PCB Designer's Notebook: Selecting Halogen-free (HF) Soldermask Materials

Estimated reading time: 7 minutes

By Vern Solberg

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.

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.

Suppliers state that experience has shown little to no difference in processing parameters for a halogen-free soldermask compared to a non-halogen-free (green) version of the same soldermask. Any difference may be attributed to a possible change in pigment that could have a minor effect upon the exposure of a photoimageable soldermask. This difference, however, usually would have minimal impact on the application process. Manufacturers typically have a wide selection of soldermask products readily available (liquid and dry film) and note that nearly half of North American production used HF soldermask in 2008 because most of the current PWB products offered are also designated as RoHS-compliant and qualify as halogen-free.

In regard to product labeling and declaration systems for environmental performance, manufacturers must retain detailed information for each product placed on the market. Because of the relatively short product lifecycles for consumer products games, wireless handsets, entertainment systems labeling accuracy will be monitored closely. The industry has developed voluntary declaration systems that allow suppliers to present environmentally relevant information in a standardized and traceable manner, so that a purchaser can make an informed decision. The IPC-1750 standard provides the principles and details for declarations necessary between members of a supply chain. Although this standard contains only generic information between trading partners, when combined with a specific sectional standard, the entire document set can be used to define and maintain the declaration type information. The requirements pertain to both hardcopy and electronic data descriptions with provisions for the creation of a record that will serve as a commitment between companies and may be used to establish due diligence in any dispute in third-party litigation. As stated in the document, the purpose of the standard is to establish a methodology for any declaration process between customers and their suppliers. It benefits suppliers and their customers by providing consistency and efficiency to the declaration process. In addition to the physical descriptions of a declaration site, information also is captured regarding personnel and their responsibility or role in providing clear and unambiguous information required by any of the sectional standards within the declaration document series.

The intent of the IPC-1750 standard set is to promote the development of consistent data exchange formats that will facilitate and improve data transfer along the entire global supply chain. Because producers may choose to audit information provided under this standard, audit methodology, verification procedures on material, supplier capability, quality systems, and code of conduct all also are described within the standard. Each section of the standard has a specific focus and shall be used, as appropriate, to describe a particular declaration process. As far as interpretation of the IPC standards, the word "shall," the emphatic form of the verb, is used throughout the standards whenever a requirement is intended to express a provision that is mandatory. Deviation from a "shall" requirement may be considered if sufficient data is supplied to justify the exception. The words "should" and "may" are used to express non-mandatory provisions intended to be recommendations. "Will" is used to express a declaration of purpose related to the text description.

As far as general terminology, "compliance" refers to the suppliers' demonstration of meeting a government regulation, law, or directive; "conformance" is the supplier assurance that the product meets a company, national, or international specification; and "due diligence" is proof of actions exercised and reasonable steps taken to avoid misrepresentation or committing of an offense.

The materials declaration process now covers about 80% of customer inquiries, and includes reasonable and IT-industry-accepted criteria found in plastics and some of their additives including flame retardants.

Vern Solberg, an SMT Editorial Advisory Board Member, is a technical consultant specializing in surface mount and microelectronic design and development. Additionally, Vern holds several patents for IC packaging innovations and is a member of many industry organizations, including IPC, IMAPS, SMTA, and the JISSO International Council. He may be contacted at (408) 568?3734; vsolberg123@aol.com.

Sources for Further Information:www.ipc.orgwww.halogenfree-flameretardants.com www.itecodeclaration.orgwww.epeat.neteuropa.eu.int/comm/environment/gpp/ www.epa.gov/dfe/pubs/projects/pcb/index.htm

Related Articles:Part 1: PCB Designer's Notebook: Fine-pitch and Die-size Array Packages Design and AssemblyPart 2: PCB Designer's Notebook: Stencil DesignPart 3: PCB Designer's Notebook: IC Component Package Evolution and the Impact of Lead-free Soldering