I finished re-updating the bonding diagrams of SOT753P package. So, till today, I had re-updated SOT353P, SOT363P and SOT753P package, with one package having approximately 110 devices. An inexorable sense of achievement gushed up upon me. Next, I was given a new package, SC74 to update the bonding diagrams. I duly obliged and finished the task within considerably short period of time. I finished off the day with the ever-intriguing ball shear measurement. This time around, I had to perform it alone without the supervision of seniors or operators. It was challenging and fun. I put my flippancy aside and began to acquaint myself with the machine. Owing to my inexperience and slipshod nature, some dies were wrecked; some ball bonds were flattened and so forth. It would be preposterous to suggest that I was not traumatized. Fortuitously, the “debacle” that I had instigated was indiscernible.
27 April 2010
I was given the fifth package to update the bonding diagrams, SOT457P and was able to finish it within today. Subsequently, I was brought to the line where I gained knowledge on several things. Firstly, cross sectional view tests are performed on molded products or finished products to check for any copper interconnect and improper die attachment. Probable causes of imperfect die attachment are excessive bond force and exceedingly high temperature. One example of flawed die attachment is tilted die. Tilted die, along with lifted die or similar defects will culminate in the product be labeled as Rth units. These units will have high resistance, due to the voids between the die and the lead frame. On the other hand, copper interconnect can be reckoned as imperfect diffusion taken place between the die back metal and the lead frame substrate. Lead frames are made from copper. Under copper interconnect conditions; the back metal has copper particles diffused into it.
Next, Miss Manggala provided me with some SEM images which were done using devices far more superior than those powerful microscopes that I had used before. The SEM images were extremely clear. Besides that, high magnification can be attained.
I learned a new term today, by the name, collet. Collet is the protruding edge of a die bonding machine used to pick up individual die from a wafer. If the collet is filthy, the dies may be picked up in undesirable orientation or position, which may result in tilted dies. Hence, it must be cleaned regularly using propanol.
There are myriads of aspects that can engender defects, such as imperfections to the die, chipped die, cracked die; imperfections to the wire, offbond, exposed lead and so forth. Therefore, we need delayering analysis, where one can inspect and determine the root cause of failed products.
Lastly for today, some dies are large, which may pose inextricable difficulties in adhering them well to their respective bondpads. Hence, WBC or Wafer Back Coat is used to assuage this predicament. This is one type of glue bonding.
28 April 2010
I started off the day performing and finishing both ball shear measurement and wafer inspection. As I had mentioned in previous blog entries, a wafer may consists of few hundred thousands of dies. There is a possibility some dies are chipped, cracked or physically imperfect. That is the purpose behind me performing the wafer inspection.
Next, I was asked by Miss Manggala to have a roam around the assembly line. A casual stroll in the assembly line provided me with opportunities to gain some valuable knowledge. First off is the sporadic green light shining on lead frames in ADAT die bonding machines. By the way, ADAT is the acronym for Advanced Die Attach Technology. The occasional green light is meant as a visual or lighting aid for PRS, which is Pattern Recognition System. This system is designated for the ADAT machine to recognize good as well as bad dies. The picture below illustrates the PRS concept.
I was also introduced to Pepperpot by Mr Leong, an industrious engineer working along the line. Pepperpot is actually the vacuum head of the ADAT machine, which is used to ensure that the mylar does not stick along with the die when the die is picked up by the collet. It does so by adhering the mylar to the vacuum head.
Besides that, I gained a deeper understanding why BSOB is momentous in double dies wire bonding. Again, BSOB means Ball Stitched On Ball. Usually, in wire bonding, a ball is bonded on the die (ball bonding), while a wedge is bonded on the leadframe (wedge bonding). When double dies are involved, BSOB is implemented. One of the dies will be bonded first with a ball, then with a wedge on top of that ball. This combination is called a bump and acts as a cushion that protects the die from the high force entailed during wedge bonding.
Subsequently, Mr Leong explained the purpose of having two rectangular boxes just after several ADAT bonding machine. These boxes serve as ovens to dry or solidify the glue used for die bonding. Before I go into details of the boxes, let me deviate to the available types of die bonding. Die bonding process can be categorized into eutectic process, epoxy process and soft solder process. In NXP Semiconductor, soft solder process is not used. The discrepancies between eutectic and epoxy process are that eutectic process entails melting and solidifying two components (alloy), forming tri-alloy, such as AuSiAg (Gold Silicon and Silver) and AuGeAg (Gold, Germanium and Silver).
Epoxy process or glue bonding, however, involves the use of adhesive resin. Furthermore, epoxy or glue process is relatively cheaper than eutectic process. In addition, Epoxy process is conducted at a comparatively lower temperature than eutectic process. Now, back to the ovens; So, with the die bonding information in mind, it is equitable that only epoxy process will be needing ovens. If the glue is not dried, the die may budge or move around.
A pictorial representation of the types of die bonding process:
Besides the boxes, the discerning eyes of me caught sight of the changing colours of lead frames in different part of the process. This is due to the oxidation process. Apart from that, the relatively slow speed of some ADAT machines caught my attention too. After some cogent explanation from Mr Leong, I was able to know that these machines are producing power products which use dies with larger size. The low speed is indispensable to ensure that sufficient amount of time is present for the die attachment to take place properly.
I had also gained further understanding of die bonding process. There are 5 prominent issues with die bonding:
1) Must not transmit destructive stress to fragile chip
2) Must make intimate contact between chip and substrate with no voids and adhere well to both
3) Able to withstand high temperature
4) Must have good thermal conductivity: to remove heat generated within chip
5) Must be a good conductor or insulator, depending on application
I was also told by Miss Manggala that there are 3 parameters that are significant in die bonding process, which are temperature, bond force and process delay. Generally, die bonding process can be subdivided into 4 modules, die select, die pick up, die transfer and die attach.
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