Placement repeatability study of two flip chip die attach machines: leadscrew vs. linear motor

In the development of fine pitch flip chip technology, the die attach placement is considered as one of the most critical steps, since it dictates the assembly yield of the fine pitch process. The purpose of the study is to evaluate the placement repeatability of two flip chip die placement machines: one using leadscrew, the other using a linear motor. The study was conducted using a clear quartz die screen-printed with solder bumps. The quartz die were flipped, and placed onto a substrate. The die was firmly secured on the substrate using a transparent double-sided tape. The bumps near the four corners were measured using a high accuracy, automatic focusing optical microscope, and misplacements were calculated. By plotting X and Y misplacements of the four designated bumps on the quartz die, the rotational and translational placement errors were decomposed, and independently obtained. There are four spindles in each machine. By comparing the errors with the placement specifications, the placement capability was obtained. It was found that Machine B (linear motor) was more capable than Machine A (leadscrew), for all four spindles in both translational and rotational movements. Machine B was capable of delivering a 6-sigma process for a pitch of 225 μm, with ±45 μm specification limits. However, it was not quite capable of delivering a 6-sigma process for the fine pitch (150 μm, with a ±30 μm specification limits). It was also found that the reflected light from the spindle heads affected significantly the placement accuracy, because the light reflection influenced the effectiveness of the machine vision. Since the error analysis was relatively easy to perform, the error handling technique was used to optimize the die placement accuracy by altering the die placement sequence