In case you missed it.

This month's abstracts come from the SMTA International conference in September (smta.org).

Component Placement

"Process Window Research on 0201 And 01005 Chip Component Assembly"

Author: Syber Chen; [e-mail].

Abstract: This study was performed to research the assembly process window of 0201 and 01005 chip components. This paper presents the experiment procedures and results of 0201 assembly with Type 3 Pb-free solder paste, including evaluations of components, designs of pad and stencil aperture, screening experiment for better pad design, and the influence of offset solder paste printing and placement offset and the reflow profile. The soldering defect comparison of different factors such as component types, layout orientations, layout gaps, and PCB platings are also presented.

Reliability Testing

"Acceleration Factors for Lead-Free Solder Materials"

Author: Olli Salmela; [e-mail].

Abstract: This paper focuses on analytical models proposed by Norris-Landzberg, Pan and Salmela. Test vehicles used are ceramic BGA modules attached to FR-4 PWBs using SnAg3.8Cu0.7 solder. The TVs are 1 mm thick ceramic substrates on which daisy-chained pads are arranged as two concentric circles. Two assembly lines are used, one a prototype line and the other a mass production line. The different lines result in different solder ball thicknesses and slight quality deviations. After assembly, the TVs are split in three lots that are addressed to three different thermal cycling tests: -40 - +125[degrees]C, 0 - +100[degrees]C and 30-80[degrees]C. The TVs are monitored continuously in-situ and failure data are analyzed statistically. The failure mode recognized is solder fatigue. It is expected the reliability prediction models must be recalibrated in order to be useful in conjunction with Pb-free solder materials. This is accomplished by using the failure data recorded. Based on the experiments, also the Norris-Landzberg model is also recalibrated.

"H.A.L.T. Test Vehicle for Lead Free Surface Finish and Solder Paste Selection"

Author: Tim Murphy; [e-mail].

Abstract: Thomson has developed a test vehicle and protocol to enable engineers and management to easily measure and compare interactions between surface finish, paste flux and solder alloy. This thermomechanical test method measures the average energy required to fracture SMT solder joints using nine different strain conditions. The ease of replication and simple test geometry allows practical multifactor experiments to be conducted on paste alloys and their flux vehicle. The method permits measurement of thermal stability, packaging and shelf life studies on PCB finishes and permits measurement of their complex interaction with paste alloys; i.e., wetting, fillet formation, intermetallic phases distribution and, alloy dilution. Data presented have expanded upon Thomson's earlier work and now include evaluations of suppliers of immersion silver and tin coatings, OSP, ENIG and Pb-free HASL. Additional reliability data on 3 and 4% SAC alloy are presented along with comparisons to non-silver-bearing paste alloys, SnCuNi and SnZn.

Underfills

"No-Flow Underfill Material Evaluation and Reflow Process Development of a Flip Chip in Package Device Using No-Flow Underfill"

Authors: Sangil Lee, Sungmin Suh, Steve Markey, and Daniel Baldwin, Ph.D.; [e-mail].

Abstract: Flip chip in package using no-flow underfill can produce high yields under optimized assembly process conditions. Optimized flip chip assembly parameters depend on underfill material characteristics and device interconnect configuration. The test vehicle has high I/O density (over 1300), large chip size (up to 10 mm), and small pitch (down to 150 [micro]m). Since process performance of commercial no-flow underfills was limited on these high-density TVs, newly developed underfill materials from commercial suppliers were considered for this production TV. New methodologies are explored to better narrow the feasible process conditions while limiting number of TVs required establishing process conditions promoting high yield. Several experiments are conducted to investigate reflow profile parameters, material thermal limits, and outgassing characteristics using a parametric approach. Conduction and convection mode thermal testing determine temperature range limits of each underfill material. A formal wetting study was used to identify potential reflow parameters. The results of these initial studies are used to trim reflow profile with low-cost commercial TVs and this pre-qualifying experiment will be used to optimize no-flow underfills processes prior to using the commercial TVs. The main concern of the prequalifying experiment is high electrical yields with a low-cost TV. The high assembly yield process for a low-cost test device is determined in order to ensure that the process can be used effectively in production. Therefore, new methodologies developed in this project produce baseline assembly process to get high yields with newly developed underfills and test devices.

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.