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RoHS implementation: an OEM and EMS case study: the process by which a Pb-free line was evaluated is detailed.

Ed.: For the full column, please see circuitsassembly.com/cms/content/view/2579/

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Each OEM/EMS team approach to Pb-free conversion will be different. These differences can be generated by limiting factors such as company resources, RoHS budget, influence within the supply chain and logistics. Due to the smaller size of each company in this Pb-free case study, these limiting factors affected how roadmaps, testing and implementation plans were developed.

This article delineates the process by which a component designer and EMS firm worked together toward RoHS compliance. The companies were Rabbit Semiconductor (formerly Z-World), a fabless semiconductor company, and SEP Productionworks Ltd., a PCB supplier and contract manufacturer.

Integral to the success of converting to Pb-free/RoHS was the inclusion of our contract manufacturer on the team from the start. Being able to share knowledge and prepare for qualification testing made the process easier. The advantage to working with an EMS is that a Pb-free/RoHS process is qualified and a source for producing Pb-free/RoHS product by July 2006 is defined. For the EMS, it is an opportunity to gain market share in the Pb-free/RoHS assembly business early in the industry transition.

Since SEPP is based in Shenzhen, preliminary setup and process parameters were set by the engineering staff at SEPP to establish a process before onsite test runs were performed. Procurement of RoHS parts was planned in advance to ensure timely delivery and to maximize the potential of having a 100% Pb-free BoM. Working with the paste supplier, SEPP ran tests on various modified profiles and stencils for a SAC305 paste. The 10-zone reflow profile was set up with one soak period and had a peak reflow temperature of 248[degrees]C with 70 sec. above liquidous. A 0.005" stencil was used and the aperture design was modified for 105% in the width and 100% in the length.

A four-day time frame was planned to perform the testing. To ensure complete separation of Pb-free and leaded processing, the Pb-free process is on a separate floor in the manufacturing plant. This aided in controlling potential logistical issues of mixing the two processes. The test plan evaluated 280 assemblies made up of two products--high and low Tg RoHS compliant laminate--and used >95% RoHS compliant components. Five components were not Pb-free/RoHS:

* Rabbit chip: 30 Pb-free chips were available for testing and populated on specific assemblies. The balance of the samples used leaded chips.

* Two headers did not meet the 260[degrees]C temperature spec.

* One resistor.

* One TSOP.

Although the headers did not meet the reflow temperature specification, they did not show signs of melting or deformation. Red and blue soldermask on the PCBs distinguished the high and low Tg RoHS laminate (Figure 2). Testing high and low Tg RoHS laminate was critical due to the characteristics of the test assemblies and the potential for panel warpage. The test vehicle had:

* 0.063" final thickness.

* V-scored 10-up array.

* PCB dimensions: 1.15 X 1.6".

* Double-sided SMT.

[FIGURE 2 OMITTED]

Overall, the testing was successful. Initial results indicated that the Pb-free process was qualified.

* 97% of assemblies passed functional test. Failures were not a result of Pb-free processing.

* There was no significant difference between high and low Tg laminate for these test vehicles.

* No paste, print or stencil problems were evident.

* No component issues resulted from higher reflow temperatures.

To fully quantify the quality of the Pb-free solder joints, a sampling of the Pb-free assemblies was sent to a third-party laboratory for cross-sectional analysis. Each sample was inspected and photomicrographed. Samples were cross-sectioned, selectively etched and photographed using a scanning electron microscope (SEM). XED analysis was performed on the solder joints to determine composition by weight percent. Table 1 is an example of the analysis results.

To further test the reliability of the Pb-free assemblies, the samples will be subjected to thermocycle and HALT.

In-House Process Qualification

From information gleaned in seminars and conferences, the impression preceding the initial test phase was that Pb-free reflow could not be performed in anything smaller than a seven-zone oven. With two five-zone oven process lines, this was a major concern and needed to be evaluated specifically with our products. In addition, the current process uses water-soluble paste. To reduce process steps and production cost, conversion to a no-clean process was evaluated.

Phase IA testing evaluated the following:

* Five-zone oven without [N.sub.2].

* One profile based on industry recommendation for five-zone oven.

* Three no-clean pastes (two SAC305 pastes and SAC387).

* Test vehicle (10-up panel, double-sided SMT).

* Laminate (high and low Tg RoHS compliant).

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As Pb-free assemblies were built, specific in-process responses were evaluated. These were mainly visual observations including paste consistency, print and placement quality, wetting, solder joint appearance, panel warp and twist, and any component problems (Figure 3).

[FIGURE 3 OMITTED]

After top and bottom SMT were completed, all defects were recorded, a failure rate was determined and each sample was functionally tested. Phase IA testing results indicated significant differences between the pastes with regard to flux residue and solder ball defects. The Tg of the laminate did not have any impact on the test results.

Phase IB testing further evaluated the "best" paste from Phase IA. Prior to sending samples to an independent laboratory for cross-sectional analysis of the joint microstructure, additional reflow profiles were tested. If cross-sectional analysis of Phase IA samples indicated that the solder joints were unacceptable, there would be no indication on which direction to adjust the process.

Based on the profile from Phase IA, the profile was set lower on the maximum temperature by -10[degrees]C and higher by +15[degrees]C. With the five-zone oven, the actual changes in peak temperatures were -6[degrees] and +8[degrees]C from the standard profile.

Samples of each test configuration from Phase IA and IB were sent to an independent laboratory for cross-sectional analysis. Although no significant difference in the average composition of the solder between samples was evident, the lab results indicated a significant difference in the microstructure of the solder joint materials. This was due to the low, standard and high temperature reflow profile and the effects on the rate of cooling. Even with the microstructure difference, all the joints were acceptable as compared to standard leaded joints (Figure 4, online).

Phase II testing evaluated the water-soluble versions of the pastes from Phase IA. The same visual and defect responses were recorded. Although consistency was different among the three paste suppliers, no print or placement issues were observed. The major defect with no-clean was solder balls; with water-soluble paste, it was shorts. The water-soluble pastes rated exactly the opposite of the no-clean. Thus, paste vendor selection would change based on clean vs. no-clean processing. Lab analysis results indicated good intermetallic structure and that all joints were acceptable overall.

Thermocycle and HALT testing will be performed on all samples from each phase of testing to provide more data on reliability and solder joint integrity. Based on the results thus far, a five-zone oven can be used for both clean and no-clean Pb-free processing for this specific product form factor. Additional testing will be performed on optimizing specific pastes and testing of alternate product families which include BGA technology.

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Ed.: This paper was first published at Nepcon South China in August 2005 and is used with permission of the authors.
Table 1. Pb-free Assembly Cross-Section Results

 SP-52 SP-115 SP-03 SP-43

PCB quality Good Good Some Cu Some Cu
 in fibers in fibers

Solder voids Some Some Several Few

Wetting Good Good Good Good

Solder microstructure Small Large Medium Large
 grains grains grains grains

Solder composition SnAgCu SnPb SnAgCu SnPb


Guy Martindale is outsource program manager at Rabbit Semiconductor (rabbit.com); guy_martindale@rabbit.com. H.R. Chai is assembly manager at SEP Productionworks Ltd. (Contact Bob Ellis, director U.S. operations, SEP Productionworks Ltd.; sepusal@sbcglobal.net.)
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Title Annotation:Global Sourcing
Author:Martindale, Guy
Publication:Circuits Assembly
Geographic Code:1USA
Date:Jan 1, 2006
Words:1328
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