Every company involved in manufacturing static-sensitive components has horror stories to tell about problems with ESD. Even a large company like Motorola is not different when it comes to battling the various problems that ESD can cause. At Motorola, most of the problems are addressed by individual departments that assign a person to investigate, identify, and resolve the problems.
I was chosen to resolve issues associated with ESD in my manufacturing area. The appointment came after a serious problem arose while trying to manufacture a military version of the MC6820 microprocessor.
Although we already had installed several safeguards to protect our product from ESD, this new product was much more susceptible to static charge. Previously, our threshold for static discharge on products was greater than 1,000 V. The MC6820 has a threshold of 200 V. Here are some of the problems we encountered with the very sensitive microprocessor and some solutions we implemented.
Let’s start from the floor and work our way up. Since a very large amount of our manufacturing area used conductive flooring and some of the personnel needed to wear heel straps to properly perform the job functions, the floor seemed like the logical place to start. I used a surface resistivity meter with two 5-lb weights separated by approximately 3 ft to make measurements in approximately 15 locations.
The first thing noticed was inconsistency in the readings. They varied anywhere from 106 W to 1012 W. This was caused by a wax buildup on the floor. The old wax and dirt were not being removed before new wax was applied; consequently, wax built up on the floor.
We resolved this issue by instituting a cleaning program in which all the wax was removed. Then the floors were cleaned daily and buffed one time each week. Once the use of wax was eliminated, the readings throughout the manufacturing area were 106 W.
The only other issue we had for this area was the operators’ heel straps—they were not being properly cleaned and worn. This problem was solved by training users about the proper method for attaching, and removing, and cleaning the heel straps once a day to remove dirt buildup.
Next, we concentrated on the worksurfaces where the product would reside in open containers. Again using the surface resistivity meter, I placed the two 5-lb weights 1.5 ft apart and made four measurements on each tabletop. Some of these measurements were so high (>1013 W) that my meter wouldn’t register the results.
Similar to the floor, I was getting different readings from one table to another. This was caused by dirt buildup on the tabletops.
Another problem was the condition of some of the soft, static-dissipative mats which had become torn over time. The damaged mats were replaced, and the staff was given an antistatic topical solution to clean the tabletops weekly. We checked all the tabletops after they were cleaned, and all the readings were between 107 W and 109 W.
Some worksurfaces were cluttered with paperwork that was less than 12 in. from the product. Everyone was trained to keep static-generative materials away from the product.
The equipment seemed like the next logical thing to concentrate our efforts on. Since we used all-metal conductive carts with chains that drag along the conductive flooring, we decided to investigate how effective this method was.
Using an ohmmeter, I connected one lead of the meter to the cart and the other to the conductive floor. The readings were close to 0 W, which is what you would expect with a totally conductive cart. Then I began to roll the cart a short distance and noticed a big problem. The meter jumped from 0 W to 1012 W. We simply didn’t have a good connection between the floor and the cart.
Because the chain has links, it was very difficult to maintain a good connection. To improve contact, we replaced the drag chains with a wire cable and added a weight to the end touching the floor. This provided the proper connection to the floor.
The handlers in the final test area were checked for continuity to ground from where the product entered the machine to where it exited. One type of handler we used required a bowl-feeding method for the product. This method was used so bulk product could be dumped into the bowl and, with the use of vibration, the parts would work their way around the bowl and into the temperature chamber of the handler.
The bowl was set on four rubber pedestals to get the required effect of the vibrator. But doing so isolated the bowl from the handler’s electrical ground. Since no connection existed, it measured an open circuit. This problem was resolved by installing a ground cable from the metal bowl to the chassis of the handler.
Ionization is a unique and important part of controlling ESD. Even with clean, grounded tabletops and operators’ wrist straps connected to ground, sometimes a static charge still can be created.
The carriers used for some of our products required a Velcro strap to keep the product trays together. When separating the Velcro, a static charge of approximately 5,000 V was measured using a static field meter.
The only way to neutralize the charge was to place a table ionizer 12 to 24 in. from the trays while the Velcro was being removed. The voltage measured after ionization was installed was less than 20 V.
Another application where we were required to use a table ionizer involved a piece of equipment that mounted the silicon wafer onto tape before cutting each die for package assembly. The machine placed the tape on a metal ring, and then the wafer was placed on the tape. Using a static field meter, 20,000 V were measured on the tape as it was placed on the metal ring. After the ionizer neutralized the charge, we measured only 20 V on the tape.
These are only a few of the problems encountered when we were developing a better ESD program. There are many factors involved in protecting product from this silent killer. Look at everything, but you must attack ESD in strides since it can be a very large task.
ESD didn’t happen overnight, and it won’t be controlled quickly. But if you work at it, you will control the problems over time.
About the Author
Michael R. Hoogstra works in the New Products Development Group for Motorola’s Semiconductor Components Group. He has been employed at Motorola for 17 years. Mr. Hoogstra received a degree in electrical and electronics technology in 1981. Motorola Semiconductor Components Group, Technology and New Product Development, 2100 E. Elliot Rd., Tempe, AZ 85284, (602) 413-3957.
Copyright 1998 Nelson Publishing Inc.