At one time, expiration dates weren’t as regulated on perishable groceries as they are now, which led to some adventures in eating that I would rather not repeat. For example, there were more than a few times when the carton of milk that I opened yielded something considerably more disgusting than what I was expecting. I’m not sure, but I think that Captain Crunch may have gone on full war alert at the threat of having that milk poured on him. Because of this, I still tend to sniff everything that I pull from the refrigerator to the great amusement of my entire family.
When you are having a printed circuit board built, however, the sniff test just won’t cut it. Some materials and components will have more stringent shelf-life specifications than others. If those specifications aren’t followed, it could result in a lot of problems with the production of the board. Here is some more information on the shelf life of printed circuit boards as well as their building materials and components, and how your contract manufacturer should be working with them.
Why Does the Shelf Life of Printed Circuit Boards Matter?
The primary concern of the PCB assembly process is being able to create a good solid solder joint between the leads of the components and the bare board. Over time, oxidation will compromise the soldering surfaces in both the bare boards and the components, causing poor solder joints. These soldering failures could cause intermittent connectivity problems in the assembled PCB or outright failure of the finished board.
To prevent these failures, both the components and the bare boards will include an attached shelf life designating when they should be used. This shelf life is based on the plating and solder surfaces, and with proper packaging, traditional lead-based finishes will last for 2 years. Surface finishes that are lead-free should be used within a year, while more exotic finishes such as immersion silver (IAG) or organic solderability preservatives (OSP) have a shelf life of 6 months or less.
In addition to the components and the bare boards, the different materials used in the assembly processes of a PCB will have a shelf life attached to them. These materials include adhesives, solders, and other chemicals. The shelf life for these materials has a large range, and the length of their shelf life depends on what they are doing. Typically, those materials that are more toxic or designated for higher performance applications will have shorter shelf lives.
Moisture-Sensitive Devices Used in PCB Assembly
Another concern of the assembly process is how the components will handle the harsh environments of the assembly process. The components mounted to circuit boards will be exposed to a lot of heat in the reflow ovens, and any trapped moisture in a component will expand and turn to steam. As this steam escapes, it can cause internal problems within the component, or it may even cause the component bulge and pop.
Those components that are susceptible to moisture are known as moisture sensitive devices (MSDs), and they will come shipped in vacuum-sealed moisture barrier bags. This packaging prevents them from absorbing moisture from the air prior to their use. Once the package is opened, the component must be assembled to a board within a designated period of time, or the component must be “baked” to remove any moisture. There are different classifications of moisture sensitivity levels in components that are designated by the IPC/JEDEC J-STD-020E standard, while the handling and baking of these devices are covered in the IPC/JEDEC J-STD-033C-1 standard.
Here are the shelf life classifications for MSDs according to J-STD-020:
|Moisture Sensitivity Level|
|MSL 2||1 Year|
|MSL 2a||4 Weeks|
|MSL 3||168 Hours|
|MSL 4||72 Hours|
|MSL 5||48 Hours|
|MSL 5a||24 Hours|
|MSL 6||Mandatory bake before use|
How Your Contract Manufacturer Manages PCBs, Materials, and Components
Controlling the shelf life of the bare boards, components, and materials used in the assembly of printed circuit boards is an important part of the PCB manufacturing process. To ensure that shelf life isn’t exceeded, a PCB CM will carefully track the part numbers of all of these different items using customized inventory systems. These systems are documented, and the employees are well-trained in their use and are monitored with periodic audits.
At VSE, we understand the importance of PCB component and materials shelf life is to the quality of the finished circuit board, and have designed our processes to support that. Here are some of the steps we take to make sure that only fresh materials and components are used in the assembly of your circuit boards:
- First in and first out (FIFO) inventory control.
- Component labeling for tracking.
- Strategic purchasing to prevent long-term storage in-house.
- Scrapping of materials like adhesives that don’t get used by their expiration dates.
- Monitored and alarmed refrigeration for cold product storage such as solder pastes.