SMTA International Conference Proceedings


Author: Sunil Gopakumar et al.
Company: Brocade Communications
Date Published: 9/25/2005   Conference: SMTA International

Abstract: Despite the fact that most high density printed circuit boards (PCB) nowadays have less through-hole components than before, Pb-free wave soldering is still an area of research that merits considerable attention. Extensive research by a number of consortia has yielded considerable data on the issues related to Pb-free wave soldering. However, successful implementation of Pb-free wave soldering for high reliability products such as networking equipment, is far from complete. More work is needed to develop an acceptable Pb-free wave solder process. This paper focuses on the issues involved in developing an acceptable Pb-free wave soldering process window for thick, multilayer boards using a selective soldering pallet.

A new laminate capable of withstanding Pb-free soldering temperatures was used to construct a 0.125-inch thick multilayer board. The board has 18 layers with 8 ground planes and 10 signal layers. This experiment utilized two popular Pb-free surface finishes commonly used in the industry Immersion Silver (Imm-Ag) and high temperature Organic Solderability Preservative (OSP). The SAC 305 alloy with a composition of Sn3.0Ag0.5Cu was used for wave soldering.

Due to the complexity of the design, typical of sophisticated telecom products, a specially designed wave soldering pallet was used to create a selective soldering application using a wave soldering machine. Issues such as solderability, hole fill and defect levels were studied using transmission 2D Xray and 3D X-ray. A suite of reliability and destructive physical analysis (DPA) tests were carried out to establish the quality of the soldering as it compares to a eutectic Sn/Pb assembly, which was used as a baseline. The impact of increased soldering temperatures on the board, components, and reliability of the product were also studied.

Initial results pointed to a reduced wetting of the plated through hole barrels as compared to Sn/Pb wave soldering process. Defects such as insufficient hole fill, bridging and voiding were observed. The process window for Pb-free wave soldering is narrow and required considerable optimization to attain reasonable hole fill. Additionally, the solderability and hole fill was significantly better on Imm-Ag surface finish as compared to the OSP surface finish.

Key Words: Lead-free, Pb-free, Wave Soldering, SAC, DPA, Optimization, RoHS.

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