P.Tamminen, J.Paasi, H. Kojo
VTT Technical Research Centre of Finland

Purpose
• To study ESD control tools and methods for electronics assembly lines
– Status of the assembly line ESD control
– ESD properties of the assembly lines
– Critical path of ESDS devices
– Experimental methods
– Use of the ESD control tools

ESD and Electronics Assembly
• Automatization of assembly lines reduces manual handling of ESDS devices.
• Large volumes of ESDS items are handled.
• Devices with withstand voltages below 100V are common.
• Amount of ESDS sub-assemblies are increasing.
• Embedded components in the boards are increasing.
• CDM and MM model discharges will emphasise.

ESD Properties of Assembly Lines
• All assembly lines do not fulfill ESD requirements.
• Standards concerning ESD protection of production equipment are not sufficient
• Standards are not detailed enough for very ESD sensitive devices.
• In line ESD verification of the production line is often too late.

ESD Properties of Production
Equipment
• Specific ESD capability of the equipment should be provided by the vendor , but
 there should be specific understanding of ESD protection demands of AHE.
• Understanding of both electrostatics and production requirements are needed.
• Also product materials should be taken into consideration during the design phase of equipment.

Controlling of ESD in assembly
• Assembly lines have a lot of measurement locations.
 Environment varies.
 Construction of the lines varies.
 Product materials varies.
 Level of automation varies
• Standard ESD control tools are difficult to apply to assembly lines.
 New or modified standard practice measurement methods are needed

Critical Path of the ESDS Devices
• Target is to reduce number of measurements during the ESD verification of assembly lines.
• Path of the ESDS items through equipment determine required measurement locations.
 The critical path depends on the type of line and the construction of products.

Defining of the Critical Path
• Define process phases and “ESD functions”.
• ESD function is a process phase with a change of
– Ionization,
– Capacitance,
– Triboelectric charging,
– Induction and
– All other charging or discharging events of
ESDS devices.

Reduction of Measurements in the
Critical Path
• Select common measurement methods
– Reduce amount of different measurements required.
– More efficient data handling.
• Select common “ESD functions”
– Many of the process functions are similar for many equipment on line.
– Same measurement methods and tools can be used.

Examples of ESD Control Tools
Component assembly phase
• Critical path functions are:
– Input conveying
– Clamping of PWB
– Component pick up
– Placement
– Release of the PWB
– Output conveying.
• Electrostatic potential and charge measurements are used.

Experimental Methods
• Charging of the PWB
– Charging depends on equipment and PWB materials.
– The capacitance of the PWB may change.
 High electrostatic energy may exist.
• Potential of the PWB should be monitored through all functions of the critical path.

Experimental Methods
Electrostatic potential measurement
 Potential is a good indicator of risk locations on line.
 Challenge is to take into account the influence of grounded conductors nearby.
Charge measurement
 Valuable when estimating risk of ESD damage.
 Useful especially with CDM type of discharges

Use of ESD Control Tools
Conveying and clamping of the PWB

Use of ESD Control Tools
Numerically simulated electrostatic potential of a PWB on a conveyor when grounded clamps are closed.

Use of ESD Control Tools
Pick up and placement of components
Components

Use of ESD Control Tools
Electrostatic charging of one component

Component Assembly Phase, Results
• Conveying and clamping of the PWB
– Over 200V electrostatic potential may exist on copper layers of the PWB
– Capacitance sets potential and energy levels of the PWB
• Over 150V potentials may exist on components before placement
– Components became charged during pick up
– Charge of the component packages can be measured with Faraday cup
• Charge of the components can be used to estimate CDM type of ESD risks

Conclusion
• Target should be to develop new standard practises of ESD verification to be used by equipment vendors and end-users.
• Use of new or modified ESD control methods is needed, since equipment and product materials of assembly lines are varying.
• Number of ESD measurements can be reduced by using the method of a critical path.