PCB Fabrication

Thermal Reliability of PCB Solder Mask Materials

    A polymer material hardened at the time of bare-board fabrication is used in solder mask formulation the PCBs of today. Solder masks ensure the protection of surface layers of the conductor. Solder masks too have peculiar thermal properties that need to be looked into during assembly.

    The base material of a solder mask is polymer and it succumbs to the problems that other polymers face. Polymers and their properties are not a concern for the designers but to analyze the reflow cycles and reliability of the solder mask is nevertheless important while you're building PCBs which are to be used in high-grade tech which have to run for prolonged periods in extreme conditions and unpredictable environments.

    Hence the fact of the hour is that the material integrity of the solder mask has to be kept intact in order to get a desirable end result.

    Factors That Determine Reliability of Solder Mask:

    Being a complex mixture of chemically active ingredients like pigments, fillers, solvents, resins, both photoreactive & crosslinking and surfactants, their properties are the deciding factors of the intensity of the bond between solder mask materials and PCB. The solder mask hardens into a fine film while it is curing. Environmental ageing factors and degradation of chemical components can also affect the mixture.

    Long term reliability of a cured solder mask can be affected by multiple factors. They can be broadly stated in 3 categories-

    • Process of curing and the parameters decided while curing
    • Process of assembly, temperature while soldering and reflow cycles
    • Ageing occuring by moisture, chemicals and heat

    The Process of Curing:

    The process of curing generally affects thermal as well as thermomechanical properties of materials used in solder mask via hardening during crosslinking. Hence, a vendor will always recommend a curing temperature and time line. If there is even a slight deviation then it can deviate from the initial value given by the vendor. Prolonged ageing can take the Tg from 150°C to higher levels like 170°-180°C.

    The Effect of Reflow

    Reflow soldering includes putting the board for thermal cycling in an oven. A double sided PCB will have to be reflowed twice. There isn't a limit to the number of thermal cycles a board can experience.

    Reflow and thermal cycling during reflow is a form of extreme aging. Evidently, controlling the multiple reflow cycles and temperature is necessary otherwise there is a substantial decrease in the life cycle of the equipment of technology it's a part of.

    The Embrittlement Issue

    Hardening of the solder mask occurs when a polymer ages in a particular environment wherein there is crosslinking and loss of volatiles. This phenomenon leads to embrittlement and causes the mask to break and flake away. Larger excursion are capable of fracturing and delaminating from the PCB.

    Nearby conductors are not an issue during flaking as solder mask materials are insulators. Nevertheless, it exposes internal materials along with copper which can experience corrosion while being exposed to moisture and/or chemicals.

    Embrittlement is inevitable but it can be delayed by proper storage of solder mask liquid and bare board prior assembly. The end environment has to be considered as an important parameter while choosing the solder mask material. The vendor might be the most trusted source for guidance in these cases.

    Majorly, there are no specifications as to which solder masking material has to be used. Designers usually choose a standard performance material while they design the documentation. Multiple materials meet with standardized requirements. Sometimes when higher temperature resistance is required, some specific decisions have to be made.

    A proper knowledge of processing parameters and influence on materials has to be attained by the designer in case they want to take control over thermomechanical properties. Consistent testing can ensure a precise curing procedure to produce a fine film which can assure prediction of the reliable lifetime. These values can be a benchmark to conduct investigations for reliability.

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