ESD impacts upon productivity and product reliability in every area of the electronic environment, affecting production yields, manufacturing costs, product reliability, product quality and most importantly profitability. It is estimated that average product loss due to ESD ranges from 8-33%, resulting in billions of dollar losses to the electronics industry annually.

Precautions such as ESD packaging, conductive wrist and foot straps for line workers and anti static floor coverings are all in place to remove build up of static charge during manual handling of completed circuit boards. The same precautions however are not taken at nozzle and component interaction.

The majority of SMT nozzles are made from a wide range of materials which may be either polymer, metal, or ceramic or any combination of these, so the problem faced in removing ESD is as varied as the tool tips themselves. Plastics and ceramics are most commonly used and act as electrical insulators, slowly dissipating any charge build up. However if the rate of generation is greater than that of charge dissipation then there is an increased chance of an ESD event as the charge finds a path to earth. Controlling the route that the discharge takes can make a real difference to a line’s profitability, especially at the final stages of assembly. The preferred route is for the charge to be carried away from the assembly and towards the SMT machine, where it can be safely handled. This is achieved by controlling the electrical dissipative resistance of the SMT nozzle, by careful material selection and fabrication. Metro Technologies has the experience and capability to build ESD discharge protection into your nozzles.

Figure 2: Component skew, due to drag forces

Electrical resistance can help to define how a material can dissipate any charge that it may carry. Two values are usually given for this: Surface Resistivity (Ω cm2) and Volume Resistivity (Ω cm3) and typical values are shown in Table 1.

These ranges of values can be used when selecting materials for new and existing nozzle designs to ensure they are within the desired dissipative resistance value range for electronic assembly (104 - 109 Ω).

Figure 3: 50X Magnification Damage Caused By Electrostatic Discharge

Design engineers at Metro Technologies use powerful 3D finite element analysis (FEA) software to determine the following factors in ESD:

• Static build-up
• Discharge
• Component and board damage, cost Standard means of mitigation (cost,
  reliability)
• Design robustness into the nozzle
• Material selection and resistivity
• Conduction paths, assembly, and overall resistance

For example, the surface and volume resistivity can be predicted for any nozzle geometry, material types and electrical and mechanical properties. The combined effects of: discharge path length, cross-section, material resistivity and density, and contact resistance can all be calculated, allowing safe resistivity levels to be designed in to the nozzle maximising the ESD protection.