Authored By:
Denis Barbini, Ph.D.
Universal Instruments Corporation
Advanced Process Laboratory
Conklin, NY, USA
Summary
To address the requirements of component miniaturization, high-density board designs and ever-increasing throughput and yield, solder paste technology is evolving. Not only must next-generation solder materials offer meaningful improvements in process performance, but also provide more manufacturing flexibility by expanding the process window. The ability to effectively accommodate manufacturing environments where assembly processes are interrupted, where the luxury of long start-up times are nonexistent and where materials are often handled nonoptimally, materials capabilities in the modern age of manufacturing are critical.
Results from an investigation that studied and characterized a novel lead-free solder paste as compared to traditional solder materials will be presented. As basis for the analysis, solder materials were exposed to harsh environments to understand if the new solder paste technology is capable of withstanding the realities of modern manufacturing processes for fine pitch components (0.3 mm CSPs and 01005/0201 passives). Evaluation of solder paste printing performance was a primary focus of the investigation, taking into consideration numerous common board finishes and stencil aperture designs. Paste volume measurements acquired by SPI were used to verify solder paste volume on pads to quantify performance. Challenging manufacturing processes were simulated by aging the pastes at room versus elevated temperatures and then printing at defined time intervals. Simulated extended continuous printing was also examined. A detailed statistical analysis identifies the relationship between the condition of the solder paste and the paste volume on the pad of a given component type.
Conclusions
Although significant differences in performance were observed across the different pastes, a number of common trends were documented. As aperture size increases, the deposit volume distributions typically narrow (Cp increases), eventually stabilizing for component apertures of 0402 and above. All pastes exhibited a reduction in process capability (Cpk decreases) with increasing aperture size due to a shift in the center of the deposit volume distribution to above the 100% target. This is a result of the hydrostatic characteristics of solder pastes and yields deposits that are higher than the thickness of the stencil. The extra paste may also contribute to mid-chip solder balling. Paste A consistently yields the greatest Cp and Cpk values when compared to the other pastes.
For apertures with Aspect Ratios less than the IPC 7525B guideline of 0.66, only Paste A was able to yield sufficient deposit volumes to produce a viable process, even down to 01005 apertures which have an Aspect Ratio of 0.46. Aperture shape was found to be significant, as square apertures with rounded corners and higher aspect ratios performed more poorly than the 01005 rectangular apertures.
All pastes required an initial print after one hour of inactivity, and multiple prints after eight hours to return the average print volumes experienced on the initial prints. Cp and Cpk values for paste A also returned to similar levels as those achieved for the initial prints.
Initially Published in the SMTA Proceedings
|
