An overview of surface finishes and their role in printed circuit board solderability and solder joint performance

Abstract

A overview has been presented on the topic of alternative surface finishes for package VOs and circuit board features. Aspects of processability and solder joint reliability were described for the following coatings: baseline hot-dipped, plated, and plated-and-fused lOOSn and Sn-Pb coatings; Ni/Au; Pd, Ni/Pd, and Ni/Pd/Au finishes; and the recently marketed immersion Ag coatings. The Ni/Au coatings appear to provide the all-around best option in terms of solderability protection and wire bondability. Nickel/Pd finishes offer a slightly reduced level of performance in these areas that is most likely due to variable Pd surface conditions. It is necessary to minimize dissolved Au or Pd contents in the solder material to prevent solder joint embrittlement. Ancillary aspects that included thickness measurement techniques; the importance of finish compatibility with conformal coatings and conductive adhesives; and the need for alternative finishes for the processing of non-Pb bearing solders were discussed. IntroductionSuccessful manufacturing operations that use solder joining processes require the bonding (or "faying") surfaces of the circuit board and package VO (lead, termination, or castellation) to have adequate solderability. It is important to understand the functional definition of solderability in order to appreciate the role of surface finishes. Solderability includes two underlying properties, wettabizity andJlowabiZity or spreadability. Wettability is described as the capacity of the molten solder to interact with the base material by means of interfacial tension effects or, interfacial tension effects in combination with interfacial reaction processes, so as to develop an adherence between the liquid and the base material surface. The adherence must continue after the molten solder has solidified. The terms flowability or spreadability pertain to the molten solder being able to spontaneously cover

An Assessment of Immersion Silver Surface Finish for Lead-Free Electronics

“…25,41 The shelf life of OSP can be 6-12 months in office-type storage environments (20°C to 30°C, 30% to 70% RH), but OSP is relatively permeable to air and degrades quickly with sustained exposure to higher temperatures and high humidity over prolonged time periods. 3 Due to the chemical stability of gold, an ENIG finish has a shelf life of at least 2 years, 8 which is higher than other pad finishes, such as ImAg, ImSn, and OSP.…”

“…Once the silver layer is completely dissolved, the molten solder wets the underlying base metal surface, which is usually copper. 3 An ImAg finish is also highly solderable, and its solderability is maintained through multiple reflow cycles. The contrasting colors of silver and copper make it easy to inspect for defects.…”

“…These IMCs are typically brittle and their excessive growth can degrade the reliability of solder joints. 3 The formation of an intermetallic compound is a diffusioncontrolled process. Some surface finishes inhibit solid-state intermetallic compound layer growth, which may occur during high-temperature operations.…”

Material issues in electronic interconnects and packaging

“…In addition, metallization of the electrical components and the electrically non-conducting substrates, based on the processing methods and parameters used can pose problems. [6][7][8] Although issues related to other properties like dielectric constants of the polymeric materials become important due to the ever increasing speed of the electronic devices such as Electronic packages consist of metallic, polymeric, and sometimes ceramic materials as integral entities. Individual physical and mechanical properties of these constituents, and their influence on each other's behavior, affect the overall reliability of the electronic packages.…”

Influence of Noble Metal Coating on Wettability of Copper Substrate by Sn-Ag Eutectic Solder

“…1) Much insecurity about the mechanical and electrical reliability of Pb-free soldered connections remains, because the wettability of Pb-free solder is quite inferior to that of a Sn-Pb eutectic solder. [2][3][4][5] While Pb-free coating materials, for example, Au/Ni, Pd/Ni, Sn, Sn-Ag and Sn-Bi, on the electrode surface of a printed wiring board (PWB) and electronic device have been investigated, [6][7][8][9] the wettability of the Pb-free coating material on the electrode surface by Pb-free solder has not been sufficiently clarified.…”