2005 Articles

Based on DoEs, a look at how different pastes stack up in six areas.

Speedline Technologies has printed Pb-free solder pastes for more than six years. We work with most of the major solder paste suppliers in evaluating process performance characteristics of SnPb and Pb-free solder pastes. We have conducted a number of formal designed experiments (DoE) and statistical studies on Pb-free printing. We have also worked with a number of customers on Pb-free process implementation and have even built our own working Pb-free product (128 MB USB memory module) to enhance our practical experience.

Printing Pb-free pastes involves identical process development methodology as printing SnPb solders. The only method to optimize any printing process is to perform detailed DoE that will identify and optimize the printing parameters (squeegee speed, squeegee pressure, etc.). Here is brief overview of what we have learned.

Solder paste volume. We printed several different Pb-free solder pastes on circuit boards with OSP, immersion silver and nickel gold finishes using the same stencil and the same printing equipment, at virtually the same time. In each instance we measured the deposited solder paste volume. There was no statistical difference between the volume of SnPb and the many different Pb-free solders.

Solder paste release from the stencil. This is an interesting area of printing to judge. As noted, the printed volume of SnPb and Pb-free paste are the same. But can you accurately quantify how paste releases from a stencil? Using a high-speed camera, we recorded the release of several SnPb and Pb-free pastes from a stencil. Our review found no physical difference in the release of SnPb and Pb-free solder pastes.

Stencil design. The primary issue with designing a stencil for Pb-free solder is that it does not solder or wet as well as SnPb. Using normal aperture reduction rules may result in exposed copper after the reflow soldering process since the Pb-free solder paste may not spread and cover the entire PCB pad. Each Pb-free solder paste has its own wetting characteristics; to determine these, experiments should be performed on any particular Pb-free solder paste. We have seen a marked improvement in the wetting of some new Pb-free solder pastes. All the solder paste suppliers we are working with are on their third, fourth, fifth or higher generation of their primary Pb-free solder paste. They have made significant improvements.

Stencil fabrication. We have printed many Pb-free solder pastes using stainless steel laser-cut and electroformed nickel stencils. Variations in stencils depend on the laser used, its calibration or lack thereof, the supplier’s process control, etc. However, we have discovered no compelling data that suggest any particular stencil fabrication technology will optimize the Pb-free solder paste printing process.

Enclosed print heads. All enclosed print heads have some things in common, but also differ in some ways. They share the goal of protecting the solder paste from the environment to reduce paste waste. However, they work in different ways. The design of enclosed print heads is dramatically different from supplier to supplier.

It has become evident since enclosed print heads were introducted that some solder pastes work well in some enclosed print heads and some do not. A particular paste that works well in a particular enclosed print head design may or may not work well in another enclosed print head design. We have found this to be true with Pb-free solder pastes as well. The performance of each Pb-free solder paste must be evaluated in that particular enclosed print head.

Printing accuracy. We performed a detailed study using Pb-free and SnPb pastes from five different suppliers to determine if printing Pb-free solder pastes requires a more accurate printing process. We designed a stencil with apertures offset at varying distances from the PCB pads. We printed the Pb-free and SnPb pastes and measured how well they wet back to the pads after reflow.

We learned that, in general, Pb-free solder pastes do not wet back to the board pads as well as SnPb paste. Some of the Pb-free solder pastes wet back very well, others did not. Also, there is a significant difference in the wet back capability of different Pb-free solders. This underscores the need to make the Pb-free printing process as accurate as possible. Equipment should be calibrated, cleaned and kept up to date with preventative maintenance (PM).

Developing a Pb-free printing process requires the same attention to detail as developing a SnPb printing process. In general, the same tools and methods (stencil design, stencil fabrication, squeegees, support tooling, etc.) can be used. Likewise, well-maintained, calibrated printing equipment can be used, while newer, more accurate equipment can be considered to optimize printing process capability.

Joe Belmonte is project manager, advanced process development, at Speedline Technologies (speedlinetech.com); jbelmonte@speedlinetech.com.