In case you missed it.

All abstracts come from this month's SMTAI show (smtai.org).

Conformal Coating 

"Conformal Coating, Application and Environmental Relationships"

Author: Timothy Harmon; [e-mail].

Abstract: The fluid nature of conformal coat makes it susceptible to exposed dispensing and environmental parameters. The dispensing application and environmental parameters maintain a strong variable relation. Consequently, the process window is small and requires significant process calibration. This paper presents a series of design of experiments in relation to the important variables of the conformal coating process. Specifically, the data correlate optimal environmental characteristics for spraying conformal coat with some emphasis on relating environmental parameters to the viscosity of the material, as this directly affects dispensing results. Also provided: comparison data of automated and manual conformal coat dispensing as they relate to the optimal setup of each process. Acrylic and silicone conformal coating materials are evaluated. Conclusions include optimal process setup and ultimate coating reliability from the aspect of the material types and the process parameters.

Lead-Free Soldering

"Copper Dissolution Rate in Pb-free Solder Fountain Systems"

Author: Fritz Byle; [e-mail].

Abstract: A flowing "mini wave" or solder fountain is often the only effective method for mounting multilead through-hole components or for rework of these components when they are not compatible with standard wave-soldering processes, or where the assembly cannot be wave soldered. Connectors are typical components soldered or reworked by this method. The high-Sn alloys and higher temperatures used in Pb-free soldering processes increase the dissolution rate of copper from traces and pads exposed to the liquid solder. Under common conditions, the dissolution rates may be so high that traces are almost completely dissolved within 20 to 30 sec. Trace cross-sectional area will be reduced to levels violating IPC guidelines in even shorter times. The defects generated may not be detected electrically, since the solder will remain wetted to copper remaining at the bottom of the "tooth" of the laminate surface. This paper presents results of tests intended to evaluate the various factors affecting the copper dissolution rate. Data and test methodology are discussed in detail, showing how dissolution rates can be changed by a factor of four or more depending on process and materials. The data developed in these tests are also applicable to any process where a PWB surface is exposed to flowing solder; e.g., wave or selective soldering.

Process Optimization 

"Process Characterization of the 01005 Component Package"

Author: Tom Borkes; [e-mail].

Abstract: This paper investigates the technical, quality and cost impacts associated with the 01005 component package. It presents results of an experiment to investigate the general process characteristics associated with reflow soldering 01005s. These include empirically derived specification limits for component placement accuracy and reflow soldering time/temperature profiles for SnPb eutectic and Pb-free alloys.

Solder Joint Reliability

"Applicability of Various Pb-free Solder Joint Acceleration Factor Models"

Authors: Ron Zhang and Dr. Jean-Paul Clech; [e-mail].

Abstract: Solder joint acceleration factor models can quickly assess initial interconnect designs before hardware becomes available for testing. Accurate values of AFs are also needed to extrapolate test results to field conditions, permitting an upfront estimate of board-level reliability. In this paper, an analytical approach is used to calculate AFs from field conditions to standard temperature cycling conditions on different Pb-free packages and board assemblies. The results are compared with several Pb-free AF models as well as actual test data. AF predictions are compared for algebraic, compact and finite-element based models. Conclusions are drawn as to the accuracy of Pb-free AF models and conditions under which particular models are applicable.

"Novel SACX Solders with Improved Drop Test Performance"

Authors: Dr. Weiping Liu and Dr. Ning-Cheng Lee; [e-mail].

Abstract: Adoption of SnAgCu alloys has been hampered by the fragility of solder joints, as reflected by their poor drop test performance relative to SnPb. This is an issue for portable devices employing area array packages such as BGAs and CSPs. Although reducing the silver content in SAC alloys is helpful, the performance is still inferior to that of SnPb eutectic. In this work, a family of SAC alloys modified with small amounts of additives such as manganese, cerium, bismuth and titanium was developed and studied. Results indicate these alloys exhibit improved drop test performance. Other properties such as thermal aging performance and microstructure are also discussed.

CIRCUITS ASSEMBLY provides abstracts of papers from recent industry conferences and company white papers. With the amount of information increasing, our goal is to provide an added opportunity for readers to keep abreast of technology and business trends.