Surviving Lead Free PCB Manufacturing
by: Tony Garramone
The RoHS Directive and similar legislative acts in other parts of the world are fostering changes in the fabrication and assembly of printed circuit boards. The elimination of the six hazardous materials listed below from the bare board is easily accomplished. If only it were as simple as removing the offending materials there would certainly be less turmoil throughout the industry. Unfortunately the elimination of lead from solder used for both board manufacturing and assembly has forced a number of process, raw material, component and equipment changes. Before I focus on bare board manufacturing issues created by the RoHS directive I want to review the elements of the legislation. The legislation limits the use of lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls or polybrominated diphenyl ethers in new electrical and electronic equipment beginning July 1, 2006. In other words any products brought to market after that date must meet the requirements of the law. Repair parts, rebuilt equipment and inventories that were marketed prior to July of 2006 are exempt from the restrictions. Similar restrictions will go into effect in California in January of 2007. Of the six "regulated" materials the removal of lead is placing the heaviest burden on the industry. As far as the board manufacturers are concerned it is the replacement alloys for the lead in solder that are creating the biggest problem – higher processing temperatures.
Process Issues Arising from Lead Free Conversion
The major changes for most board shops will be the mainstream use of different types of laminate materials and final finishes that will withstand higher processing temperatures. The types of laminates that can be used with the higher temperatures have not been in high demand until now. As panels move through the manufacturing process a number of changes need to be made to the existing process parameters. For example multi-layer press processes have higher temperatures, different pressures and processing times. Changes to drill "feeds and speeds" will need to be made as well as limitations on drill hits. These same panels will also behave differently in the typical desmear and electroless processes. Extended dwell times along with other process changes may be necessary to effectively process panels with the higher thermal capacity. To meet RoHS requirements solder mask and legend inks will have to be hazardous material free and withstand the higher temperatures without noticeable discoloration. Because final finishes can vary so much its not clear which option, if any, will emerge as a "standard". Plated nickel gold, electroless nickel immersion gold, OSP, immersion tin or silver or palladium – which one of the optional finishes will pass muster for the customer? On top of all these changes, board manufacturers may be required to qualify process temperature and material changes through Underwriter's Laboratories.
The laminate picture is not as clear as one is lead to believe. All of the talk is centered on using materials that have higher thermal capacities than the "typical" 150°C Tg range materials. However, there are midrange Tg materials that are acceptable for processing as long as the assembler is aware of their thermal limitations. I know of a small number of board assemblers that are processing RoHS compliant products using 170°C Tg (DSC) material for their boards and having great success. On the other hand, I have talked with others who do not want the liability and are seeking out the highest thermal capacity that they can reasonably afford. Certainly, there have been laminate materials available for some time that have the capability to withstand the increase in time at higher temperatures being proposed for lead free assembly. These materials were designed to withstand the extended higher temperatures that the "standard" FR4 materials may not. A number of characteristics have been identified for the laminates and prepreg that will successfully survive the new assembly processes. Those include Td, T-260 & T-288, and CTE as well as Tg (DSC) ratings. The combination of these properties and their respective test results should provide the information necessary to select a laminate material with the thermal capacity you require. In the past, Tg or the glass transition 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 for material selection. Essentially, designers and assemblers are being challenged to consider the temperature and number of thermal excursions each part may be expected to undergo and survive as a fully functional product. Industry experts are suggesting that decomposition temperature (Td) is much more important than Tg and that T-260, T-288 performance and Z-axis expansion (CTE) should also play a larger part in material selection. As a board fabricator, we began our lead free research by investigating a large number of available materials and their various attributes. We made the decision to qualify three higher thermal capacity laminates through Underwriter's Laboratories. It became evident that a number of changes would need to be made if we were to provide the same quality product we do now. In our situation we found that processing multi-layer panels through the press would require higher press temperatures and a longer processing window. As a "quick turn" PCB manufacturer, adding time to a process is difficult to swallow. Once the parameters for the press cycle were established we began processing a number of sample panels, among them our UL qualification boards. We have discovered that the usual "stackup" or number of prepreg sheets that we currently use may not match up with the new materials and will cause a review of the various multi layer builds that we use.
Studies have shown that the higher thermal laminates require variations in drill feed and speed rates. Some have different chip load requirements, others need different bit geometry and the group, as a whole, can force lower drill hit counts to maintain good hole quality. As these materials begin to make up more of our daily production load we will continue our process evaluation and testing. Possible lower hit counts and lower feed rates will add more time to processing panels. One issue associated with drilling is deburr and debris removal from the hole. At Advanced Circuits we made the decision to replace an older piece of equipment with one that has ability to deal with any debris issues that might be associated with the new laminates.
Desmear, Electroless & Final Finishes
When the drilled holes have been cleaned and deburred the panels move on to the desmear and the electroless processes. Desmear is the removal of laminate resin smeared across the face of inner layer interconnects during the drilling process. This process is similar to etchback in that it removes laminate resins. Electroless is the chemical deposition of a very thin layer of copper across the panel and through the holes. This layer, about 50 millionths thick, electrically connects the copper on both sides of the panel and the exposed interconnects. The thin metallic layer will facilitate electro plating the panel later on in the process. Again higher thermal capacity laminates set themselves apart from the "norm". They can require longer dwell times and possibly higher temperatures to process correctly. Depending on the laminate material, they may even require special processing like plasma etchback. In any event we are again faced with the real possibility of additional time and a process dissimilar to other laminates that we currently use. After the electroless process the panels are ready for image processing. Here we begin "normal" processing methods and little if any variation will be necessary until after the etching process takes place. Variations will begin to appear again depending on the final finish. Full panel processing with ENIG, NiAu, immersion tin or silver may take place prior to the application of solder mask. The same processes can also be done after mask application – coating only the exposed metallic surfaces. When lead free solder is used the panels will be coated with mask prior to applying the solder. OSP application is usually done after bare board testing to prevent probe damage to the coating.
Lead Free Solder
When making the decision to operate a lead free solder operation a number of choices must be made. First, will you manufacture both lead free and standard pcbs and how will you maintain both operations? We made the choice to run a lead free HASL in tandem with our current leaded solder machine. A second machine will be our leadless solder line and we will continue to run our current leaded machine as long as the product is viable. If you choose to manufacture a lead free solder board you must also decide on an alloy. We have investigated the processing parameters of a small number of alloys and kept abreast of both the INEMI and IPC solder projects. What is most interesting to me about the majority of the solder investigations is that few if any have recommended an alloy for PCB manufacturers. Solder pastes, flux, solder alloys, reflow processes, wave processes, rework, and more have all been covered in depth for assembly operations. Despite this lack of discussion the leading candidate for use in our facility is a tin copper alloy due to its processing temperature of 260°C. It's nearly identical to the temperature of the process currently running on our leaded machine. Another reason for this choice is value – this alloy is less expensive than others and it retains compatibility with the majority of assembly alloys under investigation. The processing of panels through a leadless process is not that much different than existing procedures and does not present any major issues.
Rout, Scoring and Quality & Test Issues
There have been some discussions surrounding router bit and score blade longevity and a few industry sources have voiced concerns regarding fracturing at breakaway tabs and poor separation characteristics along score lines. I am not aware of any definitive data regarding these issues. Once removed from the panel the parts move on to test, inspection and shipping. It would seem that these materials would present a relatively benign addition to these final processes. However, there have been indications that test probes on various types of bare board testing equipment may see a shorter life span. Apparently tin residue from lead free solders left on probes can damage them if not properly maintained. Surface coating damage from various types of test pins is a real issue and has raised some concerns. In addition, inspection criteria will change slightly to allow for a different finish that the "no lead" solders can present. There may also be concerns over coverage and dewetting, which can crop up more often than with the leaded solders. Shelf life, oxidation and humidity are just a few issues surrounding packaging for shipment and stocking that must be addressed.
Traceability, Training & Communication Issues
Along with all of the process issues there are other problems to solve. Traceability becomes a major issue. The ability to identify specific lots of raw materials as well as manufacturing process dates and times will help substantiate lead free claims. As the manufacturer we have to certify that the product we are shipping is indeed RoHS compliant. To do that we need the support of our suppliers and vendors in maintaining supplies that conform to our requirements and are backed by certificates of conformance. We have already contacted our vendors regarding their products and provisions for conformance certification. The usual date code identifier and traveler information will help trace orders to specific process dates and times. By using a different UL type designator following our logo, customers will be able to easily identify those parts that are produced to meet the higher thermal capacity. Eventually we will be providing a certificate of conformance that states compliance to the RoHS directive as well as customer specifications and requirements outlined in the purchase order. Another issue is having knowledgeable employees throughout the facility. Beginning with the sales staff there has to be a certain level of understanding of what RoHS compliance is, what is necessary to produce the "lead free" order that they are reviewing and what questions should be answered to understand exactly what a customer needs or wants. Anyone who reviews orders, reads drawing packages, creates process travelers or deals with any pre-production process must be aware of the requirements. Training the staff to recognize the differences, the materials and processes required to meet the customer's needs is a priority. Production staff will need to be clear on what can or cannot be done with lead free panels. Cross contamination is a concern that we have to deal with as a possibility. I have recently had conversations with customers asking how they should write specifications that will be meaningful to a board manufacturer and result in a product that will meet both the thermal requirements and RoHS. A large number of fabrication notes I've read are not very clear or specific. Certainly to just specify RoHS compliance leaves the laminate choice up to the fabricator. There could be problems if a customer expects a Td in the 350° range and gets something in the 290's. On the other hand, I have read purchase orders that have gone to the extreme. One I read recently specified a choice of 5 laminates by name, listed the Tg Td and CTE parameters and provided a selection of 4 or 5 final finishes in addition to reproducing nearly the entire RoHS directive.
The board designer should provide specific information in the electronic package and the purchase order should confirm the material type necessary, final finish and the requirement for RoHS compliance at the very least. All things being equal I would like to see organizations like IPC push to have designers and assemblers clearly state their needs in their purchase orders and "readme" files. This would alert our sales, programming and production staff to the specific needs for each part number being manufactured. The designer's role in this whole scheme is critical. Both the RoHS and WEEE directives encourage design changes that support a greener role for all types of electrical and electronic equipment. Of course, some of the immediate changes are being driven by the use of alternative materials. For instance, a board designed to operate using the impedance values for standard FR4 may not function well using one of the higher thermal capacity laminates. Components that survive assembly temperatures currently in use may not be available in a lead free package or may not survive the higher temperatures needed for some of the lead free alloys. A designer who is aware of the industry's need for change will be able to provide products that will support recycling, reuse and meet the restrictions of current and proposed legislative acts. The future of electronic devices should not be dictated by the various legislatures throughout the world. It's time for the industry as a whole to step up and take the initial steps to eliminate hazardous materials from their products and pcbs. There is a need to keep an eye out for future issues that are already beginning to be raised. Like halogens and their place in the scheme of electronic components, raw materials and manufacturing processes. It's even possible that some of our current saviors, like silver, that have a less than stellar reputation in environmental issues may eventually come under fire. We, as an industry, need to strive to be proactive rather than scrambling to meet deadlines that we had little or no part in establishing.
As a printed circuit board manufacturer our role has been to provide a quality product capable of meeting the specifications of a wide range of customers. With RoHS it's not as simple – now we have customers who do not know what they need. I spend the majority of my "lead free" time answering strikingly similar questions from customers. They include designers with years of experience, buyers that are totally inexperienced, engineers, operations and quality managers. Some assemble 100 boards a month, some 100 in an hour, some have engineered a hundred boards others are making their first one. They all have one thing in common - newfound responsibilities to obtain RoHS compliant products. Some know what they want and need but don't know how to tell us that. Some just need reassurance that they're on the right track and that we can support their needs when they get done. Most have just begun and haven't got a clue. The key is and always will be communication – in all directions. If you have questions and concerns voice them to the person responsible. Verify the impedance needs with the designer, settle on maximum processing temperatures with your process engineer, clarify your board manufacturer's capabilities and then put it down in black & white in language you and your vendors understand. When I took on the responsibility of spear heading the RoHS and WEEE compliance investigations for Advanced Circuits I had no idea what a challenge it would become. There's nearly a year left to go and I expect that that may not be the end of it. The next few months will certainly test the mettle of all of us in the industry.