10 May 2010

I started off the day by gaining cognizance about the term “partial wafer”. Partial wafer is exactly the same as ordinary wafer except that only a certain amount of dies from a partial wafer will be used or picked up during production. That amount will be dependent upon customers’ demands. After a designated amount of dies had been picked up, that wafer will be kept for further use.

Besides that, I went to the front end where Mr. Leong is stationed. He was monitoring the production of a new product. An ineluctable predicament surfaced in the form of the absence of numbering of die pick up starting position. To rub salt on wound, some of the dies in propinquity of the edges were not entirely defective. This will pose some impediment to the effectiveness of the Pattern Recognition System (PRS) in identifying good (usable) dies. After numerous attempts, the indefatigable spirit of Mr. Leong prevailed as he successfully estimated a proper starting position by trial and error.

Mr. Leong also taught me that the Pattern Recognition System (PRS) of ADAT2 die bonding machine works on the principle of binary scale. In other words, it discerns patterns based on black and white images of the die. On the other hand, PRS of ADAT3 machine not only works on binary scale, but also grayscale. This renders the PRS of ADAT3 machine significantly efficacious than ADAT2 machine. Nonetheless, higher cost of ADAT3 machine prevents its widespread use.

My learning experience continued after lunch break. After the front end process, the die bonded as well as wire bonded lead frames will be channeled to the mid-end process. Before that transfer takes place, unusual lead frames will be identified using stickers with different colours to inform personnel in mid end. Firstly, red coloured sticker indicates that certain part of the lead frames contain new device or new lot. Within front end itself, lead frames having new device will be noted with pen marking on lead frames to avert or assuage mixed device possibility. Next, green coloured sticker is used when a portion of the lead frames is unwanted scrap of production. Lastly, blue coloured sticker is used to represent lead frames that had been welded. Welded parts may be uneven and therefore it is not advisable to mold them.

During a casual stroll around front end, I came across several customer complaints. A customer from Korea complained about partial lifted ball bond. The cause of this is believed to be floating of lead frames due to copper build-up at heater block. During wire bonding process, as the capillary exerts lateral force on the ball at the bondpad, the force will not be spread evenly. Next, a Dutch company complained about offset ball bond, which means that the balls bonded on dies are out of designated positions. Thorough investigation reveals that the probable cause is the failure during Pattern Recognition System (PRS) after lamp burnt (dark field) incident.

Circular pulley-like parts are ubiquitous between each machine. According to Mr. Leong, this is meant to provide operators with more time to troubleshoot and amend errors in case of machine breakdown. During that time, other functional, parallel machines can continue production using inputs stored or winded on the pulleys.