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To prevent static-related problems, countermeasures need to be taken by appropriately measuring static. Management of static measurements is also important for maintaining the efficacy of the countermeasures.
 

Static measurement in the electronics industry

 

Types of static measurement equipment

 

Static meter

Measurement principle of static meter

The figure to the left shows the measurement principle of the static sensor used for a static meter. The sensor utilizes electrostatic induction. When the detecting electrode receives the strength of the electrostatic field Eo (which is proportional to the charge potential Vo of the charged object) from the charged object, an induced charge q is generated. When this strength of electrostatic field Eo is changed periodically by using a vibrating electrode, the induced charge q also changes periodically. Displacement current Is resultantly flows from the detecting electrode to the grounding electrode. This current is converted into AC voltage signals Vs by the resistance Rs. The charge potential Vo of the charged object is found from these AC voltage signals Vs. This is how the static sensor works.

 

Features of static meter

• The measurement distance is specified. When the measurement distance is changed, the indicated voltage will also change.
• The measurement area differs among different types of static meters. If measuring an object that is smaller than the specified measurement area, the meter will indicate a value smaller than the actual charge potential. à Use of a static meter geared for small objects is necessary.

 

 

• The electrostatic capacity of the measured object needs to be taken into account. Even if the quantity of electric charge (Q) remains constant, the charged voltage will change from (V1) to (V2) when the electrostatic capacity of the measured object is changed from (C1) to (C2).
• The quantity of electric charge depends greatly on the environment. Therefore, the measurement location and conditions (such as temperature and humidity) need to be added to the measurement data.

Coulomb meter

Models of damage from electrostatic discharge (ESD) to electronic devices, a typical example of which is LSI, are largely divided into the following:

1. Human-body model (HBM)
   Damage from ESD from human body
2. Machine model (MM)
   Damage from ESD from machine frame, etc.
3. Charged device model (CDM)
   The device itself is charged and causes electrostatic discharge to the ground, etc., resulting in damage to the device.

A variety of countermeasures against HBM and MM have been developed. Those against HBM include anti-static floor, anti-static shoes, and wrist straps, while those against MM include grounding of manufacturing devices and metallic objects on the work floor. CDM is currently reported to be the main model of ESD damage.

If CDM is explained by using LSI as an example, it is a phenomenon in which the electrification charge of the internal conductor flows at high speed with a high peak current when it contacts the ground, which is caused by induced charge of the internal conductor due to charging of the package surface or the surrounding area or by direct electric charge of the lead, for example.

A static measurement method for preventing ESD damage of CDM is to use a static meter to measure the charge potential of the surface of the electronic device. However, measurement with this method is becoming difficult because electronic devices are becoming less tolerant of static and their package size is becoming smaller.

A Coulomb meter directly measures the intensity of discharge from an internal conductor such as the lead and pattern, thereby allowing immediate judgment of whether the electronic device will be damaged. This measurement method precisely fits the CDM. A Coulomb meter can also be used for grounding check of metallic objects used in the production process.

 

Examples of guides on managed voltage of static and charge-to-breakdown in various manufacturing fields

•  Semiconductor device manufacturing process (CMOS IC)
  Design rule 0.18 μm: 50V or below (charge-to-breakdown: 0.5-2.0 nC)
  Design rule 0.25 μm: 50V (charge-to-breakdown: 0.8-3 nC)
  Design rule 0.35 μm: 50V (charge-to-breakdown: 1-4 nC)
• Digital camera assembly process (CCD): 50-100V (charge-to-breakdown: 2-4 nC)
• Optical pickup manufacturing process: 30-50V (charge-to-breakdown: 1 nC)
• Optical disk drive assembly process: 50-150V (charge-to-breakdown: 1 nC)
• Hard disk drive assembly process:
  MR head: 10V (charge-to-breakdown: 0.2 nC)
  GMR head: 5V (charge-to-breakdown: 0.2 nC or below)
• LCD manufacturing process: 50-100V (Cell manufacturing process: 1,000-1,500V)