Practices for handling ESD- and MSD-sensitive components.

“If the thunder doesn’t get you, then the lightning will.” – Jerry Garcia

We are all aware of electrostatic discharge. To varying degrees, most assemblers have taken precautions. While on one hand, ESD is a science and therefore a specialty in itself, preventing ESD damage is not necessarily rocket science.

ESD-sensitive components are most vulnerable when handled in their unmounted condition. Diligence in handling and storing components is in order. Parts are delivered to the assembler in anti-static bags, matrix trays, tubes, reels, etc. Preventing the placement machine operator from touching the parts generally goes a long way.

However, there is a misconception that once ESD components are mounted, your troubles are over. Not true. Indeed, this is where the assembler has to take the greatest precautions. Again, post-assembly packaging in anti-static bags is normal procedure, but there is much to be done in between.

First, all equipment, including workbenches, must be grounded. Operators coming in contact with assemblies should wear wriststraps connected to a known good ground. Anti-static smocks/lab coats, anti-static footwear or ground straps, plus anti-static floor finishes (paint, tiles) go a long way, but are only part of the picture. A good process auditor looks not just at whether assembly personnel are “outfitted” correctly, but whether they are indeed tested. There should be an ESD test station at the assembly floor entrance, and anyone entering the floor should pass the test and log in. (Traceability is essential). World-class operations have personnel test every time they enter the assembly floor.

Again, anyone coming in contact with the assembly at any operation should wear a wriststrap connected to a good ground – and this should be tested routinely. This includes manual component stuffing, loading and unloading AOI and ICT, touchup and rework. Skimping here can be deadly.

Depending on the nature of the component, its threshold “voltage of pain” and the voltage endured, ESD damage can be immediate or latent. Typically, ESD-sensitive devices that have been exposed to their respective “killer voltages” will be damaged to the extent that they will cause the assembly to fail functional test. They can thus be replaced and the damage source identified and rectified. However, some devices fail through a slow death. Exposure to damaging voltage will, in such cases, deteriorate the component’s functionality and result in a field failure. This can happen over hours, days or months. So although a component was zapped in the assembly process, the effects may not be fully apparent for perhaps hundreds of hours.

Moisture-sensitivity is another silent killer that comes into play during assembly. That problem dates to the onset of SMT, yet widespread attention ebbs and flows. IC thermoset epoxy bodies appear to be impermeable to moisture. They aren’t. Over time, the plastic encapsulant absorbs moisture from the atmosphere, the degree based on the actual plastic compound, shipping and storage conditions, and ambient humidity on the production line. Things get steamy during reflow soldering as components are exposed to rapid temperature change. Trapped moisture turns to superheated steam, and the sudden change in vapor pressure in turn causes the package to swell. How much expansion takes place again depends on the composition of the plastic, the amount of moisture actually absorbed, the temperature, heating rate and plastic thickness. When the resulting pressure exceeds the plastic compound’s flexural strength, the package may crack, or, at the very least, interfacial delamination can occur. Bear in mind that the rate of diffusion varies with each package. In extreme cases, components could detach from the assembly. But in many cases, the defect goes undetected until the product fails in the field.

J-STD-033 addresses the problem of moisture-sensitive devices. It contains handling, packing, shipping and implementation recommendations and guidelines. The manufacturer of an MSD component must test and classify the device based on a maximum floor life at 30°C at 60% relative humidity (prior to reflow). The component is dry-packed with proper identification prior to shipment to the assembler. Per the spec, once the dry bags are opened, the components must be assembled and reflowed within specified time limits.

One interesting misconception about MSDs is that the exposure time clock stops when previously exposed MSDs are resealed in drypacks or stored in controlled-environment cabinets. The return of MSDs to a dry environment when not on the placement machine is common (and good) practice, but life is never that simple, is it? In reality, once components have been exposed to ambient conditions for more than an hour, the additional moisture is in there. Moisture diffusion at ambient temperature is a very slow process. In fact, it has been shown that the existing moisture gradient will continue to diffuse toward the center of the package, nearer to the die interface, where it has the propensity for doing the most damage.

Therefore, proper handling requires the total cumulative exposure time for the component be tracked through the manufacturing process, from incoming, repacking (i.e., from matrix tray to reel), on and off the placement system until the component is placed and headed for the reflow oven. Once the assembly has been reflowed, the clock resets on device moisture absorption. This is not a problem unless that component sees temperatures above 100°C (again) (which it just might, for instance, during rework or repair of the assembly, whether in the plant or the field). Thus, proper MSD care and handling can be a logistical challenge. Tracking can be performed by simple means such as a traveler, recording the time components are removed and returned to moisture barrier bags or dry cabinets, and the cumulative elapsed exposure time. Or, if you prefer, more sophisticated measures such as a barcode or RFID tag system (the closest to foolproof) can be employed. But track you must.

If that’s not ominous enough, bear in mind that Pb-free reflow thermal excursions expose components to higher peak temperatures. This approximately 25°C difference amounts to a significantly greater amount of vapor pressure. As a result, MSD ratings have been reclassified, increasing by one to three levels for parts used in Pb-free assemblies.¹ Many components now carry dual ratings – for SnPb reflow and for Pb-free temperatures.

Preventing ESD and MSD damage involves due diligence. If you have MSD- or ESD-sensitive components, this translates to “do or die.” Remember, we’re all in this together.

References

1. Tom Adams and Steve Martell, “The Moisture Sensitivity Standard Goes Pb-Free,” Circuits Assembly, January 2007.

Phil Zarrow is president and SMT process consultant with ITM Consulting (itmconsulting.org); itm@itmconsulting.org. He still bears the scars, physical and mental, of reflowing convection/IR ovens. His column appears bimonthly.