Contract manufacturers tend to showcase their latest equipment for printed circuit board (PCB) assembly. As a buyer, you should also audit the less exciting aspects of your manufacturing partners' operations, to see if these areas are keeping pace with the more visible changes.
One such aspect is humidity control. Here a manufacturer has to balance opposing goals. On the one hand, we want high humidity to reduce the risk of electrostatic discharge (ESD) damage. On the other, we want low humidity to maximize the floor life of moisture sensitive devices (MSDs). Both of these factors became more important as the industry moved to smaller SMT components, which are more sensitive to both ESD and moisture.
A good, safe, quality work environment has plenty of exhaust systems and a steady supply of incoming fresh air. In many parts of the country, for much of the year, this fresh air needs to be constantly humidified to maintain the IPC-recommended range.
For example, on a typical winter day here in western New York, the outside air is around 20°F with 75% relative humidity. When we bring that air inside and heat it to 70°F, the relative humidity drops to just 12% —warming the air increases its capacity to hold moisture. That's pretty dry: for comparison, the Sahara Desert averages 23% relative humidity.
Dry air means static. While any good PCB assembly facility uses heel and wrist straps along with grounded floor mats, carts, and work areas to protect components from static discharge, a surprising number overlook the importance of maintaining relative humidity above 30%. This is an essential first line of defense against ESD damage.
What might you look for in a humidity control system? First, see if they have one. In warm, humid climates a separate humidity system is probably not needed; an air conditioning system will take care of it. In areas with cold winters or dry summers, a dedicated humidity control system is a must.
Next, check out the relative humidity in the assembly area. The control system should have an integrated display to tell you the current relative humidity. If the system is running and the humidity is below 30%, the system is either set too low or it is too small to keep up with the heating and exhaust systems. This is a common problem as manufacturers add more equipment with corresponding exhaust systems, increasing the air turnover in the facility.
Fortunately, new humidity control systems include small, high-pressure misting systems that are quiet and more energy-efficient than old-fashioned boilers, while still providing the necessary manufacturing humidity levels.
Now, with relative humidity above 30%, we have to worry about keeping components dry enough. Today's smaller components and ball grid arrays (BGAs) are more likely to require special handling as moisture sensitive devices (MSDs). MSDs have a limited number of total hours they can be out of dry storage before soldering. This number depends, in part, on the humidity of the ambient environment. For this reason we try to keep the humidity at the low end (30%) of the IPC-recommended range. Above 60%, the floor life of MSD has to be derated.
MSDs are kept in dry storage when not in use on the pick-and-place lines. Traditionally we seal them in moisture barrier bags, along with a desiccant and a moisture indicator card. Dry cabinets are now becoming more common as well, especially in "high mix" operations where a tube or reel of MSDs could be pulled out of storage and used several times a week over several months. The dry cabinet stores the tubes and reels at 0.1% humidity while providing more convenient access for quickly kitting frequently-used parts.
Humidifiers and dry cabinets may not be exciting, but they belong on your list of things to look for in your contract manufacturers' operations. Take a look at the video below to learn more.
From an ESD perspective, low RH is acceptable if you have an ESD program in place such as ANSI S20.20. Surface resistivity of materials evaluated by the ANSI/ESD S7.1 methodology will be conditioned at 12% RH for 72 hours before measurement. This approach is applicable to all sorts of materials. This emphasizes the importance of using properly grounded dissipative materials for flooring, shoes, workstation surfaces, packaging and transport materials on the production floor. If you cannot avoid using materials prone to triboelectric charging effect, then a mitigating approach is to neutralise them with an ioniser. Now from a human comfort perspective, I suspect ASHRAE recommendations will not go as low as 12%RH and this should bound your minimum RH conditions.
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