An alternative method to verify the quality of equipment grounding

There has been an increasing trend on the use of Automated Handling Equipment in the semiconductor as well as electronics industry. At some point in the production processes, a device, module, circuit boards or a sub-assembly will undergo one or more types of automated handling which involves contact and separation (potential risk of charging up by tribo-charging). Machine components especially those robotic arms or moving parts of the machine which are separated from the chassis (main machine body) by bearings of any kind should be grounded properly to prevent tribo-charging or better to guarantee a proper dissipation of any generated charge. Conventionally, a good grounding is always associated with Resistance to ground (Rtg) measurement. The only available ESD Association reference which exist as STANDARD PRACTICE on Automated Handling Equipment (ESD SP 10.1-2000) does not specify a clear Rtg limit too. This Standard Practice document indicates that all machine components should be grounded in a manner that will provide a constant ground path (1 times 10⁶ ohm or less) regardless of rotary or transitional rate. It is specified under Appendix C (suggested equipment grounding guidelines). It should be pointed out that this Standard Practice was released in the year 2000. Prior to this, there was no National or International References on equipment grounding at all. What is the resistance limit which can be considered as good and acceptable grounding for moving parts of the machine was really not clear. Actual monitoring conducted on semiconductor packaging equipment for example wire bonders where the metallic bond tool are in direct contact with the bonding pads of IC chips showed Rtg value ranging from about 1 times 10³ ohm to 1 times 10¹⁰ ohm. As evident, there was a huge variation in the Rtg values. What is the reasonable and acceptable Rtg limit that should be implemented on the production line for equipment grounding was a major- concern. AC voltage measurement as an alternative method to verify the quality of equipment grounding was investigated. The results of the study were promising and the AC voltage measurement was implemented in the manufacturing facilities world wide within the company for verification of equipment grounding. The failure limit used during initial implementation was 0.5 V_AC. After further investigation and detail study, we came to the conclusion that the limit of 0.5 V_AC can be further tighten to 50 mV_AC.