Edwin Bradley

Effect of PCB finish on the reliability and wettability of ball grid array packages

Organic protective coatings (OPC) and metallic plating chemistries have emerged as alternatives to traditional hot air solder leveling (HASL) to ensure the solderability of printed circuit boards (PCB). This study examined a number of commercially-available printed circuit board finishes to determine their intrinsic solderability and their effect on the solder joint reliability of ball grid array (BGA) packages. The PCB finishes included OPC (Entek Plus CU-106A and MEC-seal), immersion Au over electroless Ni (LeaRonal, MacDermid, Shipley, and Technique), electroless Pd over Ni, electroless Sn over Cu, immersion Bi over Cu, and HASL. Solderability tests were performed by reflowing 20-mil diameter solder spheres on variously aged test coupons and then quantitatively measuring the area and shape of the solder after cooling. The immersion Au finishes generally exhibited excellent wettability as a function of N/sub 2/ reflow aging as compared with the OPCs and HASL. The Pd-Ni, Sn, and one of the Ni-Au finishes exhibited poor wetting when exposed to an 85/spl deg/C/85% RH environment. Solder joint reliability was measured for a 41 I/O ceramic BGA using cyclic thermal shock (nominally -55-125/spl deg/C) and three point bending. All finishes tested behaved similarly in thermal shock. The Au finishes generally performed worse in three-point bending as compared to OPC and HASL. The wetting and reliability results are correlated with the structure and composition of the finishes and the solder joints.

A novel mechanical shock test method to evaluate lead-free BGA solder joint reliability

Portable electronic products such as cell phones are often subjected to drop tests to simulate consumer usage and to ensure required field reliability is met. Product-level drop tests can cause solder joint failures in both lead-free and tin-lead BGA packages due to the combination of highly localized strain created by high strain-rate mechanical shock. In order to understand this phenomenon at the package level, a 4-point dynamic bend test was developed to produce repeatable and scalable strain levels on printed circuit boards assembled with BGA packages deformed at high strain rates (surface strain rates >5/sec). This test method requires a very simple set up that can be easily replicated. Initial determination of board strain level at the point of BGA solder joint failure was characterized for lead-free (SnAgCu) BGA assemblies. This test, in conjunction with a Weibull failure model, provides a method to effectively evaluate BGA solder joint reliability in dynamic environments.

Effect of Reflow Profiles on the Board Level Drop Reliability of Pb-Free (SnAgCu) BGA Assemblies

The mechanical reliability of BGA assemblies is sensitive to the intermetallic compounds that are formed at the solder/base metal interfaces. With Pb-free assembly using SnAgCu solder alloys, the reflow temperatures are higher and may degrade the performance of the BGA assemblies compared to tin-lead. In this study, BGA packages with Sn3.8Ag0.7Cu solder balls and either Ni/Au or Cu substrates were assembled to printed circuit boards with Cu pads protected with organic solderability preservative using a standard Pb-free solder paste of the same composition. The reflow profile variables investigated were peak temperature and number of reflow excursions. A subset of the assemblies were subjected to 4-point dynamic bend at high strain rate (>5/sec) to determine the effect of the reflow variables on the board level drop reliability. BGAs with Ni/Au substrates showed improved performance with increasing peak reflow temperature, while those with Cu substrates demonstrated the opposite. There were weak trends seen with number of reflows for both substrates. The intermetallic compound layer development due to the reflow variables is also covered.

Lead-free solder assembly: impact and opportunity

There has been major interest in Lead-free soldering within the electronics assembly industry for the last several years, and this will continue with the agreement on the language and implementation dates of the WEEE/ROHS legislation in the EU. This paper will focus on several topics critical to the implementation of lead-free soldering. These topics include the impact of Tin-silver-copper as the alloy of choice for leadfree assembly both with respect to component and solder joint reliability, temperature exposure, and lead-free finishes. Results from the recently completed NEMI Lead-free Solder Project, the author’s own work and other published data are discussed. Background The recent history of lead-free solder for electronics assembly has been a tumultuous period of research on the technical merits of various candidate alloys as well as and discussion of the actual benefits of eliminating lead from solder. Much of the initial push for lead-free came from Japanese electronics companies that perceived lead-free soldering as an opportunity to differentiate their product to consumers and increase sales [1]. Around the same time, the EU proposed legislation that are now known as WEEE (Waste from Electrical and Electronic Equipment) and ROHS (Restriction of Hazardous Substances Directive) [2,3]. The WEEE sets targets for take-back recycling while ROHS would severely limit the use of lead in electronics. The language of the legislation was finalized in late 2002, and if passed, the restrictions on lead would be enforced as of July 1, 2006. The bulk of electronic products are affected by the lead ban in ROHS, with only the following applications exempted at the present time: ‐ Lead in glass of cathode ray tubes, electronic components and fluorescent tubes. ‐ Lead as an alloying element in steel containing up to 0.35% lead by weight, aluminium containing up to 0.4% lead by weight, and as a copper alloy containing up to 4% lead by weight.

Characterization of the melting and wetting of Sn-Ag-X solders

There is tremendous interest presently with Pb-free solder assembly in the surface mount assembly industry in response to recent Japanese and European initiatives and proposed governmental restrictions regarding Pb usage and disposal. Many different solder alloys have been proposed as potential Pb-free solder replacements and the most promising of these fall into the general alloy families of Tin-silver (Sn-Ag), Tin-silver-copper (Sn-Ag-Cu) and Tin-silver-bismuth (Sn-Ag-Bi). Published melting point data on some of these alloys indicates that they should be capable of reduced reflow temperatures relative to the commonly available Sn-Ag alloy, which melts at 221/spl deg/C. Differential Scanning Calorimetry (DSC) and reflow visualization was used to characterize the melting and wetting of the Pb-free alloys and generate the practical reflow temperature requirements. This was compared to the DSC data to gain insight on the meaning of the DSC melting data for surface mount applications. The results show that, in general, the wetting performance of the Sn-Ag-Bi alloys are more similar to Sn-Ag and Sn-Ag-Cu than would be predicted by the major onset melting temperature data as measured by the DSC.

Relationship of tensile interfacial strength to lead-free BGA impact performance

With the increased use of Lead-free alloys in BGA packages and assemblies, new reliability issues have become critical. A common mode of failure during impact conditions for BGA assemblies is brittle failure at the interface between the BGA substrate pad and the bulk solder joint. However, the strength characteristics of these interfaces, which include an intermetallic compound (IMC) layer, are not well understood as most existing tests focus on the strain of the entire joint at failure (or number of drops to failure). The present study uses high-speed tensile tests to isolate and characterize the interfacial strength of BGA assemblies. These results are compared to the impact performance of the BGA assemblies as measured by a four-point dynamic bend test and the relative merit of the test methods are assessed. In addition, the interfacial mi era structure of the different assemblies is characterized and related to test results