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Lead-free: Making the Adjustments
December 31, 1969 |Estimated reading time: 5 minutes
The EU’s RoHS Directive, as well as similar global legislation, is fostering change in the fabrication and assembly of PCBs. Unfortunately, eliminating lead from solder used in board manufacturing and assembly is forcing a number of changes in processes, raw materials, components and equipment.
By Tony Garramone
The RoHS Directive and other global environmental legislation are fostering change in PCB fabrication and assembly. These regulations require the elimination of six hazardous materials from the PCB production process. The elimination of lead from the solder used for board manufacturing and assembly is forcing several changes in processes, raw materials, components and equipment.
Legislation requires products to be compliant by July 1, 2006, limiting the use of the restricted materials in new electrical and electronic equipment. Any new product brought to market must be compliant; however, repair parts, rebuilt equipment and inventories that were marketed prior to the July deadline are exempt from these restrictions. Similar legislation will go into effect in California in January 2007. For PCB manufacturers, replacing lead solder alloys is creating a number of process changes and adjustments.
For most board shops, the major change will be the use of laminate materials that can withstand higher processing temperatures and different final finishes. Until now, laminates with the ability to endure higher temperatures were not in great demand. Because materials behave differently, there will be a number of process changes necessary to effectively process panels with higher thermal capacity.
Selecting an appropriate laminate is not an easy task. The industry is focusing on using materials with thermal capacities higher than the “typical” 150°C glass transition (Tg) range. However, some mid-range Tg materials can be acceptable for processing if the assembler is aware of thermal limitations. Materials designed to withstand these higher temperatures are available. Some board assemblers are processing RoHS-compliant products using 170°C Tg material, while others are seeking the highest affordable thermal capacity.
In the past, Tg temperature of the laminate was the main criteria used in laminate selection. With the increase in soldering temperatures, Tg is no longer the only consideration. Designers and assemblers are challenged to consider the temperature and number of thermal excursions each part may be exposed to during assembly while remaining functional. Industry experts suggest that decomposition temperature (Td) is more important than Tg; and T-260, T-288 performance and Z-axis expansion (CTE) should also play a part in material selection.
The combination of these properties and respective test results provide the information necessary to select a laminate material with the desired thermal capacity. Depending on the laminate selected, additional time may be added to the production process, an unappealing thought for designers and quick-turn PCB manufacturers.
Changing laminates also affects drilling processes. Higher thermal laminates require variations in drill feed and speed rates. Some have different chip-load requirements, others need different bit geometry. Meanwhile, lower drill-hit counts may be required to maintain quality. Changes in hit counts and feed rates could also increase processing time.
After drilled holes have been cleaned and deburred, the panels move on to de-smear and electroless processes. De-smear involves the removal of laminate resin smeared across the face of the inner layer. Electroless is the chemical deposition of a thin layer of copper across the panel and through the holes. This thin copper layer connects electrically to the copper on both sides of the panel. The thin metallic layer allows the panel to be electroplated later in the process. Higher thermal capacity laminates set themselves apart from the “norm,” requiring longer dwell times and potentially higher temperatures.
Lead-free Solder
Investigations into a solder for lead-free production have not highlighted an ideal alloy for PCB manufacturers. Because of its processing temperature of 260°C, the leading candidate for use in many facilities is tin/copper. This temperature is nearly identical to that of processes running on many leaded machines. Many board manufacturers rely on optional finishes, such as ENIG, instead of lead-free solder.
Rout, Scoring, Quality and Test Issues
Industry experts have voiced concerns regarding fracturing at breakaway tabs and poor separation characteristics along score lines, as well as router-bit and score-blade longevity. These materials should present a relatively benign addition to final processes. However, there have been indications that bare-board test equipment probes may see a shorter life span. Tin residue left on test probes from lead-free solders can cause damage. Some of the optional surface coatings are also susceptible to damage from test pins or probes.
Problems such as traceability, tracking and training must be addressed. The ability to identify specific lots of raw materials, as well as manufacturing process dates and times, is needed to substantiate lead-free claims. The manufacturer will need to certify that the product is RoHS-compliant. Suppliers and vendors must offer assistance in maintaining supplies that are backed by certificates of conformance and test data. The usual date-code identifier and traveler information helps trace orders to specific process dates and times. A certificate stating compliance to the RoHS Directive outline in the purchase order is necessary.
Throughout the facility, knowledgeable employees are a necessity. There must be a level of understanding of RoHS compliance, what is needed to produce a lead-free order and what questions should be answered to understand customer requirements.
Designers are asking how to write specifications that are meaningful to board manufacturers and result in products that meet thermal and RoHS requirements. Simply specifying RoHS compliance leaves the laminate choice up to the fabricator, which could be problematic. On the other hand, orders can go to the extreme, for example, specifying a choice of five laminates by name, listing the Tg, Td and CTE parameters, providing a selection of four final finishes and repeating nearly the entire RoHS Directive.
Board designers should provide specific information in the electronic package. Minimally, the purchase order needs to confirm the material type and final finish, as well as requirements for RoHS compliance.
The designers’ role is critical. RoHS and WEEE Directives encourage design changes that support a greener role for all electrical and electronic equipment. The use of alternative materials is driving some immediate changes. A board designed to operate using impedance values for standard FR4 may not function well using a higher thermal-capacity laminate. Components that survive current assembly temperatures may not be available in a lead-free package. A designer who is aware of the industry’s need for change can provide products that support recycling and reuse while meeting restrictions of current and proposed legislation.
Conclusion
PCB manufacturers have had to provide a quality product that can meet specifications for a range of customers. However, RoHS complicates matters. All manufacturers have one thing in common - new responsibilities to create RoHS-compliant products on behalf of the customer. Everyone has questions about lead-free compliance.
RoHS compliance will no doubt increase manufacturers’ challenges as they strive to implement new processes and adjust methods to meet requirements. Unfortunately, changes often negatively affect time to completion. With intended redundancy and efficiency in streamlining processes, the additional time and impact customers feel can be minimal.
Tony Garramone, corporate training manager, Advanced Circuits, may be contacted at (800) 979-4722; e-mail: tonyg@4pcb.com.