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Vladimir Kraz, 3M
In this second part of the two-part series, topics include ESD preventive measures and their properties. Part I covered ESD exposure on different steps of board assembly.
IonizationIonization is the only realistic way to reduce charges on insulators and floating conductors. In short, ionization is a flow of conductive air. When the ions in this conductive air envelop a charged object, the charge is partially or fully neutralized. Ionizers' fundamental properties are decay and balance. Decay is measured in seconds, and determines how fast the ionizer will dissipate charges from a charged object. Balance (or offset) is a function of the ratio of positive and negative ions and, ultimately, to what voltage the board (or other item) will be charged after being under the ionizer for some time. Ideally, the balance should be close to zero. Decay always needs to be checked first since zero balance can also mean a dead ionizer, not just a perfectly balanced one. ESD Association (ESDA) ANSI 3.1 standard defines specific tests for both parameters.Practical ionization implementation requires basic understanding of the technology's effect on PCB fabs and ICs.
PCBsPCBs are made of insulative materials (typically FR-4) and can easily be charged and retain the charge. While the energy on the FR-4 material itself won't generate much of discharge current dangerous to the components, capacitive coupling can charge the traces. Figure 1, Part I explains this phenomenon in detail. When a component is placed on the conductive traces a discharge is likely. When a board with the components already installed goes through wave solder, a charged board can cause discharges as well (see Figure 3, Part I).
Ionization can help alleviate this problem, as long as the ionizer is properly selected and positioned. Not all ionizers are suitable for the PCB assembly process. The best are either DC or AC ionizer blowers. Ionizer positioning is critical. Place it too close to the devices and they may experience excessive variations of balance. Place it too far away and there won't be enough ions to dissipate charges properly. Place the ionizer in such a way that the airflow hits a metal surface and the ions of opposite polarities immediately recombine. If this metal surface is grounded, the remaining ions immediately discharge and the result is just a flow of neutral air turning an ionizer into just a fan.
For PCB fabs as insulators, it is essential to ensure that both sides of the board are bathed in ionized air. Dissipation of charge on one side of the board does not guarantee that the opposite side is free of charge. Often this requires more than one ionizer. It is also better to have ionizers placed just before the next operation. If an ionizer is placed on the exit of the board from a tool, the board still can be charged just by moving it along on the conveyer to the next process.
ICsSmall devices, such as ICs, receive far fewer ions from the ionization flow than a large PCB. This makes ionization more challenging. The decay time of an ionizer as measured with standard methods, according to ANSI 3.1, should be as short as possible (preferably within one second) to provide a viable decay of charge on a tiny component. In automated tools, the time between component pick-up from tray, tape, etc. and placement on the substrate may be a small fraction of a second. This short time does not give even the best ionizer a fighting chance to do its job.
For best possible ionization, an ionizer should provide the longest coverage distance for the moving device. This includes the moment of separation to the moment of placement. Unfortunately, often an ionizer is positioned to cover the device only for a brief moment of its transition.
Ionizer MaintenanceThere is no such thing as a maintenance-free ionizer. Ionizer emitters (tips) get contaminated and worn out until they are producing inadequate amount of ions. Ionizer balance drifts with time and humidity. At some point, an ionizer may charge boards and components rather than dissipating the charge. While periodic scheduled maintenance of ionizers helps, it consumes resources and interrupts the production process. Also, there is a significant possibility of ionizer failure between maintenance efforts. For proper ionizer performance and lower maintenance costs, use ionizer controllers and monitors. These provide either complete control of ionization parameters or, for those ionizers that cannot be controlled, real-time monitoring of ionization parameters with an alarm for out-of-spec operation. Figure 5 (for images 1-4, see Part I) shows an ionizer controller.
WriststrapsThe most common method of ESD protection during electronic assembly by operators is wriststraps. Do they really help? Properly working, good-quality wrist straps do. Improperly worn, questionable ones do not. The best way to ground an operator with the wriststrap is to add a wriststrap monitor. Plenty of models of wriststraps and monitors are available on the market. The most significant distinction between them is single or dual wire designs. Single wire wriststraps and monitors are the most cost-effective; however, they provide very little factual assurance of proper grounding. They determine whether an operator is wearing a wriststrap properly by measuring parasitic capacitive coupling between an operator and ground on the floor. This method provides only an approximation of the operator's connection. More expensive and much more relevant dual-wire wriststraps and monitors ensure operator connection. Dual wire wriststraps have two separate electric contacts with the operator's body, and the monitor measures resistance between these two points, verifying that the operator is electrically connected to a wriststrap.
Figure 6. Static field from an operator wearing single-wire wriststrap.
Figures 6 and 7 illustrate the difference between the two wriststraps. The data were taken at a manual operation station where an operator inserted a camera module into a mobile phone PCB assembly. An ESD monitor with the ability to measure static voltage was used to detect and record the static field from the operator as she moved in the process.The chart in Figure 6 was taken when this operator wore a single-wire wriststrap. The wriststrap was quite loose and barely contacting her wrist. This was enough for a single-wire wriststrap monitor to indicate a connection, but not sufficient contact to quickly dissipate accumulated charges. As seen, the resulting voltage was quite high and the occurrences of static voltage frequent.
Figure 7. Static field from an operator wearing dual-wire wriststrap.
Figure 7 shows the static field from the same operator wearing a dual-wire wriststrap plugged into a monitor. As seen, the occurrences of static voltage during the same operation are much less frequent and the static field is significantly lower. It is evident from these charts which wriststrap does a better job in reducing static charges on an operator.
It should be mentioned that no wriststrap can guarantee zero volts on an operator. The only thing that a wriststrap can do is to quickly dissipate accumulated charges to reduce static voltage on the operator.
ESD Shoes and Heel StrapsIf a worker has to move around during normal operation, a wriststrap becomes more of a liability than an aid. In such cases, an alternative way of dissipating charges on an operator is to use special ESD footwear. It provides a dissipative path from the operator's body to a ground via conductive footwear and the floor. For ESD footwear to work, the entire conductive chain must be present: human body to footwear, footwear to conductive (or dissipative) floor, connectivity throughout the floor and, finally, floor to ground. If any of these links are weak, there will not be sufficient dissipation of the charges. Every element must be periodically tested and thoroughly checked.
ESD shoes offer the best conductivity. The electrical contact between operator's feet and shoes is very good due to close and broad contact. If the ESD shoes are not an option, heel straps, or heel grounders, will do the job. Heel straps provide electrical contact with the body via a conductive fabric strip that is inserted inside the shoe or between the sock and the foot. Keeping ESD footwear clean improves the chances of consistently good contact.
It should be mentioned that as good as ESD footwear can be, it is still not as effective as a wriststrap. The human body has certain electric resistance and if it is grounded via footwear, the dissipation of charges from hands to feet may be slow and the resulting voltage on the operator too high in comparison to wriststrap results. In many cases, such as where an operator does not touch the components or the board, this may still be acceptable.
There is a dangerous practice seen in a number of facilities around the globe: operators wearing their wriststraps around their ankles. This practice has at least two serious problems. First, safety. An operator can be easily tripped by the cord and possibly fall and incur injuries that could be quite severe in the industrial environment. Second, it does very little for ESD. The ankle usually has very dry skin with high resistance, which would significantly slow charge dissipation.
GroundingProper grounding offers the most return on investment in ESD protection simply because of the cost of a wire from a tool to ground is negligible compared to the experienced benefits. Consult ANSI/ESD S6.1-2005 and ANSI/ESD SP10.1-2007 from ESDA for recommended guidelines.
Any production tool is a composite one, comprising separate parts (robotic arms, frame, load port, conveyer rails, etc.) that may or may not be properly connected to a ground. Typical failures to connect to ground involve, but are not limited to:
- • Not knowing where good ground reference is. Often grounding is done to the ground contact in a power outlet. In many cases this is sufficient; however, in some cases neutral and ground in the outlet are unintentionally switched, negating proper grounding and creating a safety hazard. Sometimes a good ground is identified with "Common Point Ground" but it is still helpful to verify this.• Assuming grounding via anodized aluminum parts. Anodized aluminum is typically an insulator and all the connections must be made to the body of the metal, not through a contact on the surface. • Grounding of moving parts via ball bearings. These are, essentially, separate pieces of metal covered with an insulative film of lubricant. When the tool is at rest, this thin film is squeezed out and the electrical contact between the parts of the bearing may be good. Grounding checks performed when the tool is at rest do not accurately represent what happens when it's in motion. Test ground connections when the tool is operating, observing all requisite safety precautions. It may help to attach a flex wire to the moving part to simplify measurement.
Figure 8. Continuous ground monitor.
Ground wires can break or become disconnected for any reason. Periodic checks of proper grounding are necessary to identify problematic connections. However, to maintain continuous good grounding, use ground monitors (Figure 8). These instruments continually monitor grounding quality on essential parts of the tool, and are capable of generating an alarm and even stopping the tool should any of the monitored grounds fail.
Proper ESD/EOS Management PracticesQuite often EMS manufacturers work in the open loop without any way of verifying a safe ESD environment. If an assembly's component has a damage level of 100 V CDM, how will a manufacturer know whether the component was exposed to higher levels during the manufacturing process? Simply buying ionizers, wriststraps, and heel grounders cannot not assure a manufacturer, or their customers, that their components are safe. The only practical way to manage ESD is by results, not by assumption. If ESD events and/or EOS exposure cause damage to the devices, just having wriststraps is a useless assumption of safety. It is essential to measure and monitor the parameters that cause ESD and EOS damage. Verification instruments, as shown in Figure 9, provide a quick and reliable way to identify ESD events and their origin. ESD monitors track ESD events and static voltage in multiple locations simultaneously and provide numeric values of ESD exposure anywhere in the process. With such information, a manufacturing manager can determine which tools are qualified for what ESD exposure, with the confidence that devices are not being exposed to dangerous levels of ESD and EOS.
ESD and EOS detection and measurement tools are used for tool qualification, process qualification, material qualification and for verification of ESD-protective measures. If you are considering investing in an ionizer to reduce accumulated charges, what better measure of return on investment than to see how much it reduces ESD exposure to the devices? This is also the most practical way to justify purchase, installation, and maintenance of any ESD-protective measures.
Vladimir Kraz, 3M, may be contacted at (831) 459-7488; mobile: (408) 202-9454; firstname.lastname@example.org.