluxing process had to be developed. The chips then needed to be underfilled. The underfill dispense pattern and substrate heating were evaluated. The interposer was attached using eutectic solder paste stenciled on the back of the substrate. A BGA style attach process was set up to do this. The result of this process development activity was the successful assembly of 21 modules with no defects. Lessons were learned during the assembly process development regarding the difficulties associated with attaching components with high lead solder. Key words: flip-chip attach, BGA, multi-chip module.">

Pan Pacific Symposium Conference Proceedings


ASSEMBLY OF A SERIAL LINK TEST VEHICLE USING FLIP-CHIP, SMT, AND BGA ATTACH PROCESSES: A CASE STUDY

Authors: Wendy Wilkins, Barry Gilbert, and Erik Daniel
Company: Mayo Clinic Rochester
Date Published: 2/25/2005   Conference: Pan Pacific Symposium


Abstract: The Mayo Foundation Special Purpose Processor Development Group has been supporting a serial link development effort. In order to characterize serial link performance, two versions of a multi-chip module (MCM) test vehicle were assembled to study the effects of the substrate and the integrated circuit input/output buffer technologies on basic signal integrity performance parameters at data rates of 2.5 GHz.

The difference between the test vehicles was the substrate technology used: ceramic and laminate. This paper describes the steps taken to develop an assembly process for the ceramic version of the test vehicle.

The ceramic test vehicle consisted of a high thermal coefficient of expansion ceramic substrate, 2 CMOS chips with IBM high lead content (hereafter: "high lead") bumps, 16 surface mount (SMT) capacitors, and an interposer. Assembly processes needed to be developed for SMT attach of the capacitors, flip-chip attach of the chips, and ball grid array (BGA) attach of the interposer.

Based on Mayo's suite of assembly equipment, three process flows could have been used to attach the capacitors and the chips to the substrate:
1. High lead attach of the capacitors and chips;
2. Eutectic attach of capacitors, high lead attach of chips;
3. Eutectic attach of the capacitors and chips.

Assembly process flow 1 required the least number of steps. Therefore, it was initially selected as the process flow. However, this process flow was found to be unsatisfactory due to problems with flux charring around the capacitors and a high incidence of chip to substrate opens. Assembly process flow 2 was then evaluated. Assembly process flow 3 was considered the least desirable because eutectic solder paste had to be coated on the substrate die pads and Mayo did not have capability to do this.

In order to develop the capacitor and die attach processes and to set up the interposer attach process, two assembly test vehicles were built on glass substrates. A capacitor attach process was devised that involved stenciling the eutectic solder paste, placing the capacitors, and then reflowing the assembly. The flip-chip attach process development required greater effort. Experimental matrices were run that allowed an assessment of bond strength as a function of flux quantity, bond force, and reflow temperature and time.

Water-soluble flux was used for the chip attach process, so a de-fluxing process had to be developed. The chips then needed to be underfilled. The underfill dispense pattern and substrate heating were evaluated. The interposer was attached using eutectic solder paste stenciled on the back of the substrate. A BGA style attach process was set up to do this.

The result of this process development activity was the successful assembly of 21 modules with no defects. Lessons were learned during the assembly process development regarding the difficulties associated with attaching components with high lead solder.

Key words: flip-chip attach, BGA, multi-chip module.



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