ESD procedures for Flexible Circuit Assembly

Companies that do component assembly to flexible circuits need to be aware of ESD sensitivity issues and take steps to address them.

What is ESD?

Anyone who has walked across a carpet on a dry winter day and got “zapped” when they went to touch a door knob has experienced the phenomena known as Electrostatic Discharge (ESD).    The discharge occurs when a material builds up enough charge to overcome the dielectric resistance of ambient air and grounds to another material that has a lesser charge build up.

Some materials tend to easily “give up” their electrons and some materials tend to “take them”.   When two dielectric materials come in contact, electrons pass from one material to the other.  Some materials will tend to build up a negative charge when electrons jump to another material.  Conversely some materials tend to build up a positive charge.   Materials are ranked on a Triboelectric Series scale.  Materials near the top tend to be positive, and materials near the bottom tend to be negative as shown below.  Source “Triboelectric Generation: Getting Charged”  by Ryne C. Allen

triborelectricseries

 

 

 

The listing or ranking of the above materials can actually vary, depending on factors such as surface roughness and temperature.   The most dramatic transfer of charges occurs when a material near the top of the list comes in contact with a material near the bottom.

Many electronic devices are highly ESD sensitive:  a very small charge can transfer to the device and fuse or burn the internal materials to the point that degrades its functionality.   Integrated Circuits (IC) devices can be especially susceptible to ESD because of their small size, lack of large open areas to dissipate energy and the materials used such as metal oxide.  Source: “Susceptibility of Integrated Circuits to Electrostatic Discharge

It is believed that a spark from a human finger to a door knob requires around 6000 volts; some IC’s can be affected by as little as 3 volts.

 

While even the densest and thinnest of flexible circuits are totally resistant to ESD, many of the components that are assembled on the circuit are not.   Therefore flexible circuit manufacturers need to employ ESD procedures to protect these devices.  These procedures may include the following:

  • Design: The best way to protect a device is to make sure that any susceptible areas are grounded to drain off the excess charge. Flex circuits can be built with ground planes that perform that exact task.
  • Materials:   There are a variety of anti static materials available that can minimize the ability of a surface to accumulate a charge.   This includes base substrates, overlays and potting materials.
  • Handling: The biggest risk for ESD damage comes during the assembly of the IC.   There are a number of methods that reduce the risk of ESD damage.
    • Grounding straps for operators.   The human body can accumulate a charge of over 50,000 volts; a well grounded operator will stay neutrally charged.
    • Ionized Air Flow: Some workstations can come equipped with ionized air blowers. The ionized air will neutralize any surface that it comes in contact with. There are a number of portable devices that are also available.
    • Humidity Control:   Moisture tends to conduct, so a humid environment will have less ESD issues than a dry environment. The level of humidity needs to be balanced with other factors such as operator comfort, the hydroscopic nature of materials and corrosion. 40% relative humidity is considered ideal for electronic assembly.
    • ESD flooring/mats: Conductive surfaces that are grounded will help dissipate a charge when a material is placed on it.
    • ESD clothing and Shoes: Some types of fabrics can generate static; operators should wear anti- static socks and shoes or shoe coverings that have solid contact to the operator’s skin.
    • Monitoring/ Auditing:   There needs to be a good program for ESD monitoring and auditing that assures that all ESD procedures and equipment are fully functional.
    • Operator/management Training: There needs to be an effective training and certification program for all operators that handle ESD sensitive devices.   Management and supervisors need to be trained and certified to ensure that ESD procedures are understood and enforced.

All Flex routinely assembles ESD sensitive components to flexible circuits.  Many of our certifications such as AS9100C and ISO9001:2008 and MIL-P-50884E cover the protection of ESD sensitive devices during assembly.   All personnel that come into contact with ESD sensitive products are trained in ESD procedures.    All Flex routinely monitors its ESD compliance.