The Dissimilar Self-Alignment Characteristics of Smaller Passive Components in the Length and Width Directions

Abstract

Motivation: As passive components’ size gets smaller, quality rejects due to overhang and misalignment after the reflow appear more frequently. This situation is partly because the pass-fail criterion is set based on the offset concerning the component dimensions. Therefore, understanding the self-alignment characteristics of electronic components becomes very critical for surface-mount assembly yield. This research investigates the dissimilarity of self-alignment in the length and width directions.
 Approach: To avoid the argument of sample to sample variations, data are collected from 81 printed circuit boards (PCB) and 182,250 assembled components. Within a PCB, 25 different solder paste printing offset locations and 81 component placement offset settings are implemented. Component-placement positions before and after the reflow are monitored. The results are compared to identify different component sizes’ self-alignment characteristics in the length and width directions.
 Key findings: The misalignment of smaller passive components, e.g., R0402M(0.40 mm × 0.20 mm), is worse than the larger component under the identical solder paste printing and component placement conditions. Furthermore, the self-alignment characteristic in the length direction of these passive components, e.g., R0402M, to R1005M (1.00 mm × 0.50 mm) is superior to that of width direction. The observations are not consistent with the results found in earlier research that reported on larger components, e.g., C0402M(0.40 mm × 0.20 mm), to C3216M(3.20 mm × 1.50 mm).

A pick-and-place process control based on the bootstrapping method for quality enhancement in surface mount technology

“…However, it can also enhance the assembly quality by helping misaligned components to be located near the pad center [9]. Therefore, understanding the self-alignment characteristics of small components during the reflow process has become critical for achieving high yield in SMT [8]. However, understanding self-alignment characteristics is challenging since no distinct patterns or particular distributions exist, as shown in Fig 4. Even though it is a challenging task, it is a significant physics phenomenon that must be considered to reduce small components' misalignment.…”

“…As passive components' size gets smaller, quality rejects due to overhangs after the reflow appear more frequently. This is because the dimensional pass-fail criterion is set based on the amount of component misalignment regarding the size of the component and pad [8]. APC is a method of placement control that collects solder location data from SPI and transmits this data to the mounters.…”

“…PB places components at the midpoint between solder paste and pads in the length direction if the solder paste offsets are greater than 20 µm. This rule was developed based on an experiment, which showed a strong selfalignment effect in the length direction compared to the width [8]. Since PB places components between the solder paste and pad center, PB results in a smaller average and standard deviation in misalignment compared to APC and PP.…”

“…To enhance the assembly quality by decreasing the final component misalignment after soldering, it is important to understand the characteristics of selfalignment. Fig15 (a) and (b) show the length and width self-alignment distributions of the DOE dataset in[8]. Components tend to move toward the solder paste during the reflow process.…”