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While electronic devices continue to advance, in terms of technology and miniaturisation, new challenges emerge for manufacturers when it comes to assuring the reliability of printed circuit boards. One of the biggest threats to PCBs is moisture.
This doesn’t mean dropping a mobile phone in the bath. Far more subtle, this kind of damage relates to moisture-sensitive devices, known as MSDs, which are highly susceptible to humidity. In a relatively short space of time, this can render a device useless.
As our electronic devices get smaller, leading to much slimmer PCBs, the problem of humidity can get bigger. A thinner case on any device can increase a printed circuit board’s risk of being damaged by moisture.
What is an MSD?
Moisture sensitive devices are components packaged in plastic that are sensitive to atmospheric humidity and the damage it can cause. When the electronic components are not sealed hermetically, they can absorb moisture from the environment. This can expand rapidly and damage the device – sometimes irreparably.
The permeable nature of the packaging enables moisture to enter via diffusion. It then builds up at the interfaces between the different materials. This causes product defects and a reduced production yield that can significantly hamper a manufacturer’s bottom line.
As a result of various factors including a reduction in the package body thickness; higher reflow temperatures; an increase in the use of plastics; and manufacturing in locations where there are high humidity levels; the challenges of moisture sensitivity have increased.
How do manufacturers combat this?
There are two manufacturing standards to help electronics manufacturers to combat the problems of moisture sensitivity.
Standard IPC/JEDEC J-STD-020E provides a classification system to determine the initial reliability level of the device. Once identified, the manufacturer can package, handle and store the devices in an appropriate way to reduce mechanical and thermal damage during the reflow process.
Standard IPC/JEDEC J-STD-033D provides guidance aimed at end-users about the packing, handling, shipping and application of MSDs. It also offers advice on procedures to limit reliability degradation and to boost yield.
The standards classify the levels of moisture sensitivity, based on a scale of one to six, depending on how susceptible the devices are to moisture-induced damage. Devices with the highest levels have the shortest floor life. These devices remain safe to reflow only with the smallest amount of exposure to the environment.
An increase in moisture-related damage relates directly to the modern drive for smaller packaging and more compact electronic devices. Consumers want lighter packages, but thinner devices are more sensitive to humidity by their nature.
In addition, a move towards lead-free alloys means 15% to 25% more energy is needed to manufacture boards with lead-free reflow. Increasing the temperature can lead to more damage and botched yields.
Tackling moisture sensitivity
Manufacturers can take various steps to tackle moisture sensitivity in PCB manufacturing. These largely relate to keeping sensitive equipment dry and protected from the environment.
The most common solution today is baking the components and monitoring them carefully to make sure the moisture is controlled. Known as a dry pack, the process uses ovens to remove the moisture from the packages. Once the moisture has gone, the packages are heat or vacuum-sealed inside anti-static bags.
A humidity indicator card is also enclosed, along with a desiccant to keep on top of moisture during handling. The bag’s interior humidity must stay below 10% at a temperature of 25°C. Should any of the components be accidentally exposed to dampness, they may require a re-bake, aimed at reversing any moisture the device has collected.
As an alternative to bags, manufacturers can use dry boxes to provide humidity controls for a longer period of time. Dry boxes should keep the humidity lower than 5%. They are controlled digitally and can also combat handling errors.
In the most extreme high-risk cases, nitrogen cabinets are used to remove oxygen and eliminate moisture. While extremely effective, this method is considerably more expensive, so it may not be cost-effective for the average manufacturer.
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