Using Bill of Material Management to Prevent Problems with Moisture Sensitive Devices

Up here in the Pacific Northwest we are once again in the rainy season. My family doesn’t like it, my friends don’t like it, and even my dog doesn’t like it. The other night I had to pull the ladder out of the garage to unclog a gutter during a rainstorm. That was definitely not on my list of fun things to do, especially at 11:00 at night.

We are used to the rain though. We know how to direct the water run-off so that it doesn’t pool up, we have our raincoats, and we do our best to keep our gutters cleared. By staying ahead of the moisture we can avoid problems. In the same way, by staying ahead of moisture problems with the components on a printed circuit board, we can help our manufacturers to build a better board.

As PCB designers we understand what it takes to place and route a board, but we don’t always understand the processes used to manufacture the board. One of those processes that we may not be that familiar with is dealing with moisture problems in PCB components. A closer look helps to diagnose the problem and to understand how to move forward in solving it. Obviously, you can’t combat this kind of a thing with a raincoat, but knowing more about moisture problems in PCB components will help you to work with your manufacturer to control it.

What Are Moisture Sensitive Devices?

Moisture sensitive devices, or MSDs, are PCB components that are encapsulated by plastic compounds or other organic materials. Depending on the component and the materials that it is made from, moisture from humidity can be absorbed into the part. When these trace amounts of moisture are exposed to the heat of a solder re-flow oven, they can expand and cause damage to the component.

To protect these devices from absorbing moisture, their exposure to the open air and humidity must be limited. What makes the job of the manufacturer more difficult is that different components and different materials will absorb moisture at different rates. To help manufacturers work with these different components, moisture sensitivity classifications have been defined that components are assigned to. These definitions guide manufacturers as to how long each component can be exposed to the open air. In addition, joint IPC and Jedec standards exist to define the best handling practices for MSD components.

Problems That Can Happen On Your PCB Due To MSDs

When the trace amounts of absorbed moisture heat up and expand in a PCB component, it can cause a problem known as the popcorn effect. The component may actually “pop” due to the expanding moisture, just like a boiling pot popping its lid. In addition to expansion when heated, moisture can also be the cause of the following problems:

• Damage to the component substrate, die, or the wire bonds due to the popcorn effect.
• Condensation during reflow causing bad solder joints.
• Oxidation of the components’ metal feet, which can also cause bad solder joints.

Many of these problems are extremely small and difficult to detect. The damage is often invisible requiring X-ray equipment to find. This can result in intermittent circuit board failures that may not show up until the board is in regular use.

Bill Of Material Management Can Help

Problems with MSDs often occur because incorrect components are specified on the bill of materials (BOM). The wrong components may get ordered and then sit for too long, or the wrong MSD classification may be attached to the ordered parts. Manufacturers identify CAD to BOM mismatched data as being one of the leading problems in manufacturing delays.

BOM management tools can help to eliminate these problems by coordinating design, procurement, and manufacturing together within the same system. Engineering will find and request the components that they need from the BOM management tools ensuring that they are selected from approved vendors. Procurement will order these components as soon as the online request comes through from engineering. All of this data is uploaded real-time to the online bill of materials where manufacturing has immediate access to it.

With manufacturing having immediate access to the online bill of materials, they will be prepared to deal with moisture sensitive devices when they are ordered. Part inventories will be ready and your PCB design will be manufactured without the delays caused by incorrect components due to errors in the PCB BOM.