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Lead-free Rework: Are You Ready?
December 31, 1969 |Estimated reading time: 12 minutes
Adoption of lead-free will have a major effect on rework and repair. Operators will encounter new component and board finishes with soldering characteristics unlike those of typical tin/lead finishes. This article highlights some of the changes operators will face in the move to lead-free.
By Douglas J. Peck
Manufacturers in the U.S. face major near-term production challenges. Those shipping product into any EU country are bound by the RoHS mandate, effective July 1, 2006. RoHS bans the use of four heavy metals: cadmium, mercury, lead, and hexavalent chromium in electronic products. It also bans the use of two flame retardants: polybrominated (PBB) and polybrominated diphenyl ethers (PBDE) used in some PCBs. RoHS will have a major effect on electronics rework and repair operations.
Several lead-free alloys have been identified and evaluated by worldwide organizations and consortia. Of these, the International Electronics Manufacturing Initiative (iNEMI) recommends the use of Sn3.5Ag0.6Cu (SAC) for reflow soldering, and Sn3.5Ag and Sn0.7Cu for wave soldering.
These lead-free alloys have melting (reflow) temperatures much higher than tin/lead alloys. Where eutectic tin/lead solder reflows at 183°C, the iNEMI SAC alloy reflows at 217°C. This effects rework and repair operations in that both component and PCB temperatures must increase. Further, RoHS mandates that any lead-free solder connection should not contain more than 0.1% lead by weight. Under RoHS, lead contamination of lead-free solder connections becomes a major rework and repair issue.
Complying with RoHS is more than raising the temperature in lead-free soldering. The Directive mandates that operators master a new body of knowledge and practices in rework and repair. These operators must become the gatekeepers in determining if and when products meet RoHS demands.
Hand Soldering
The key elements for hand soldering include a source of solder, a source of flux, and a heat source. Like tin/lead solders, lead-free alloys are available in conventional wire diameters with RMA, no-clean, and water-soluble flux cores. However, 10-mil-diameter lead-free solder is brittle, difficult to spool, and expensive. Operators accustomed to using 10-mil solder may have to adapt to 15-mil lead-free. Manufacturers also recommend RMA, no-clean, and water-soluble flux and flux pens for lead-free soldering. However, at higher reflow temperatures, no-clean fluxes are less effective in producing quality lead-free connections. Water-soluble flux should not be used in rework/repair unless the PCB assembly can be machine-washed. Hand cleaning water-soluble flux residue is ineffective and can lead to field failures.
Most rework and repair is done using a soldering iron or hot-air pencil. When possible, use a double-chisel soldering tip, rather than a conical tip. In general, tip-width should match the land/pad width. This maximizes the heat-transfer surface area between the soldering tip and the connection. Not all lead-free soldering applications will require higher solder-tip temperatures. If possible, begin with a double-chisel tip operating at a lower tip temperature and increase as needed.
Solder-tip oxidation increases as temperatures rise. Tip care becomes extremely important in lead-free soldering. Soldering tips should be tinned between soldering cycles and with lead-free solder. To maximize tip-working life, turn the iron off during longer breaks. Lead-free tip cleaners also are available for restoring tip condition. After conditioning, shock the tip on a damp sponge to remove tip-cleaner residue and re-tin with lead-free solder.
When soldering a connection, the solder should not touch the soldering tip. Heat the work with the iron and use the heated work to reflow the solder. Once solder begins to reflow, heat the connection for two to three seconds more before removing the solder and soldering tip. This assures adequate formation of tin/copper intermetallic for solder to bond to the copper and a homogeneous SAC element mix in the solder joint. If a solder connection is overheated, homogeneous SAC elements tend to separate and concentrate, resulting in a brittle solder joint.
Some operations use the hot-air pencil in rework operations. Typically, these are set to 700°F. Air flow is adjusted such that a wipe is discolored to a light brown from a 700°F tip at a distance of about ¼". It is important to keep the hot-air-pencil tip moving to prevent damage to the solder mask, PCB, or component.
Use of a hot plate for preheating PCBAs may be key to successful lead-free rework and repair. Preheating raises the temperature of the entire assembly, reducing the connection heating requirement and the time to reflow the connection.
Reflow Soldering
Typical tin/lead peak reflow temperatures are 205°-215°C. Lead-free candidates reflow at 217°-221°C. Target peak reflow temperatures for lead-free solders are between 235° and 245°C. Figure 1 compares typical tin/lead and lead-free reflow profiles and process windows. In response to tighter lead-free process windows, component suppliers are re-qualifying components to higher maximum exposure temperatures, as prescribed by IPC/JEDEC J-STD-020. With these higher exposure temperatures, lead-free process windows remain tight. ICs and BGAs must be reprofiled for lead-free rework.
Figure 1. SnPb and SAC process windows.
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Moisture Sensitivity
Most ICs and BGAs used in surface mount are encapsulated in non-hermetic plastic, making them sensitive to moisture-induced stress. During reflow, vapor pressure from absorbed moisture increases significantly and can cause internal package failure. Component suppliers also use IPC/JEDEC J-STD-020 to qualify each product to a specific moisture sensitivity level (MSL). Table 2 shows maximum allowable floor life for each MSL under typical floor conditions.
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Lead-free Solder Inspection
Lead-free solders do not wet as well as eutectic solders. Reflow is more sluggish and solder tends to reflow as placed, rather than flowing and wetting an entire land. Poor spread in tin/lead soldering has been an indicator of poor wetting. This is not the case with lead-free alloys. Lead-free wetting angles are greater and contours are more convex. Solder-connection surfaces tend to be dull with a grainy appearance. Poor lead-free solder appearance may tempt operators to resort to adding more heating in hand soldering to improve cosmetics.
As a result, an untrained operator/inspector could reject what are normal lead-free connections and specify unnecessary rework. This poses a training task to ensure that all personnel who inspect lead-free product be adequately trained for the task. The latest revision of IPC-610 D, Acceptability of Electronic Assemblies, provides guidance for lead-free soldering and includes numerous visual comparisons of tin/lead and lead-free solder connections under various process conditions (Figures 2 and 3).
Figure 2. Chip-cap tin/lead and lead-free solder connections. Photo courtesy of IPC.
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Figure 3. Tin/lead and lead-free gull-wing solder connections. Photo courtesy of IPC.
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Lead-free Material Issues
Many changes in materials and their processes arise in lead-free rework and repair. FR-4 has become a standard in PCB fabrication, owing to its favorable cost and thermal properties. PCB materials are rated by their glass transition temperature (Tg). When heated above Tg, a board’s mechanical properties begin to rapidly deteriorate. The hard, brittle material can change to a soft, rubber-like substance. Peak lead-free reflow temperatures of 235°-245°C are well above the Tg of modified FR-4 materials, rendering all vulnerable to thermal damage. Extreme care will be necessary to avoid substrate process damage during reflow, and rework and repair.
Today, PCBs typically use a tin/lead HASL coating or planar land finish with OSP protection on land and pads. To comply with RoHS, new PCB finishes must be lead-free. A wide selection of surface finishes has evolved that may be specified on products. Leading candidates include OSPs, electroless nickel and immersion gold (ENIG), immersion silver (ImAg), immersion tin (ImSn), and tin-copper HASL. Some respond to multiple rework cycles better than others. Solderability, wetting, and thermal degradation properties of each are also important factors in conducting lead-free rework and repair.
One phenomena arising from use of tin plating is the formation of tin whiskers. For decades, tin whiskers have been identified as the source of numerous avionic and satellite electrical failures. Prevention of tin whiskers has taken on new urgency, as tin plating will be a common lead-free plating finish. Tin whiskers can grow out of most pure-tin plating deposits up to 10-mm long, or in clusters 1-2 µm on a side. They are created by compressive stresses in the tin plating that force tin from a weak spot in the plating.
ENIG plating applies electroless nickel over copper surfaces. Nickel provides a barrier to copper migration and protects the copper surface from oxidation. A thin layer of gold plated over the nickel prevents nickel oxidation until the board is processed. During soldering, if the nickel-tin intermetallic layer becomes brittle and fractures, a phenomenon called black pad may occur. Black pad may appear in various shades of gray or black. Pad/land repair by wicking and retinning may or may not be successful. There is no practical method of prescreening PCBs for black pad. It occurs as non-wetting or a premature joint failure due to a weakened interface at the board surface.
Most components use tin/lead plating on terminations and leads. In response to RoHS, the industry has developed a new family of lead-free component finishes. Leading candidates among these are matte-tin for products with short (five-year) lifecycles, matte-tin plating with nickel barrier, Sn3Ag0.5Cu for solder-balled components, tin-copper, and tin-silver. Other finish candidates include tin-bismuth, tin-silver, nickel-palladium, and nickel-palladium-gold. Either tin plate or tin dip will be used for plated thru-hole (PTH) components.
Tin used in lead plating exists in two states. Above 13.2°C, it is called white tin because of its appearance. Upon cooling below 13.2°C, white tin turns into gray tin, gradually losing about 20% of its original density. After extended exposure to low temperatures, gray tin can develop visible warts and gray powder on its surface called tin pest. Applications in cold environments, such as telecommunications stations, outdoor recreation, and automotives, are at risk of developing tin pest.
Lead-free Transition Alternatives
The transition to lead-free will differ among producers. One transition method is ideal - transitioning directly from lead to lead-free products. This is the most direct path and has the fewest rework and repair issues. Lead-free solder, solder paste, and bar solders are specified, as are lead-free components and PCB finishes. Lead-free bills of material (BOMs) are in place. Lead-free boards and parts are kitted. Avoidance of lead contamination also is minimized.
Two other transition alternatives are available. Both present multiple challenges in rework and repair operations. Producers of consumer, computer, and computer-related products will likely choose a backward-compatible path, which retains use of tin/lead solder initially. Producers of servers, storage array systems, and network backbone systems are exempt from RoHS until 2010. These producers will likely follow this path as well, but more gradually. Fortunately, most lead-free finishes are forward- and backward-compatible. However, BGAs using the SAC alloy are not backward-compatible because SAC solder balls do not reflow completely at lower SnPb reflow temperatures. Modified profiles may be required for SAC BGAs. Alternatively, SAC balls may have to be removed and replaced with tin/lead balls.
A forward-compatible transition poses numerous obstacles. The use of lead-free solder means all rework and repair soldering must be done at higher lead-free reflow temperatures, even though components and boards may use tin/lead. Reflow at higher temperatures can create excess voiding in tin/lead solder balls. This transition requires that rework inspection track and adapt to transition variations in solder wetting and appearance. Rework kits must identify the solder type to be used correctly. BOMs must evolve during the transition. Potential contamination of lead-free solder by lead or other lead-bearing sources requires detailed rework procedures to eliminate this occurrence.
Some assemblers will opt for dual assembly lines: tin/lead and lead-free. The lead-free line would process new products, while the tin/lead line processes current and legacy products. New kitting procedures are required to assure that materials are not interchanged between lines. Throughout this transition, rework and repair personnel must be able to differentiate and adapt to lead and lead-free components and assemblies.
Component and Assembly Labeling
The transition to lead-free has evidenced some conflict between component suppliers and their users. Most users prefer that component suppliers produce two versions of each component - lead and lead-free - using separate part numbers. Rather than doubling their potential product lines, more than half of component suppliers have transitioned to lead-free without changing part numbers. Other manufacturers have established unique part-number variations to designate lead-free. Using manufacturer data, some component distributors test incoming manufacturer parts and renumber these as tin/lead or lead-free.
Two labeling standards are available. iNEMI has issued JES97 Marking, Symbols, and Labels for Identification of Lead (PB) Free Assemblies, Components, and Devices. IPC has released IPC-1066 Marking, Symbols, and Labels for Identification of Lead-free and Other Reportable Materials in Lead-free Assemblies, Components, and Devices. Both establish distinctive symbols and labels to identify materials that are lead-free and capable of providing or have lead-free, second-level interconnects, such as solder connections between a chip resistor and its SMT lands, and thru-hole connections between an axial resistor and its pads.
When space permits, component-marking designations are prescribed with the category designation enclosed in a circle or ellipse. Otherwise, the category must be indicated on the lowest-level shipping container using the second-level interconnect label.
The Benchtop and Lead-free
For most assemblers, the lead-free transition from tin/lead to lead-free will occur over time. Most will establish parallel tin/lead and lead-free rework areas. Each requires dedicated tools and materials to minimize the possibility of lead contamination.
A systematic approach must be identified and followed when preparing for the transition. We must root out all sources of lead contamination in lead-free rework and repair. This changeover must involve all rework personnel - no exceptions. Potential sources of lead contamination include tin/lead solder, flux pens, solder wick, soldering tips, rework hand tools, sponges, cleaning cloths, alcohol bottles, and cleaning brushes. All lead-free items should be labeled as such.
Conclusion
Rework and repair personnel face major changes in the lead-free transition. Lead-free solders, component leads, and PCB finishes will require new soldering and inspection procedures. Higher lead-free reflow temperatures pose sources of potential thermal damage for components and PCBs. New supplier MSL component ratings will require strict adherence to the out-of-bag exposure period and entail new pre-bake procedures. IPC and iNEMI labeling standards will become important in directing use of rework and repair materials compatible with PCB assembly materials. A challenging lead-free transition will see component and PCB material evolution, ongoing changes in inspection criteria, and numerous documentation issues. Personnel must become sensitive to potential lead contamination and be able to identify and eliminate all potential contamination sources.
For a complete list of figures, please contact the author.
Douglas J. Peck, director, AEIC Inc., may be contacted at (603) 859-2342; e-mail: dougaeic@aol.com.