UL testing may be required to produce and assemble boards with lead-free surface finishes. This article describes the base material, surface finish, and process changes to consider when moving to a lead free product that will require UL Certification. In addition, suggestions are made on how to plan for and more quickly move through the UL Certification process.

The worldwide movement to make environmentally friendly products through phasing out certain hazardous substances presents many challenges. Lead-free manufacturing is a process change involving the OEM (Original Equipment Manufacturer), EMS (Electronics Manufacturing Service), PWB fabricator, and material supplier. Communication across the supply chain is necessary to determine if lead-free and bromine-free is the appropriate direction for the product.

Due to the conditioning time involved for UL PWB testing, identifying the need for UL Certification should not be left to the last minute. Lead-rich and lead-free processes are significantly different based on the reflow temperatures and possible cross contamination issues. Bromine-free materials may be processed similarly as traditional
FR-4. However, the less understood alternate flame retardant systems may create questions regarding the flammability characteristics of the board assembly. Therefore, evaluation of the revised PWB manufacturing and assembly process may be required. Since many manufacturers will not find it cost effective to support two PWB production processes, manufacturers not intending to market the product to Europe may choose to recertify their product with lead-free processing.

Surface finish choices

Lead has been in the solder used for electronic product assembly for more than 50 years. Historically, solder consisted of eutectic tin-lead 63Sn/37Pb or its equivalents, 60Sn/40Pb and 62Sn/36Pb/2Ag. Today many lead-free alternatives are available, and each material must be evaluated for its benefits and challenges. The available lead-free alternatives include:

• Immersion Finishes (Gold, Silver, or Tin)
  
  
• Electroless Nickel-Immersion Gold (ENIG)
  
  
• Organic Solderability Protectants (OSP - Benzimidazoles)
  
  
• Tin-Silver-Copper (SAC) alloy pastes, and
  
  
• Hot Air Solder Leveling (HASL Non-Lead containing —Tin-Copper and Tin-Silver).

The majority of the electronic industry associations are recommending the SAC alloy as the standard lead free soldering material.

Processing—manufacturing and assembly

Lead-free materials require 30^oC to 45^oC higher melting temperatures when compared to tin-lead solder (see Table 1). Many manufacturers have benefited from the large tin-lead reflow window by using one or two thermal profiles to process a wide range of board assemblies. However, the process window for lead-free materials is much smaller due to the component maximum exposure temperature of 250^oC (this limitation is primarily due to plastics deformation).

The higher melting temperatures required during the lead-free reflow process can cause delamination within the PWB and damage a wide variety of components such as plastic connectors, relays, light emitting diodes (LEDs), electrolytic and ceramic capacitors. Precise temperature control during lead-free processing may include a ramp stage in the temperature profile in order for the temperature rate of rise not to harm components that may be thermally sensitive. PWB warping, thermal shock-induced cracks, and differences in adjacent materials' coefficient of thermal expansion (CTE) are additional potential problems.

The peak temperature and time above liquidous must be achieved without overheating the assembly or components. A longer preheat section is needed to reach the higher temperatures and avoid thermal shocking the PWB during the solder reflow process. Figure 1 shows typical lead-free reflow parameters.

Lead-free soldering makes it increasingly important to identify the components, board finishes and the solders used to assemble them. Keeping the materials identifiable throughout the assembly process is critical as is identification of the final assembly for reliable rework. Inventories must be separated to ensure lead-free components are not mixed with leaded components due to the varying material melting points and possibility of creating metallic alloys, which may cause premature failure of the board.

Base material choices

In order to facilitate the conversion to a lead-free environment, materials used to fabricate lead-free assemblies must also be studied to ensure the thermal stresses will not compromise the performance and long-term reliability of the board and assembly. The proper selection of PWB dielectric materials for lead-free processes requires evaluation of the material to determine if the material has good thermal stability and will withstand the significant increase in thermal stress. Failure mechanisms in boards processed through lead-free processing without thermal stability include delamination and barrel cracking in the plated through holes.

FR-4 epoxy based materials remain the most commonly used material for manufacturing PWBs. Standard FR-4 substrate materials with low glass transition temperature (Tg) ratings of 130^oC - 140^oC have difficulty withstanding the thermal stress of lead-free assembly. Table 2 lists the additional characteristics to consider for good thermal stability and lead-free processing in accordance with the proposed IPC4101A specification sheet for lead-free compatible FR-4 materials. Traditional thermal characteristics can be used to determine the thermal stability of a PWB material and its suitability to lead free processing. These properties include the glass transition temperature (Tg) of the material, the decomposition temperature (Td), the Z-axis coefficient of thermal expansion (CTE), and the thermal resistance at 260^oC (T260) and 288^oC (T288).

Z-Axis CTE
a. Alpha 1
b. Alpha 2
c. 50^oC to 260^oC

Thermance Resistance
a. T260
b. T288

Tetrabromobisphenol A (TBBPA) is exempt under the RoHS directive and is acceptable for use as a base material flame retardant. However, the push to make an environmentally friendly product has increased the interest in non-halogen flame retardant materials. Non-halogen materials rely on flame retardants such as phosphorus, nitrogen, and alumina trihydrate. The flame retardant mechanism of these flame retardant systems is very different from brominated systems and suggests the overall laminate material will also behave differently. Bromine flame retardants work by releasing bromine at elevated temperatures into the gaseous phase. Phosphorus and phosphorus/ nitrogen blend flame retardants form a char layer on the burning surface. The char creates a physical barrier to and gas transfer. Aluminum trihydroxide or alumina trihydrate create water vapor to reduce the heat transfer.

Non-halogen materials can be suitable for lead-free processing. The majority of non-halogen materials demonstrate a V-0 performance in the UL94 flammability test. The non-halogen materials require the same thermal stability as halogenated materials: moderate glass transition temperatures, high decomposition temperatures, lower total expansion due to solder temperatures, and good thermal resistance. The switch to non-halogen materials is not required, but may become inevitable due to the WEEE directive.

UL requirements — PWB and end product

The UL PWB certification program monitors the manufacturing process including maximum temperature and exposure times and the materials used to produce the PWBs. As board manufacturers begin using alternate surface finishes and base materials, these materials will be included in the board and manufacturing process description. Currently, most process descriptions list "solder" which is intended to be tin-lead solder. Lead-free solders will be described by their material constituents to clearly identify the type of solder. The assembly soldering process temperatures are described by the board solder limits. The solder limits represent the maximum temperature and exposure time during the assembly process. Multiple solder limits are used to represent the temperature profile for the reflow process.

Testing is required if the lead-free board manufacturing or assembly soldering process includes higher temperatures and/or longer times. Flammability, conductor adhesion, blistering, and delamination will require re-evaluation if increased processing temperatures are involved. Silver migration testing on immersion silver and tin-silver-copper (SAC) alloys is not required. The IPC
3-11g task group conducted research testing demonstrating Immersion Silver surface finish on a PCB is no more likely to migrate than other standard surface finishes. In addition, research performed by NEMI on the SAC alloys was used to show the SAC alloys are also no more likely to migrate than standard surface finishes. The product evaluation tests typically require either 4 weeks or 12 weeks to complete, depending on whether the 10-day conditioning receives compliant test results. If it does not, a 56-day conditioning is required.

Summary

The term "Solder" can no longer be assumed to refer only to the eutectic tin-lead alloy (63Sn/37Pb) during the electronics manufacturing and assembly processes. The PWB and assembly industry are recommending the tin-silver-copper (SAC) alloys as the standard lead-free solder to replace eutectic tin-lead, and intend to assign unique part designations to distinguish the lead-rich and lead-free products from one another.

The higher required melting temperatures of lead-free solders may damage boards or board assemblies, therefore, thermal stability is an important factor in material selection for PWB lead-free processing. The higher lead-free temperatures also require certain board performance characteristics (flammability, delamination, and conductor bonding) to be re-evaluated by UL. Due to many OEMs' quests to comply with WEEE in 2005 and the conditioning time involved for UL PWB testing, identifying the need for UL Certification should not be left to the last minute. Implementing the suggested actions will help to expedite the UL PWB re-evaluation process.

By Crystal Vanderpan, Principal Engineerfor Printed Circuit Technologies, Underwriters Laboratories Inc.