In the semiconductor manufacturing industry, damage and yield losses attributed to the effects of static charges are well documented along with the determination of many of the specific causes.1 If ESD controls are not implemented properly, ICs processed by handling equipment can be subjected to Charged Device Model (CDM), Field Induced Model (FIM), and Machine Model (MM) ESD failure modes. Here are the most serious failure modes we found with such equipment—ones that consequently have resulted in the largest amount of documented ESD damage.
Many different manufacturing operations use IC handling equipment. Automatic and semiautomatic equipment that can cause ESD include all of the following operations: singulation, plating, lead inspection, lead forming, ink marking, laser marking, many types of sorting with pick-and-place capabilities, electrical and optical device testing, and several types of final packaging such as placing into trays, tape-and-reel, tubes, or metal rails.
Test Sockets
Test sockets in IC handlers are commonplace throughout the industry. Two separate and typical failure modes have been verified to cause device damage. First, many of these sockets can charge dramatically during movement or friction in automatic processes, during heating and cooling functions, and when contacted by operating personnel.
When the sockets have become charged, their relatively large surface areas produce fields that can cause inductive charging of the parts about to be inserted into them. The charged parts then can be discharged upon contact with the socket pins, resulting in an FIM failure mode.
If the socket becomes highly charged, the resulting field can cause inductive charging of the PCB wiring on the board. A discharge then can occur from the board to the device as it enters the socket. This second potential damage mode actually is a mini MM failure mode. Bathing the sockets continually in ionized air during the machine operations usually eliminates FIM and MM failure modes.
Pick-and-Place Collets
By far, the highest yield losses caused by IC handlers have occurred at test sockets and during pick-and-place operations via collet problems. Many IC handlers have mechanical structures that move devices from one place to another in the machine via vacuum pickup collets. Most machine designs in the marketplace have well-grounded metal collets for this purpose.
However, the collets can be supplied to the end user with a wide variety of suction-cup boots on the tips that can create ESD problems. Specifically, the suction-cup boots usually found on this OEM equipment comprise insulative, conductive, and static-dissipative plastic/rubber materials.
Insulative
Regular plastic boots commonly shipped with IC handler collets can charge routinely to >10 kV. Many times, the color of an insulative boot is black, which is identical to conductive boots. Be careful to not make selections based strictly on appearance.
Insulative boots are notorious for causing sizeable yield-loss problems. First, when the insulative boot slides across the body of the IC during pickup and drop-off, it can charge that surface. The device leads can become charged inductively from the charge on the body of the IC, from the insulative boot itself, or both.
Typically, devices then can be dangerously discharged when they reach their target container or tray in the inner workings of the handler. In our experience, insulative boots clearly have caused the most amount of damage. In many case studies, yield improvements were realized when conductive boots replaced insulative boots.
Conductive
As the conductive plastic/rubber boots mate electrically with the grounded metal collets, no charge can be found at any time on the boot, which is good. Although being conductive does not ensure that it will not charge the IC’s insulative body during contact since grounded conductors can cause charging problems on insulators2, we historically have found very low charging onto the devices as a result of contact with the vast majority of the commercially available conductive boots.
However, in a few studies, damage has occurred from the chip being charged from other causes as it entered the pickup area and then was discharged by the conductive boot. In these cases, yield losses involved mostly bare chip handling applications, not molded, finished IC devices.
Static Dissipative
It would seem, then, that static-dissipative boots would be the ideal material for this application. However, we found that not to be the case. Although the dissipative nature prevents the quick discharges to bare chips as opposed to conductive materials, quite a number of the static-dissipative 108 to 9 ?/sq boots tended to charge the IC body quite highly during contact.
The charging certainly was dramatically higher than most conductive boots, which was an unexpected finding. In fact, we have participated in studies at facilities where insulative boots were replaced with conductive boots and yields went up. Then the conductive boots were replaced by static-dissipative boots, and the yields went back down.
As a bottom line, we recommend using the conductive boots and bathing the pickup and drop-off areas in ionized air at all times. This combination has been more effective and produced the highest yield improvements than any other combination in our studies.
We also have observed more than a few cases where storage bins for the replacement boots had been unknowingly filled with both conductive and insulative types. Visually, it was impossible to distinguish the difference between the two. Yield losses were coming and going—from machine to machine—in a totally random manor.
Imagine the difficulty for the beleaguered quality personnel in those facilities in trying to make sense of those failures. Obviously, all boots should be measured and stored carefully.
Lead Cutters/Formers
Many times we have seen CDM IC damage occur right at the point where leads of the device are either cut to size or formed in some way. A charged device can be dangerously discharged during these operations.
We recommend two implementations. First, make the cutters or lead formers out of static-dissipative materials to slow down quick discharges. Secondly, ensure that the part is uncharged, typically via ionization, as it reaches the contact points at this operation.
Metal Chutes
Many people assume that grounded metal chutes and input/output packaging rails that provide passageways for ICs eliminate any possible ESD effects. Unfortunately, that is not the case. ICs with plastic or ceramic bodies can charge greatly sliding along grounded metal surfaces. We have observed charging on ICs in the thousands of volts on occasion as they travel down chutes and into rails in handlers.
A very common failure mode in many machines follows this scenario: The device’s plastic body becomes charged by friction as it slides on its grounded metal chute. That, in turn, charges up the floating circuit leads via induction. Test sockets or lead cutters then discharge the charged device lead frame dangerously.
It is important to determine the charging on each particular device type that is handled by a machine as the overall charging can be drastically different from device to device. To be safe, ionization should be used to remove charge from the device before it reaches the discharge mechanisms.
Input Bowls for Discrete Devices
Some IC handlers have metal bowls at the input that accept discrete components in bulk fashion. A vibrating mechanism aligns the parts and sends them in single-file fashion into the input chute of the machine. Devices can charge significantly when vibrating in these bowls, leading to potential CDM damage in the bowl itself or later in the machine. Ionizers should be positioned above these bowls to bathe the parts constantly during this process.
Plastic Guards
On many IC handlers, especially those that have long chutes for devices to travel, plastic guards cover the points at which moving equipment, collets, or test sockets contact the devices. If this guard is high in charge generation, FIM ESD damage can result as the devices charge inductively.
The majority of these guards originally are supplied from the OEM with noncharge-generating plastics. However, even these ESD-safe materials can age and deteriorate over time. We have witnessed some yield-loss situations directly attributable to these guards becoming charge-generating after a few years of service. We encourage facilities to include checks of these guards in their routine internal ESD audits.
Packaging/Handling Materials
Device-charging issues also have been documented with a number of types of handling materials, and we include them here for completeness.
Devices can charge significantly when sliding in metal canisters in bulk form before placement into input bowls of IC handlers that dangerously discharge the parts. Again, metal canisters can and do charge devices sliding around inside of them. In addition, the devices can become charged by sliding on each other. This is not the best ESD container technique.
Caution also is advised with many of the ESD-safe trays commonly used in IC handlers. For the most part, the static-dissipative and conductive plastic trays are fine from a noncharging perspective. However, many of the trays with openings do not provide Faraday shielding protection. Parts can become charged during transportation and storage, even when in the center of a number of trays stacked together.
This also is the case for some of the black conductive types that people assume must be good shielding containers, especially when trays are stacked. Unless testing is done to verify good shielding performance, we highly recommend that these trays be transported and stored in additional Faraday shielding containers such as static shielding bags or conductive totes with lids.
Clear, antistatic IC tubes also can lose their properties over time and cause the parts sliding inside of them to become charged. We have observed many cases of IC handlers that reuse the same tubes for long periods of time. We recommend adding the IC tube tests to the facility’s routine ESD internal checks.
References
1. Peirce, R.J., “CDM ESD Failure Analysis,” Solid State Technology, May 2007.
2. Peirce, R.J. and Zufelt, C., “Limitations of ESD Gloves and Finger Cots,” Surface Mount Technology, February 2007.
About the Authors
Roger J. Peirce is director of technical services for Simco Ionization for Electronics Manufacture. Before joining Simco, Mr. Peirce provided ESD consulting services for 20 years in more than 2,000 semiconductor and electronics manufacturing facilities for ESD Technical Services, a consulting company he founded in 1986. He also co-founded Voyager Technologies in 1983 and started his 13-year career at Bell Labs in 1970. 215-997-3430, e-mail: [email protected]
Bradley R. Williford is global semiconductor OEM accounts manager for Simco’s semiconductor ionization products with more than 10 years of experience in the semiconductor capital equipment and materials markets. Previously, he served as an account manager for Semitool and as a region technical marketing manager for Asahi Glass Electronic Materials. Mr. Williford has a B.S. in mechanical engineering and an M.B.A. from Virginia Tech. 919-567-0145, e-mail: [email protected]
Simco, Electronics Division an ITW Company, 2257 N. Penn Rd., Hatfield, PA 18951
December 2009