Analyzing Pb-Free Defects Using Partial DoE Print E-mail
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Written by Ursula Marquez de Tino   
Sunday, 30 November 2008 19:00

Optimal assemblies don’t come about from drop-in materials replacement.

Wave SolderingAn important step in reducing and preventing defects during soldering is defining the correct process settings. It’s a good practice to design proper experiments and analyze their results to help determine these settings. When the number of test boards and components for experimentation are limited, use of Partial Factorial, Taguchi or Response Surface Experiments makes for a good approach. These techniques permit end-users to understand the process by minimizing time, resources and cost.

The objective of these experiments is to determine the factors and their levels that minimize the variation of a product around a target response. The champion settings will result in robust products resistant to change in operational and environmental conditions. The experiment begins with a brainstorming session where the problem is stated; the factors are selected; the measurable output is chosen, and the experimental design is selected. Then the experiment is run and output measured. Data are analyzed and the best settings are identified. It is important to schedule confirmation runs based on the champion settings; if these results are good, the settings can be implemented into production.

To illustrate the concept, a Taguchi experiment was used to develop a robust process for Pb-free wave soldering. The control factors selected were solder temperature, contact time, preheat temperature and flux amount. For practical reasons, there were no “noise” elements in this experiment. The output characteristic was the number of pins without bridges (maximum 200 pins/board) and through-hole penetration (2 = 100%, 1 = partially wetted, 0 = not wetted). This experiment used an L9 orthogonal array design, which means nine experimental runs with four factors and three levels investigated (Table 1).

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Materials used for this experiment were SnAg3.8Cu0.7Sb0.25% alloy with a VOC-free flux (<2% solids). The test board was double-sided (160 x 100 x 1.6 mm) with plated through-holes and Cu OSP Entek Plus surface finish. A Delta wave soldering machine equipped with a spray nozzle fluxer, three bottom side preheaters, and nitrogen was used.

Eighteen boards were run (nine runs with one repetition). The results for bridging and through-hole penetration are shown in Figures 1 and 2, respectively. In both analyses, higher scores meant better results. (Table 1 also shows codes used in the graphics.)

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For bridging, contact time and preheat temperature were more important, while for through-hole penetration, preheat temperature was the main contributor. The optimal settings are not the same for bridging and through-hole penetration. Therefore, some sacrifices must be made. For our analysis, through-hole penetration was more important.

The impact of solder temperature was minor relative to its effect on bridging, while for through-hole penetration a higher solder temperature was preferred (275°C). However, this option could potentially damage components during soldering. Therefore, a solder pot temperature between 265° and 270°C was chosen. In addition, shorter contact time yielded better results. This might be attributable to the characteristics of the flux, which were compromised at the higher preheat and solder pot settings. The champion setting was 1.8 sec.

In terms of preheating, a setting of 110°C was the champion for this process. Calrod IR elements were used in the first heating zone, which provided the adequate heat to start the flux activity without forcing the water to boil out. Forced convection heaters were used in the second and third zones to eliminate excess water before entering the solder wave and to inhibit solder balling.

For the flux, a wet flux amount of 474 mg/dm2 was the champion setting. A continuous and uniform spray across the board is essential. Higher settings may cause a droplet bouncing effect rather than improved wetting of the board surface. All these settings were validated with confirmation runs. Reduced bridging and good through-hole penetration were achieved.

Optimal Pb-free assemblies are not a simple replacement of fluxes and materials. This review can be done in a proper and fast way by conducting Partial Factorial, Taguchi or Response Surface experiments. The outcome of the experimentation will provide a practical understanding of important factors and settings by using just a small number of test boards, reducing cost and minimizing resource use.

Ursula Marquez de Tino is a process and research engineer at Vitronics Soltec, based in the Unovis SMT Lab (vitronics-soltec.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

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