SMTA International Conference Proceedings


Authors: Tom Borkes and Lawrence Groves
Company: Kurt Whitlock Associates, and Dynatech Samsung
Date Published: 9/24/2006   Conference: SMTA International

Abstract: An experiment was designed to investigate the process characteristics associated with reflow soldering 01005 component packages. The experiment’s objective was to establish the significant process variable causal relationships and was not intended to be a strict statistical treatment of the variables. For solder application, Sn/Pb eutectic solder paste sphere size and solder stencil thickness were selected as independent variables. A systematic, progressive component offset from the nominal placement locations was used to determine process sensitivity to placement variation. The results of the experiment demonstrated the following:

1. Printing a sufficient volume of solder paste and having sufficient paste pad coverage are critical both to the placement process (i.e., successfully achieving and maintaining component location during placement by providing the component with a surface on which to adhere) and for proper solder joint formation.

2. A positional placement accuracy of 0.002 in. is the placement accuracy neighborhood needed for low ppm defect rates.

3. A 0.002 in. thick electro-formed stencil provided good results with either type 3 or type 4 paste sphere size.

4. A 0.004 in. thick laser cut stencil provided acceptable results only with paste made with type 4 powder.

5. Rapid heating during reflow is important to prevent solder sphere oxidation.

As found with 0201 components1, the time/temperature reflow ramp rate emerged as the process parameter with a narrow process window. This result is attributed to the tendency of the small volumes of printed solder paste for 0201 and 01005 components to oxidize more quickly when subjected to elevated oven air temperatures than those printed for larger components. This oxidation interferes with the ability of the Sn/Pb solder spheres to properly melt when the alloy’s liquidus temperature is reached on the board. An oven profile that results in a straight, rapid temperature ramp rate of close to 3.0 deg. C per sec. was found to yield good reflow soldering characteristics. These results suggest that reflowing in an inert environment (< 10 ppm O2) would widen the reflow process window.

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