What Factors Will Affect the PCB Assembly Mechanism?Posted on 2020-07-07
From material fusion, PCB layout, prototype design, PCB engineering, PCB assembly to packaging and order delivery stage, attention should be paid to the influence of moisture in PCB manufacturing to avoid damage and other problems of PCB function. In addition, let us deeply understand the important measures to control the humidity level during the lamination process, the controls implemented during PCB assembly and control storage, packaging and transportation.
Rigid/flexible printed circuit board components, cable harnesses, boxed components or wire harness PCB components are made of various types of materials. These materials fully meet the attributes required for strong mechanical and electrical performance in electronic products used in all major industries around the world . It requires high frequency, low impedance, compact, durable, high tensile strength, low weight, multi-function, temperature control or humidity resistance. PCB is divided into single layer, double layer or multiple layers, depending on the complexity of the circuit. Among all serious issues that should be noted in the initial stages of PCB manufacturing. Humidity is the main factor that leads to space for electrical and mechanical failures in PCB operations.
How does moisture cause trouble on PCB assembly?
By being present in the epoxy glass prepreg, it diffuses in the PCB during storage, and when absorbed, moisture can form various defects in the PCB assembly. The wet process time in the PCB assembly process exists in micro-cracks or can form a home in the resin interface. Since the high temperature and steam pressure are parallel to the lead-free mechanism in PCB assembly, it will cause moisture absorption.
With the adhesive and cohesive failures in the printed circuit board leading to delamination or cracking. Moisture can make metal migration possible, resulting in a low impedance path with varying dimensional stability. As the glass transition temperature decreases, the dielectric constant increases and more technical damages occur. It causes a reduction in circuit switching speed and a high propagation time delay.
The main effect of moisture in PCB assembly is that it reduces the quality of metallization, lamination, solder mask and PCB manufacturing processes. Due to the influence of moisture, the limit of thermal stress becomes excessive as the glass transition temperature decreases. Sometimes it can also cause severe short circuits, which can lead to water ingress and ionic corrosion. Other common properties of hygroscopicity in printed circuit board components include flame retardancy or delamination, increased (DF) dissipation factor and (DK) dielectric constant, thermal stress on plated through holes, and copper oxidation.
Method for reducing moisture in PCB manufacturing
Regardless of whether PCB manufacturing uses simple or complex technologies, there are many operations in PCB engineering that require wet processes and removal of residual moisture. The raw materials used in PCB manufacturing need to be protected during storage, handling, and handling stress during PCB assembly. The following is a brief guide to implementing controls at various stages of PCB operation.
Lamination is a dehydration step in PCB manufacturing. Because the core and prepreg are stacked together, the layers are glued into the laminate. The main factors controlled during the lamination process are temperature, elapsed time and heating rate. Sometimes when the dryness is low, take measures to reduce the vacuum to reduce the possibility of attracting internal voids absorbed by moisture. Therefore, the use of gloves when handling prepreg can control the degree of moisture. This reduces cross-contamination. Non-corrosive humidity indicator cards should be flexible so that humidity levels can be resolved when needed. Laminates should have a short wash cycle and be effectively stored in a controlled environment, which helps prevent moisture pockets from forming in the laminate.
Post lamination process and PCB assembly
After drilling, photo imaging and etching operations in PCB manufacturing, the moisture absorption rate captured in the wet process is higher. Screen printing curing and solder mask baking are processed steps to ease entrapped moisture. It is even more keen to manage storage conditions by minimizing the retention time interval between steps, which is more effective in reducing the level of moisture absorption. By ensuring the early stages of PCB lamination, the circuit board is sufficiently dry to help reduce baking operations after lamination. In addition, high-quality surface treatment is used to prevent cracks during the drilling process, and the humidity of the residue is removed by baking before the hot air solder leveling process. Baking time should be maintained by considering the determined level of moisture content, the complexity of PCB manufacturing, PCB surface treatment, and sufficient thickness required by the circuit board.
Therefore, it is important to understand the latest situation of moisture effects in PCB manufacturing. To avoid failures, damage and short circuits on the PCB, while increasing rework costs. Now, researchers are about to introduce more advanced solutions that use environmentally friendly PCB technology to control the moisture element in every step of PCB manufacturing, thereby saving time, energy and cost.