When you evaluate an electronic manufacturing services provider, look beyond the appearances of electrostatic discharge (ESD) protection efforts. Complete an audit.
During your plant visit, you may be asked to don a conductive smock and heel strap before you enter the manufacturing area. There, you may see workers standing on special mats and wearing wrist straps tethered to their workbenches. These are a few measures used to keep ESD from damaging sensitive electrical components during PCB assembly.
Unfortunately, you can't know whether these measures are working without regular ESD audits. Let's look at some common ESD protection techniques, how they can fail, and what to look for in an ESD audit.
First, what is ESD?
ESD is the transfer of electrical charge between two objects. In manufacturing, the most common source is static electricity caused by friction between dissimilar materials. A charge builds up on bodies, carts, and other objects and is discharged in a sudden flow of electricity to nearby objects.
Even small discharges (not noticeable to your touch) can render an electronic component inoperable or cause latent damage that shortens its life. Therefore, ESD protection is critical to maintaining production yields and ensuring the reliability of finished assemblies. This protection involves preventing static buildup and ensuring that any buildup is discharged harmlessly to ground, not through the components.
Follow the components on their journey through the facility, looking at all potential sources of static and verifying they are effectively controlled. Here are some things we have observed during ESD audits for ourselves and our customers. This ESD audit video illustrates these examples.
- Receiving areas: Look at the way parts are received. We have seen workers carefully remove all packing materials, including the ESD-protective bags, before transporting components to the storage or work area.
- Transporting components: Carts should be made of conductive plastic or metal, with a grounding chain or conductive casters. We have seen assembly areas with regular plastic carts, which are unacceptable. A more subtle problem is when conductive carts are outfitted with racks using spacers of regular plastic. These spacers insulate the shelves from the rack frame. Conductive tape can be added to connect each shelf electrically to the frame. Verify that all bins and totes are made of conductive materials. Regular plastic totes should never be used.
- Storage areas: All storage shelving must be conductive and grounded. Storage racks are typically metal and grounded by contact with conductive flooring. We have seen older facilities with wooden floors; metal racks on such nonconductive surfaces must be grounded by other means. An audit should test the floor's conductivity and verify that each shelf is conducting to the main post.
- Workbench areas: If wrist straps are used, check that they are plugged securely into the jack. Then test the jacks. At every place we've audited, we've seen at least one instance where a workbench has been moved and the grounding wire left hanging, rendering the wrist straps useless.
If heel straps are used, two straps are needed to ensure contact with the flooring, which must be conductive. We use a conductive floor wax throughout the facility and conductive anti-fatigue mats at work benches. In an audit, measure the anti-fatigue mats to ensure that they are conductive and properly grounded. We have seen places use regular industrial anti-fatigue mats that do not meet ESD protection requirements. Both wrist and heel straps wear out in about six months with the constant bending of the wires inside. They must be tested regularly; we recommend twice per shift. Audit test records to verify compliance.
- Shipping: Audit the handling procedures and packing materials. Using recycled materials is usually a good thing, but styrofoam packing materials can induce potential on to components via electrostatic induction, even without direct contact, causing potential for later discharge when boards are unpacked. You should use all-new packaging materials known to be anti-static.
When to audit
Audits should be conducted annually at least. At Z-Axis, the manufacturing engineering team conducts quarterly ESD audits, and the quality team audits the entire facility annually. This schedule helps ensure protection as new equipment is added and work areas are reconfigured to support the changing product mix and workflows.
Finally, as an OEM, you should audit your own in-house manufacturing facility for ESD protection. Many ESD-related issues with a subassembly can be traced to its handling after it's delivered to the OEM. To ensure success of our entire combined production efforts, we audit our customers' facilities on request free of charge.