What Is Moisture Absorption for PCB?
When printed circuit boards (PCBs) are under fabrication in wet processes, exposed to humid ambient conditions an absorption of moisture will occur, which calls PCB Moisture Absorption.
Failure Mechanisms of PCB Moisture Absorption
Moisture can accelerate various failure mechanisms in printed circuit board assemblies. Moisture can be initially present in the epoxy glass prepreg, absorbed during the wet processes in PCB manufacturing, or diffuse into the circuit board during storage. Moisture can reside in the resin, resin/glass interfaces, and micro-cracks or voids due to defects.
Higher reflow soldering temperatures associated with lead-free assembly processing increase the vapor pressure, which can lead to higher amounts of moisture uptake compared to eutectic tin-lead reflow processes. In addition to cohesive or adhesive failures within the printed circuit board that lead to cracking and delamination, moisture can also lead to the creation of low impedance paths due to metal migration, interfacial degradation resulting in conductive filament formation, and changes in dimensional stability.
Studies have shown that moisture absorption can also reduce the glass-transition temperature and increase the dielectric constant, leading to a reduction in circuit switching speeds and an increase in propagation delay times. How to mitigate PCB failures caused by moisture absorption?
Effects of Moisture in PCBs
Moisture can reduce the quality of lamination, metallization, solder mask, and manufacturing steps associated with board fabrication and assembly. Moisture reduces the glass-transition temperature (Tg) so that excess thermal stresses can cause damage. Moisture also increases the dielectric constant (Dk), leading to a reduction in circuit switching speeds and an increase in propagation delay times. Moisture ingress can also facilitate ionic corrosion, leading to both open and short circuits. Additionally, moisture that accumulates at the interfaces of the resin and fiberglass can cause interfacial degradation, resulting in conductive filament formation. The common failure mechanisms attributed to moisture absorption in PCBs are listed below. Familiarity with these mechanisms is an essential input in selecting the most appropriate strategy for reducing moisture in PCBs:
- Entrapped moisture can cause blistering or inner layer delamination.
- Excessive moisture increases dielectric constant (Dk) and dissipation factor (Df), leading to changes in circuit switching speed.
- Since moisture acts as a plasticizer, it reduces the glass-transition temperature (Tg), which in turn increases stresses on PCB features such as plated through-holes.
- Oxidation of copper surfaces leading to poor wettability of finishes and solder.
- Ionic corrosion causes electrical opens or shorts.
- Interfacial degradation can result in a reduced time to failure due to conductive filament formation (CFF).
Measures to Remove Residual Moisture
Many steps involved in the PCB fabrication are wet processes and some measures are required in order to remove residual moisture.
- Controls implemented during lamination
- Controls implemented during inner layer production and after lamination
- Controls implemented during Inner layer production and after lamination
- Controls (especially board baking) implemented before PCB assembly