Selecting Halogen-free (HF) Soldermask Materials

 In previous columns, I studied a number of new RoHS-compliant circuit board materials with chemistries that reduce the halogenated flame retardants to levels that meet legislated restrictions in Europe. There has been considerable discussion and public concern about flame retardants in general and halogenated flame retardants in particular.To get more news about Halogen Free PCB, you can visit pcbmake official website.

A guidance report from HDPUG, “Halogen-free Guideline” notes that there are really no general legal restrictions on the use of all halogenated flame retardants in any part of the world, but laws have been enacted in the past several years that restrict the use of some of the halogenated flame retardants in certain electronic-related applications. The focus on halogenated flame retardants possibly started with the observation that polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs) had an adverse effect on the environment due to their persistence, toxicity, and ability to bio-accumulate. Brominated flame retardants (BFRs) became a topic of environmental concern in the early 1990s, when it was discovered that when subjected to extreme thermal stress during an accidental fire or during uncontrolled combustion some BFRs could form halogenated dioxins and furans. The resulting halogenated vapor residues (dioxins and furans), in a confined environment, could be harmful, over time, to the health of those inhaling or in contact with the residues.

The organic flame retardants (brominated and phosphorus-based) in widest use have become the subject of European Union (EU) administered risk assessments. Fortunately for the electronics industry, a European Union Risk Assessment concluded that the most used brominated flame retardant, tetrabromobisphenol-A (TBBPA), does not present a risk to human health or to the environment when used as a reactive-type flame retardant, such as in printed wiring boards (PWBs). The interaction of the flame retardant with the resin material affects not only polymer properties but also processing and environmental issues. Reactive flame retardants are chemically built into the polymer structure of the PWB and thereby provide a consistent structure for fire retardancy when uniformly dispersed in the resin systems used in PWB base materials. Halogen-containing flame retardants are effective due to their interference with the radical chain mechanism in the gas-phase combustion process. This combination of bromine and chlorine is especially favored due to ideal timing for interference, attributed to their particular bonding strength to carbon.

When specifying RoHS-compliant PWB base materials, the designer must also consider selecting a soldermask material that is compliant (as well as compatible with lead-free solder processing). Although soldermask, especially for rigid PWBs, is usually required to have an Underwriters Laboratory (UL) Fire Retardant (FR) rating, the rating of the mask coating is assessed after applied to a compliant flame-retardant substrate such as FR-4. Since the soldermask coating usually is significantly thinner than the base laminate, the coated circuit board relies on the flame retardant element within the laminate to provide overall flammability protection for the finished substrate. The designer should be aware that it may be difficult to achieve a satisfactory UL rating in cases of ultra-thin circuit board constructions furnished with ultra-thick (>2 mils) soldermask coatings.

The International Electrotechnical Commission (IEC) standards define halogen-free as meaning materials contain 900 ppm maximum chlorine, 900 ppm maximum bromine, and 1,500 ppm maximum total halogens.

Since flame retardants are not actually added to soldermask material, any halogens present are introduced as ingredients or impurities. These halogens generally come from additives such as pigments used to create the color (green in particular) and/or from residual catalysts from resin manufacturing. Pigments with lower halogen content are available for alternative colors (blue, red, black) but, during the transition to HF, these less-traditional colors may be offered at a slightly higher cost than the more common green soldermask.

Chlorine is contained mainly at a molecular level in the “green” pigment of all suppliers’ masks; therefore, it is almost impossible to get a true green pigment that does not contain some level of chlorine. On the other hand, chlorine levels are not significantly relevant in colors like blue, yellow, and red. This means that virtually all non-green soldermask products can be classified as halogen-free and may explain why many products that do meet the HF definition are tinted “blue” and have a very low and acceptable total halogen level.

Soldermask products formulated with lower halogen content have not measurably affected the performance of soldermask materials and are expected to meet all established performance criteria. The most commonly referenced specifications for soldermasks is the IPC-SM-840, “Qualification and Performance Specification of Permanent Solder Mask.” Another source for evaluation of PCB circuit coatings is the Underwriters Laboratories Inc. Standard 94 (UL-94), “Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances.” These requirements are for all soldermask coatings, regardless of composition, so they apply equally to halogen-free products. Furthermore, the basic properties of a HF version of a soldermask are not significantly different from soldermask materials that are not labeled specifically as halogen-free.