Moisture Absorption in Printed Wiring Boards

Objective

Background

Laminate manufacturing is a process consisting of encapsulating a ply (or several ply) of glass fabric within a polymeric resin.  The fabric-resin combination provides dielectric as well as mechanical and thermomechanical properties.  However, since the resins used in laminates are hydrophilic, the assembly can be susceptible to degradation and performance failure mechanisms driven by environmental moisture.

Moisture can reduce the quality of lamination, metalization,  solder masking, and manufacturing steps associated with board fab and assembly.  Moisture reduces the glass-transition temperature (Tg) so that excess thermal stresses can cause damage and increases the dielectric constant, 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 (CFF).

 In this study, laminates with resin systems of FR-4, HTFR-4, PI, CE, and BT (TABLE II) were obtained from two suppliers (A & B).  All of the laminates were woven E-glass fabric in either 0.038 or 0.053 cm approximate thickness.  Tests were conducted to evaluate and model the equilibrium moisture content of a laminate under various isothermal conditions.  The tests covered relative humidities of 40, 50, 60, 70, and 85% and temperatures of 40, 50, 60, 70, and 85^oC.  Two coupons of each laminate type were placed in isothermal environments at a constant relative humidity until equilibrium was reached, at which time the laminate mass was measured and returned to the environmental chamber.  In order to measure the moisture content of a PWB, the Rockwell Science Center provided a capacitance monitoring method to allow for in-situ evaluation of a laminate’s moisture content.
 

Work Accomplished

• The electronic industry’s most common laminates (FR-4, HTFR-4, PI, CE, and BT) were investigated for their equilibrium levels of moisture and diffusion rates and monitored for their moisture content as a function of electrical capacitance.  Isothermal sorption tests were performed on the laminates to find the equilibrium moisture content in each laminate at various environmental conditions.  A model was developed to assess the equilibrium content as a function of both temperature and humidity.

 
• Each of the laminates were characterized for their diffusivities, through sorption-time experiments.  Data from the experiments was used to evaluate a diffusion coefficient.  The Fickian model was used to facilitate the evaluation.

 
• The moisture content of each laminate was evaluated as a function of electrical capacitance via a capacitance monitoring plate attached to each laminate.  Since the data indicated that the two parameters were linearly proportional, a factor was calculated to convert the change in electrical capacitance to moisture content.

 
• The results of the moisture content calibration were subsequently applied to a PWB with a capacitor plate on each of its laminates.  The capacitance method of moisture measurement in PWB shows slower moisture ingress than the theoretical moisture diffusion based on laminate experiments.  This is due to impermeable barrier to moisture ingress in the metal copper foil of the capacitor plate which is about 80% of the plate area.  It can be concluded from this that the presence of copper plane areas in real circuit boards forms barriers to moisture movement and therefore, circuit boards with large copper planes may require longer bake-outs than standard boards.