In recent years, I’ve grown to love making stovetop popcorn. Cooking on the stovetop is not much more complicated than throwing it in the microwave, with one wrinkle: storage. Since I like to buy my popcorn kernels in bulk, I must be aware of their age and moisture content, as both affect the cooking time and can leave unpopped kernels behind.
Unlike cooked popcorn kernels, which aim to heat the moisture inside the kernel to pop, most people don’t enjoy their PCBs cracking open. The root cause is the same: trapped moisture in the PCB materials rapidly expands with high temperatures, increasing the pressure and stressing the surrounding material. To prevent PCB outgassing, the board’s careful storage and baking treatments eliminates residual moisture before high-temperature exposure.
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Moisture: Addressing PCB Outgassing at the Source
Outgassing is the detrimental and destructive process of trapped moisture within the board materials vaporizing during high-temperature processes and violently erupting from the board, causing cracks, fissures, holes, and other areas of localized material depletion that can concentrate strain and accelerate board wear. Numerous processes in PCB manufacturing expose the board to sufficiently high temperatures that cause immediate vaporization of trapped water (other volatiles can pose an outgassing issue, but water is the primary culprit), leading to board scrappage. The best prevention of outgassing is limiting the amount of moisture around the PCB during manufacturing processes, but baking the board materials or fabricated board at a temperature slightly above water’s boiling point can gently remove absorbed moisture before submitting the board for higher temperature excursions (lamination, soldering, etc.)
When exposed to high temperatures from lamination and various soldering processes, trapped moisture rapidly expands, possibly resulting in externally visible cracking. While outgassing technically only releases trapped moisture due to increasing pressure from vaporization and thermal excitation, internal cracking and delamination can be a significant board failure mechanism and are more challenging to diagnose without non-destructive evaluations. Within the laminate, moisture exists in a bound state (trapped at polar sites bonded to hydroxyl groups) or free-flowing, affecting how the moisture damages the substrate upon heating. Bulk diffusion of free-flowing water can be more destructive overall, but concentrated wicking and capillary action at interfaces can more easily lead to catastrophic failure due to the concentrated area.
Temperature calibration is critical for setting the oven cycle. It’s worth remembering that heat can damage board performance and reliability over time. The correct temperature will depend on the board materials, but follow these general guidelines:
- The bake temperature is less than the glass transition temperature (Tg). Note that moisture depresses the glass transition temperature; capacitance measurements can more precisely determine the moisture content.
- Gravimetric measurements of the laminates or fabricated board help determine moisture content during baking and when the bake cycle is complete. Measure consistently and continually during the baking process to limit thermal wear. At maximum, the baking process should take no more than forty-eight hours (J-STD-033).
- Venting and cleanliness of the oven will ensure unimpeded airflow throughout the oven; ensure oven maintenance is up-to-date and the oven’s immediate surroundings are particulate-free and vacant.
- Sufficient gaps are necessary between the boards to prevent overlapping surfaces, which can trap or lock in moisture.
- Whether horizontal or vertical, the fan and airflow path must match the orientation of the boards to maximize drying potential.
Baking Cost and Reliability Concerns
Of course, baking adds additional lot costs due to higher energy consumption while reducing throughput for high-volume fabrications. It can also undermine assembly by reducing the effectiveness of surface finishes for solderability (e.g., OSP). Additional quality/reliability concerns like excessive intermetallic growth are also a tradeoff to weigh. Surface degradation (i.e., stripping and refinishing) may be more acceptable for moisture removal than the thermal wear associated with baking. Manufacturers can determine baking suitability by evaluating the moisture content of the proposed laminate materials: lead-free processes should not exceed 0.1% by weight, whereas low-temperature assemblies should not exceed 0.2%.
Beyond moisture trapped in laminates purchased from suppliers, subtractive manufacturing processes like etching and drilling provide additional avenues for moisture absorption. Fabricators must limit the time between processes outside humidity-controlled environments that expose the board to atmospheric moisture. Post-baking, manufacturers must carefully control the cool-down environment to prevent the reuptake of atmospheric humidity. Packaging has similar concerns but must also account for a broader range of environmental conditions than a board would encounter on a manufacturing floor. Moisture barriers and desiccants keep moisture away from the laminate, but manufacturers may also include a laminate witness coupon functions as a control for the packaged board.
Is baking necessary in all cases? Proper humidity control by suppliers/manufacturers, coupled with the appropriate protective packaging and indicators, makes thermal treatment of prepreg materials less necessary before and after lamination. However, when in doubt, baking (done carefully) offers vastly more benefits than downsides for manufacturing, ensuring long-lasting quality and performance of the board.
Your Contract Manufacturer Improves Lot Yield and Quality
PCB outgassing is a significant failure mechanism: even when it doesn’t cause immediate catastrophic failure, crack formation introduces new failure modes. Preventing outgassing starts with careful materials management from the supplier down, and manufacturers must keep track of the time board materials are kept out of storage to prevent moisture accumulation. When searching for PCB quality, designers need manufacturers and suppliers who consider all aspects of defect prevention. At VSE, our engineers are committed to building electronics for our customers, including a trained staff and certified QMS facility. We’ve been realizing life-changing and life-saving electronics for over forty years with our manufacturing partners.
