The standard solder mask formulation used in modern PCBs is a polymer material that is cured and hardened during bare-board fabrication. The solder mask used in a PCB is an important tool that helps protect conductors on surface layers and it aids assembly of the bare board. But just like other materials used to build a PCB, solder mask materials have important thermal properties, and they can be affected by processing steps during curing and assembly.
The solder mask material that is used in a PCB is polymer, so it can be susceptible to the same degradation problems as any other polymer. While designers don’t necessarily need to be experts on polymer chemistry to understand solder mask degradation mechanisms, some smart design choices that determine the number/consistency of reflow cycles will affect solder mask reliability over the long term. Especially when you consider that some devices will be deployed in the field for periods spanning years (or even decades), the reliability of these materials should be considered for long term operation success.
What Determines PCB Solder Mask Reliability?
Solder masks include multiple chemically active ingredients, including many photoreactive resins/binders, crosslinking resins, fillers, pigments, surfactants, and solvents. The complex mixture of chemicals determines how the solder mask material adheres to the PCB during deposition and how it hardens into a thin film during subsequent curing. Once the mask is cured, the remaining chemical mixture can be affected by exposure to environmental factors that cause aging and chemical degradation.
There are three broad categories of factors that will affect the long-term reliability of a deposited and cured solder mask:
-The curing process and parameters during curing
-Assembly process, specifically the temperature during soldering and number of reflow cycles
-Aging induced by exposure to heat, moisture, or chemicals during operation, including thermal cycling
Generally, the curing process can affect the thermal and thermomechanical properties of LPI solder mask materials. The curing process causes the deposited solder mask to harden via crosslinking, and LPI solder mask material vendors will specify a recommended curing time and temperature. Deviations from this can affect the Tg value of the cured material, causing it to deviate from the vendor’s rated value.
It is difficult to make generalized statements about exactly how curing affects the Tg value, as well as other mechanical properties of the cured solder mask. However, prolonged agiang (hundreds of hours) generally increases Tg to higher values. Typical rated Tg values are near 150℃ before long-term aging, and they can reach 170-180℃ after prolonged aging.
How Reflow Affects Reliability
When a board is put through reflow soldering, the board will have components placed and it will be thermally cycled in an oven. For standard double-sided PCB assembly, the board will be reflowed twice, assuming no rework is needed. In terms of reflow withstand requirements, there is no specific limit on the number of thermal cycles a board would be allowed to experience. As an example, some telecom companies require that a board be able to withstand up to 6 thermal cycles without degradation of the bare board or the components.
The other thing to remember about reflow and the thermal cycling occurring during reflow is that this is a form of extreme aging. This is one reason why multiple reflow cycles should be limited and the reflow temperature should be controlled during assembly. Large excursions from repeated reflow can preage the solder mask and reduce the overall lifetime of the end product.
When a polymer is allowed to age in its particular environment, further crosslinking and loss of volatiles in the polymer can occur. The result is further hardening of the solder mask and, eventually, embrittlement. When the solder mask becomes embrittled, it will easily fracture and flake away from the substrate. If there is a large thermal excursion, the solder mask can also fracture and delaminate from the PCB.
Because solder mask materials are insulators, they do not pose a problem for nearby conductors when flaking occurs. However, this exposes the internal laminate materials and copper. The unprotected copper could then experience corrosion if exposed to moisture or noxious chemicals.
Embrittlement can never be prevented, but the onset of embrittlement can be significantly extended by properly storing the LPI solder mask liquid and the bare boards before assembly. The LPI solder mask material should therefore be stored, cured, and selected based on how it be used in the end environment. LPI solder mask material vendors can provide guidance on these issues and some expected reliability figures.