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Doing the Poka-yoke: avoiding cross-contamination through process verification and tracking.

With the EU Pb-free implementation date behind us, assemblers are asking for help managing process consumables and tracking "cradle to grave" materials that go through screen printers. This can mean anything from green-colored blades and pump bodies to bar codes on each board, tracking bar codes in the software, and transmitting linked files to a host storage location.

For example, recently several EMS companies told us about customer requests to certify any Pb-free products for compliance to the RoHS Directive. This certification can result in the EMS supplier's liability for any Pb-free products that do not comply with RoHS.

The accepted definition of a Pb-free product is equal to or less then 0.1% lead by weight of a solder joint. It is critical for every manufacturer of Pb-free products to understand the material and components used in each and every assembly, and to ensure that Pb-free and SnPb materials do not get mixed on the factory floor. For some products, medical equipment being one, regulations go much further, requiring material and product tracing throughout the life of the product.

The Japanese have a word for a mistake-proof process: Poka-yoke. The concept is to prevent mistakes by putting limits on how an operation can be performed, to force its correct execution. Designing the Pb-free process and any process that requires complete material traceability demands the Poka-yoke approach.

As discussed, cross-contamination of Pb-bearing and Pb-free solders in assembly can be costly, as this holds the possibility of rendering all assemblies suspected of cross-contamination to the scrap pile. However, in the solder paste printing process, it is difficult to achieve a true Poka-yoke operation. We must depend on visible cues, training, process discipline, and process input verification and tracking systems.

Segregation efforts should be diligent and highly visible. If possible, operations running Pb-free and SnPb processes should designate assembly lines as Pb-bearing or Pb-free only. If this is not possible, separate setup kits should be stocked for each line. The setup kit should include squeegee blades, spatulas, dispenser nozzles, soldering iron tips, rework materials, and any other components of the assembly process that offer potential for cross-contamination.

Solder pastes should be stored in separate locations (refrigerators or cabinets) to prevent operators from taking a wrong container mistakenly. Solder bar and dross should also be stored separately and clearly labeled. Solder manufacturers are helping operators to recognize the difference between the two products by changing the colors, shapes, or legends on product containers. In early stages of transition, it is wise to keep solder materials secure, giving only line leaders or supervisors access until the labor force becomes accustomed to the segregation systems.

If shared between SnPb and Pb-free processes, stencils could potentially cross-contaminate the alloys. Manual stencil cleaning does not remove all particles from the apertures. Many automatic stencil cleaners leave some paste residue. If an assembly is permanently switched to a Pb-free process, the stencil can be thoroughly cleaned and inspected under magnification to ensure its cleanliness, then labeled and moved to a designated Pb-free storage location. If an assembly will undergo Pb-free and SnPb runs, the best option is to purchase a second stencil. This will eliminate the potential of cross-contamination from stencils. Most stencil manufacturers can aid in the identification of Pb-free stencils by etching "Lead-Free Only" in the foil, and some offer frames in different colors to provide a visual cue to production personnel.

Several third-party suppliers offer material and setup verification systems that read the bar code or other information on the individual process inputs (solder paste, stencils, etc.) and compare it to the correct information for that particular assembly. If any inputs are incorrect, the system will alert the operator or process engineer.

Equipment suppliers are developing sophisticated systems that will track all inputs to the process. The most comprehensive systems will track information for each substrate down to the process variables, test data and materials. These systems can cut off some paths to cross-contamination by cross-referencing bar-coded process tools (squeegees, tooling, stencils) and consumables (primarily the solder paste) with the process program for the specific board model, preventing a board that does not match from entering the printer.

The requirement for managing product segregation in a factory seems to be growing, and realistically was a predictable outcome of RoHS and WEEE. Product processed in a machine that has not been set up correctly not only runs the risk of having the wrong materials, but worse, because materials often require different parameters in equipment, could create undetectable but unreliable assemblies. The consequences then could be much bigger than not meeting the letter of regulatory standards; they could mean unsafe products in the field. Considering how many gifts this holiday season will have been built in factories running differing chemistries, that's a scary thought.

Joe Belmonte is project manager, advanced process development, at Speedline Technologies (; His column appears bimonthly.

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Title Annotation:Screen Printing
Author:Belmonte, Joe
Publication:Circuits Assembly
Date:Dec 1, 2006
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