|The 3M EM Eye Meter from the 3M Electronic Solutions Division provides three devices in one unit — a base unit and three separate sensors for EMI, RF, esd. |
A blank printed wiring board (PWB) by itself is not considered to be an ESD risk and it isn't. However, the board can become charged, and when an ESD-sensitive component touches it, a discharge is imminent. Often, PWBs arrive at the factory wrapped in a regular plastic wrap. When the boards are unwrapped, they have quite a significant charge on them — some have been observed with voltages up to 500V. Not surprisingly, when these boards are loaded on the machine, they retain their charge even after the application of solder paste.
Even if the traces on the board are partially discharged by possible contact with the presumably grounded stencil, the insulative material will not be. Once the stencil is removed, the traces on the board get charged again by capacitive coupling. Removal of the stencil may contribute to further charging by tribocharge. Even though the stencil is made of steel, which is somewhat "neutral" on the tribocharge scale, the FR4 material from which the fab is made 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 the separating parts. This is not entirely correct. If a stainless-steel stencil is separated from the FR4 material, the stencil itself may have no charge left given that it is most likely grounded via the tool. However, this is not necessarily the case for the board. Once separated, the FR4 material is charged and can be discharged only by ionization, not by grounding.
Flexible circuits present an especially difficult case. They are usually made of polyimide which sits on the extreme of the tribocharge scale. Any physical contact with polyimide generates a high charge. Simply brushing a sleeve of a garment on a polyimide flexible circuit can produce several hundred volts. Handling of flex circuits has to be done with extreme care to avoid physical contact with the polyimide surface as much as possible. A thorough examination of the handling process would help in adjusting this process to minimize physical contact and to control tribocharge. Masking areas of the PWBs with polyimide tape may also increase the possibility of an unwanted charge on the board. This is where a good PWB designer can make a contribution by keeping such areas 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 really not capable of producing a strong discharge on contact, unlike metal-to-metal contact. The reason for concern is that a charged insulator still generates a static field and provides capacitive coupling to metal parts (i.e. traces on the board), and is therefore capable of charging them to a significant degree. A device with no charge placed on copper traces of the PWB may experience serious shocks. In a typical scenario, PWBs for mobile phones are taken from their plastic wrapping and are loaded on the assembly line. These boards then progress from one tool to the next, receiving application of solder mask, placing passive and active components and further going through the IR oven. The boards are charged once they are unwrapped. To measure static voltage on the boards in the process, sensors of the 3M EM Aware ESD Monitors were placed immediately under the conveyor. The boards remain charged throughout the process. The voltage on the boards in this particular case reached 250V. This is the voltage to which the components will be exposed once they are placed on the board.
Properly-implemented ionization would alleviate the problem to a certain degree. Several things need to be taken into account. First, ionization must be applied on both sides of the board since both sides may be charged. Second, ionization should be implemented either at the entrance to each tool or after exit from each tool, since the board may be additionally charged during the handling inside the tool.
First, let's debunk a common misconception. For whatever reason, it is often thought by PCBA manufacturing specialists 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 occurring during the shipment from IC manufacturer to a PCB assembly plant. The same device still has the same ESD damage threshold no matter where it goes. The handling of a device in the IC handler and in a pick-and-place SMT machine are essentially the same from an ESD point of view. A device gets charged by being lifted from the packaging, and can experience a discharge as it is placed on a metal surface (pads of the PWB in our case). As the devices are becoming more and more sensitive, PCB assembly manufacturers need to pay more attention to safe ESD practices.
If the PWB is completely discharged, would it be safe for the components? Not so fast. Let's consider what happens to the components themselves. When an IC rests in its carrier, it is fairly safe as far as ESD exposure is concerned. Granted, on close examination the pins of many devices do not actually touch the carrier because of elevation from a pedestal molded into the carrier. This means that if an IC has a charge, there is a chance that it may still remain on the device.
Lifting a Device
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 and its encapsulation is charged, the substrate and the lead frame of the device gets instantly charged via capacitive coupling. In this particular example the
encapsulation is charged positive and the carrier is charged negative. The charge on the carrier is likely to dissipate very quickly since most likely the carrier rests on a grounded conductive 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 PWB, a discharge is imminent. This doesn't mean that every device will be exposed to dangerous levels of ESD, however the possibility is quite real.
Resolution of this problem is rather difficult. The first step in such situations would be to properly select materials in order to prevent the charge to begin with. Even so, this may not help. Standards for carrier materials such as ANSI/ESD STM11.11 specify a test for resistivity and for dissipation of static charge. These are important properties of the carrier. However, the carriers are not normally tested for tribocharge. Granted, such tests, even when performed, still do not resolve the issue completely since the material used for encapsulation of the individual devices and the resulting charge may vary considerably.
Ionization that would normally help to dissipate charges from the device is not as helpful here as desired. The time between the device pick-up and its placement on the board 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 amount of time. 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 for as long and as much as possible. A properly-chosen and properly-placed ionizer can mitigate charging issues to some degree.
ESD During Soldering
Once placed on the PWB, are the components safe from ESD? Not completely. When a populated PC board comes in contact with solder in a wave solder machine, a discharge follows. Such discharges can be quite strong due to the combined charge of the board and of the components. These discharges are so distinct and are so damaging that the ESD Association has created a special model — CBM — charged board model. The solder is usually well-grounded through the machine's own grounding. When a charged board comes in contact with solder, a discharge is imminent.
|The 3M Wrist Strap and Ground Monitor 773 from the 3M Electronic Solutions Division provides effective workstation monitoring in applications with two metal ground requirements — grounding and monitoring two operators and two metal grounds. |
CBM discharges are considered to be more damaging than more traditional models, such as HBM (human body model) and CDM (Charged Device Model). 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 the soldering process — both sides of the board, not just the top.
Most of the PCB assemblies require some kind of manual operation after automated assembly. This would typically involve either soldering, or mechanical assembly or both. Either of these operations can expose the assembly to undesired voltages. If grounding of the soldering iron is faulty, the iron tip can have high voltage that would generate electrical overstress (EOS) to the components it touches during soldering. EOS is different from an electrostatic discharge (ESD event). Unlike an ESD event that lasts just a few nanoseconds, EOS may last as long as the contact is maintained. The energy transferred into the board assembly is quite high. IPC-A-601 Standard for Acceptability of Electronic Assemblies states that "EOS voltage as low as 0.5V can be dangerous, and voltage less than 0.3V is recommended. Therefore, proper grounding and construction of the soldering iron guarantees grounding of the tip when the iron is used on the PCBA."
Defective Building Wiring
Sometimes, the grounding failure occurs due to a bad ground in the electric outlets used by the soldering iron. Ground failure in the outlet or incorrect wiring (ground/neutral reverse wiring) can cause severe EOS. It should be noted that even if the soldering iron is perfectly grounded, the PC board may not be, and this would cause overstress during the contact. Fortunately, it is quite common in production to hold PCBAs on static-dissipative grounded surfaces — mats — and/or provide ionization that is quite helpful in removing residual charges on the boards.
Another potential source of EOS is power tools, such as an electric screwdriver that is used to fasten the board to the enclosure. The tip of the screwdriver has the same grounding challenge as the soldering iron described above. Especially problematic are the AC-powered electric screwdrivers which can inject as much as 1/2 of the line voltage into the circuit.
An easy way to detect EOS during manual assembly is to use an EOS monitor. When this particular monitor is connected to the ground plane of the PWB or to the mat on which the board rests, it provides a real-time alarm in case of excessive EOS exposure. This alarm provides two important pieces of information. First, it is an indication that something is wrong with the ESD/EOS protection on the workbench. Second, it indicates that this particular PCBA was exposed to unwanted EOS levels and, all things considered, perhaps should not be shipped to the customer.
The same monitor can also provide wriststrap and dissipative mat monitoring, thus ensuring ESD/EOS protection for the entire bench.
Once the machine soldering is finished, the next metal-to-metal "contact sport" event is lead trimming, which trims protruding leads on the PC board to a specific length. This process is not entirely free from exposure of components to damaging voltages. The lead trimming tool is essentially an electric motor with a rotating cutting blade. The rotor of the tool 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 layer between the metal parts of a bearing. Due to capacitive and inductive coupling between voltage and current-carrying parts of the tool, the rotor may have voltage on it during rotation thus making the grounding ineffective.
When the large metal mass of a blade touches the pins of the components, a strong discharge is likely to occur. Mitigation of this problem may not be trivial. First, if the construction of the tool is such that it does not guarantee good grounding of the rotating blade, little can be done short of adding a grounding path via some kind of spring-loaded grounding contact. Second, it is also possible that the blade is well-grounded, but the board is charged. The discharge during cutting can be damaging as well.
Ionization 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.
Contact: 3M Electronics, A130-3N-52, 6801 River Place Blvd., Austin, TX 78726 800-328-1368 Web: http://www.3M.com