The “Popcorning” Phenomenon Caused by Moisture Absorption in PCBs: Strict Baking Processes Ensure Substrate Assembly Quality

Introduction

In actual electronic manufacturing, moisture has always been a hidden threat to the reliability of microelectronic devices and printed circuit boards. When moisture from the environment penetrates the substrate or the interior of a plastic-encapsulated chip, these trace amounts of moisture instantly vaporize and expand during the subsequent high-temperature reflow soldering stage. This physical damage—caused by a sudden surge in internal vapor pressure, resulting in delamination and cracking of device packages or PCB substrates, accompanied by faint popping sounds—is vividly referred to in the industry as the “popcorning” phenomenon. In high-precision PCBA manufacturing, this defect is often hidden and directly threatens the long-term service life of the final electronic product.

Mechanisms of Moisture Accumulation and Physical Evolution Within the Substrate

Printed circuit boards are not absolutely dense solids, their primary materials, such as FR-4 resin and glass fiber cloth, possess certain moisture-absorbing properties. If raw materials are exposed to uncontrolled temperature and humidity conditions during storage, transportation, or while in the workshop, water molecules in the environment will penetrate—through diffusion—into the resin matrix, the interface between the resin and glass fiber, and the gaps in the copper foil grid.

When moisture-laden bare boards enter the SMT production line, they must undergo a lead-free reflow soldering process at temperatures as high as 240 to 260 degrees. At these high temperatures, the water molecules trapped inside the material rapidly vaporize. According to the ideal gas law, the saturated vapor pressure of water at 260°C can expand sharply to nearly 5 MPa. When this instantaneously concentrated vapor pressure exceeds the interlaminar bond strength of the FR-4 resin material or the interfacial adhesion between the chip encapsulant and the lead frame, microscopic delamination occurs within the substrate.

The Devastating Impact of the “Popcorn Effect” on PCBA Manufacturing Quality

The destructive power of the “popcorn effect” is often not limited to the material’s surface, its core hazard lies in altering the circuit board’s mechanical structure and electrical performance. Under the combined effects of soldering thermal stress and vapor pressure, the most direct manifestation is delamination and bubbling—that is, the appearance of visible bulges on the board’s surface or internal voids.

In multilayer board structures, this internal interlayer delamination triggers a chain reaction. It readily severs metallized vias that penetrate multiple layers, causing fractures in the copper walls of the vias. This results in intermittent contact issues or even direct open circuits. For high-density interconnect (HDI) boards, the release of internal stress can also pull on adjacent fine conductors, inducing microcracks. This microscopic damage is difficult to detect thoroughly during routine functional testing on the production line; however, once the product is put into service and subjected to environmental temperature cycling and vibration, the cracks will propagate rapidly, ultimately resulting in catastrophic early failure.

Strict Baking Process Specifications Based on MSD Class and Material

The most effective method to prevent the “popcorn” phenomenon is to perform mandatory thermal baking to remove moisture from the substrate and moisture-sensitive devices prior to assembly. Depending on the board thickness, number of layers, and copper foil thickness, process engineers must establish customized baking parameters to precisely control temperature and duration.

For standard double-sided or multilayer FR-4 bare boards, if the time since opening the package exceeds the controlled timeframe, they are typically placed in a precision oven set to 120 degrees ± 5 degrees for continuous baking for 4 to 6 hours prior to PCBA surface-mount assembly. For complex boards thicker than 2.0 mm or with more than 12 layers, the baking time must be extended to 8–12 hours to ensure that moisture deep within the board is thoroughly removed. Additionally, for moisture-sensitive components such as IC chips, it is necessary to strictly adhere to standards and perform dehumidification baking at 125°C for varying durations based on different moisture sensitivity grades, forcibly reducing the internal moisture content of the components to below the safe threshold.

End-to-End Humidity Control Ensures High-Quality Delivery

Baking is merely one stage in dynamic humidity control, comprehensive PCBA quality assurance relies on end-to-end temperature and humidity management throughout the entire production process, from material receipt to soldering. After baking, substrates and components must be put into use immediately, with surface-mount assembly and reflow soldering completed within a specified timeframe in an SMT workshop maintained at an ambient temperature of 22±3°C and a relative humidity of 40% to 60%. If production schedule changes cause materials to remain in the workshop beyond the allotted time, secondary moisture-proofing measures must be taken. Semi-finished products and bare boards that cannot be processed immediately must be stored in ultra-low-humidity drying cabinets with a relative humidity of less than 10%, or resealed in anti-static vacuum bags containing desiccant and humidity indicator cards.

By seamlessly integrating vacuum packaging, aging monitoring, dynamic baking, and an online material traceability system, we achieve digital, closed-loop control over humidity risks, thereby fundamentally eliminating the occurrence of “popcorn” defects.

Company profile

Zhejiang NeoDen Technology Co., LTD., founded in 2010, is a professional manufacturer specialized in SMT pick and place machine, reflow oven, stencil printing machine, SMT production line and other SMT Products. We have our own R & D team and own factory, taking advantage of our own rich experienced R&D, well trained production, won great reputation from the world wide customers.

In this decade, we independently developed NeoDen4, NeoDen IN6, NeoDen K1830, NeoDen FP2636 and other SMT products, which sold well all over the world. So far, we have sold more than 10,000pcs machines and exported them to over 130 countries around the world, establishing a good reputation in the market. In our global Ecosystem, we collaborate with our best partner to deliver a more closing sales service, high professional and efficient technical support.

We believe that great people and partners make NeoDen a great company and that our commitment to Innovation, Diversity and Sustainability ensures that SMT automation is accessible to every hobbyist on everywhere.