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By Vladimir Kraz, 3M
Sensitivity to electrostatic discharge (ESD) and electrical overstress (EOS) is becoming increasingly important in PCB assembly (PCBA) manufacturing. This first article in a two-part series discusses the occurrences of ESD exposure in the assembly process. These include PCB loading, component handling, soldering, and operator interventions.
There are a number of ESD/EOS threats within the SMT assembly process. Let's examine a PCB assembly process step-by-step and analyze possible ESD exposure.
PCB LoadingA blank PCB fab is not considered to be an ESD risk, and by itself this is true. However, if the fab is charged, when an ESD-sensitive component touches it, a discharge is imminent. Often, PCB fabs arrive at the factory wrapped in regular plastic wrap. When the fabs are unpackaged, they carry a significant charge, observed at up to 500 V. Not surprisingly, when these boards are loaded on the machine, they retain this charge even after the application of solder paste. Even if traces on the board are partially discharged by contact with a presumably grounded stencil, the insulating fab material will not be safe. Once the stencil is removed, traces are recharged via capacitive coupling. Stencil pull-back can further generate charge on the fabs via tribocharge. Although printing stencils are made of steel, which is somewhat neutral on tribocharge scale, the fab's FR-4 material is not. A common misconception is that if one of the materials in the tribocharge pair is a conductor, then there won't be any residual charge on either of separating parts. This is not entirely correct. If a stainless-steel stencil is separated from FR4 material, the stencil may have no charge left, as it is grounded via the tool. However, the FR-4 material is charged and can be discharged only by ionization, not by grounding.
Flex circuits are particularly challenging. They typically are made of Kapton, which sits on the extreme of the tribocharge scale. Any physical contact with Kapton generates a high charge. Simply brushing clothing or a glove on a Kapton flex circuit can produce several hundred volts, if not higher. Flex circuit handlers must exercise extreme care to avoid physical contact with the Kapton surface as much as possible. A thorough examination of the handling process helps minimize physical contact and control tribocharge.
Masking areas of rigid PCBs with Kapton tape may also increase the possibility of an unwanted charge on the board. This is where a good PCB designer can make a contribution by keeping areas that need to be masked away from sensitive components.
Why would a charge on an insulator be harmful to the device? It appears that the components do not make contact with the insulative material on a board, and an insulator is not capable of producing a strong discharge on contact, unlike metal-to-metal contact. However, a charged insulator still generates a static field and provides capacitive coupling to metal parts (i.e. traces on the PCB) and is capable of charging them to a significant degree. A device, even with no charge, placed on a PCB's copper traces may experience serious shocks.
Figure 1 illustrates a typical scenario in the case assembly of boards for mobile phones. PCB fabs taken from plastic wrap are loaded on the assembly line. These fabs progress from one tool to the next, which in turn apply solder paste, place passive and active components, and reflow the board through an oven. The boards are electrically charged when unwrapped. To measure static voltage on the fabs in the process, sensors* were placed as shown. As the Figure 1 chart shows, the boards remain charged throughout the process. Voltage reached 250 V. This is the voltage to which the components will be exposed when they are placed on the board.
Properly implemented ionization would alleviate the problem to a certain degree. Ionization must be applied on both sides of the board since both sides may be charged. Ionization should be implemented either at the entrance to each tool or after exit, since the board may be additionally charged during processing at this step.
Component ChargeFirst, let's debunk a common misconception. PCB manufacturing specialists often believe that the components they deal with are not as sensitive to ESD as the very same components during IC manufacturing. The truth is that there is no miraculous transformation during shipment from IC manufacturer to a PCB assembly plant. The same device retains the same ESD damage threshold no matter where it goes. Device handling at IC packaging and in a pick-and-place SMT machine essentially is the same from an ESD point of view. A device is charged when lifted from the transport and can experience a discharge as it is placed on a metal surface, such as pads on the PCB. As modern devices exhibit higher sensitivity, PCB assembly manufacturers must increase vigilance in safe ESD practices.
If the PCB fab is completely discharged, is it safe for components? Not necessarily. Let's consider what happens to the components. When an IC rests in its carrier, ESD exposure risk is inconsequential. When a device is lifted from the carrier, the dissipative properties of the carrier won't matter much: two dissimilar materials (carrier and encapsulation of the device) develop charge on separation. Once the device is lifted, it is physically disconnected from the dissipative properties of the carrier.Figure 2. Charge on the IC during separation from the carrier.
Once the device is lifted and its encapsulation charged, the substrate and leadframe are instantly charged via capacitive coupling, as shown in Figure 2. In this example, the encapsulation is charged positive and the carrier negative. The carrier's charge is likely to dissipate very quickly, since carriers typically rest on a grounded conducted or dissipative surface. The charge on the device, however, will remain until the device touches some conductive surface. Once the device touches the copper traces of the PCB fab, discharge is imminent. This doesn't mean that every device will be exposed to dangerous levels of ESD materials vary but the possibility is real.
This problem is hard to resolve. First, one could select materials that can prevent the charge. This is not feasible, as assembly companies have little or no control over carrier materials used for incoming components. Even if they did, this may not help. Standards for carriers, such as ANSI/ESD STM11.11, specify a test for resistivity and for static charge dissipation. Carriers are not normally tested for tribocharge. Indeed, such tests, even when performed, may be inconsequential, as device encapsulation materials and the resulting charge vary vastly.
The time between device pick-up and placement is a fraction of a second. An ionizer with decay time of several seconds won't be able to do its job in this short period. In addition, proper placement of ionizers in pick-and-place machines can be tricky due to movement of machine parts and the desire to "bathe" the components in ionized air as long and as much as possible. Still, a properly chosen and placed ionizer can mitigate charging issues to some degree.Figure 3. Electrical contact with solder during wave soldering.
ESD During SolderingOnce placed on the PCB, are the components safe from ESD? Not completely. When a charged PCB with placed components contacts solder in a wave machine, a discharge follows. It can be quite strong due to significant charge on the board and components combined. These discharges are so distinct and damaging that the ESD Association (ESDA) has created a special charged board model (CBM). Figure 3 shows a board's movementin a generic wave solder tool. As seen, solder is usually well grounded via the machine's own system. When a charged board comes in contact with solder, a discharge is expected.
CBM discharges are considered more damaging than traditional models, such as human body model (HBM) and machine model (MM). Since there is no way to prevent electrical contact between the board and the solder during assembly, the only realistic way of mitigating this issue is thorough ionization of boards before soldering. As previously mentioned, ionize both sides of the board, not just the top.
Manual OperationsMost PCBAs require some kind of manual operation after automated assembly. This would typically involve soldering, mechanical assembly, or both. These operations can expose the assembly to unwanted voltages.
If a soldering iron is incorrectly grounded, the iron tip's high voltage can generate electrical overstress to the components it touches. EOS is different from electrostatic discharge. Unlike ESD events that last a few nanoseconds, EOS can occur for the duration of contact. The energy transferred into the board assembly is high; therefore, the damage voltage levels due to EOS are significantly smaller than those caused by ESD. IEC-A-601, "Acceptability of Electronic Assemblies," states that EOS voltage as low as 0.5 V can be dangerous; voltage less than 0.3 V is recommended. When purchasing and maintaining solder irons, look for quality construction and proper grounding.
Sometimes, a bad ground in the electric outlets or incorrect wiring (such as ground/neutral reverse wiring) can cause severe EOS. Even if the soldering iron is perfectly grounded, the PCB may not be, and this would cause overstress during the contact. Fortunately, most soldering stations hold assemblies on static-dissipative grounded surfaces and/or provide ionization that is quite helpful in removing residual charges on the boards.
EOS also can come from power tools, such as an electric screwdriver in box build. AC-powered electric screwdrivers can inject as much as half of the line voltage into the circuit.
EOS is easily detected during manual assembly via an EOS monitor (Figure 4). Monitors connected either to PCB ground planes or the mat on which the board rests provide real-time alarming for user-set excessive EOS exposure. Alarms indicate that something is wrong in ESD/EOS protection on the workbench and that the particular PCBA was exposed to unwanted EOS levels. Users then determine whether the board can be shipped to the customer.
Monitors can also provide wriststrap and dissipative mat monitoring, ensuring ESD/EOS protection for the entire bench.
Lead TrimmingAfter soldering, the next metal-to-metal event is lead trimming, removing protruding leads on the PCB. This process can potentially expose components to damaging voltages. The lead trimming tool is, essentially, an electric motor with a rotating cutting blade. The rotor is not necessarily grounded; grounding via bearings (ball or friction) works only when the tool is at rest. Otherwise, there is an insulative thin film or lubricant layer between the metal parts of a bearing. Due to capacitive and inductive coupling between voltage and current-bearing parts of the tool, the rotor may carry voltage during rotation. When a large metal mass of a blade touches component pins, a strong discharge is likely to occur. Mitigating tooling charges is not trivial if the tool's construction doesn't ground the rotating blade. Not much can be done short of adding a grounding path via some kind of spring-loaded contact. If the blade is grounded but the board is charged, discharge during cutting can be damaging as well.
Ionization often used in production is ineffective in the former case but is effective in the latter one just make sure that the flow of ionized air covers both sides of the PC board.
*Sensors used were the 3M EM Aware ESD Monitors.
Part II of this series will discuss ESD protective measures and their effectiveness. Look for it in the June 10 issue of SMT WEEK
Vladimir Kraz, 3M, may be contacted at (831) 459-7488; mobile: (408) 202-9454; email@example.com.