Solder Paste Basics: A Round-up
December 31, 1969 |Estimated reading time: 10 minutes
This article asks major suppliers some of the basic questions that every user wants to ask about solder paste. Two major EMS providers, Celestica and Flextronics, also commented on how they choose which materials and what factors are important to them.
By Gail Flower, SMT
What should be a user’s key considerations before choosing a solder paste?
Celestica’s process development team qualifies paste and fluxing processes to evaluate new-generation materials in various global manufacturing settings. Phase 1 tests evaluate the performance of the paste for printability and wetting. Phase 2 test focuses on the solder joint metallurgy and mechanical properties. Phase 3 test looks at the compatibility of paste when mixed with other manufacturing chemicals by using IPC surface insulation resistance (SIR) test method. Phase 4 tests the in-circuit pin testability of paste flux residue, and phase 5 test focuses on the reliability of paste alloy in thermal cycling. The first two phases are performed on Celestica’s internally designed vehicle. “This covers a range of components including some with pitches as small as 0.4 mm,” says Simin Bagheri, corporate process development engineer. In phase 2, different types of components are assembled on the test vehicle using the thermal reflow profile recommended by the paste supplier (Figure 1). Solder joints formed are then tested for metallurgy and mechanical properties. Phase 4 is performed on an internally designed test vehicle to evaluate in-circuit pin testability of the paste flux residue on different surface finishes when different probe types and sizes are used. In phase 5, using the IPC 9701A specifications, solder joints formed are tested at 0° to 100°C for 6,000 cycles or -40° to +125°C for 3,000 cycles.
From the supplier side, Brian Smith, global sales and marketing manager at Kester Paste, agrees that on-site testing of material is necessary. “It is critical to take a paste through a typical production process before selecting it,” he says. Lab tests, while effective in screening for basic performance characteristics, may not be able to replicate actual production demands on solder pastes. “Specifically, can the solder paste print effectively over an eight-hour shift without breaking down and causing slumping/bridging defects?” he asks. Can it perform across the entire range of temperature and relative humidity in the plant? Can it work with a variety of component metallization on all of the manufacturer’s products? “I am not advising to ignore all of the more basic lab tests, but recognize them for what they are – a simple screening tool – and not a replacement for a full production evaluation.”
Rick Lathrup of Heraeus adds that printability, wetting robustness, takiness, storage stability, flux residue appearance, hot slump, voiding level, profile sensitivity, enclosed print head compatibility , cleanability for water wash, odor, a abandon time, stencil life, SIR, electromigration, and halogen content are just a few considerations.
Figure 1. Celestica’s Phase 2 paste evaluation test vehicle.
Tippy Wicker, director of product management, Qualitek International Inc., agrees that manufacturing capabilities are important considerations. Key questions should be: Does the supplier manufacture their own powder and formulate their own paste flux or do they purchase from a second source? Does the supplier manufacture in big or small lots? Is viscosity tested? Having global sites to support manufacturing is needed in a global market. A complete solder line with many offerings is important as well. She also suggests that paste characteristics (shelf life, printing capabilities, reflow and post-reflow characteristics) and reliability data are points to consider.
Mitch Holtzer, global product manager for Cookson Electronics, suggested that considerations start with the basics: lead or lead-free, no-clean or water soluble, in-circuit pin testing, halogen free. Another consideration is the finest feature of an assembly process and if a soak profile is required.
John Vivari, applications engineering supervisor at EFD Inc. reduced considerations to four main ones: temperature-related limitations, joint strength requirements, the ability of the flux to wet well to medals being soldered, and the ease of cleaning the method of choice required.
The alloy is key, says Masato Nakamura, business development manager, Nihon Superior. “The alloy will determine the solder joints’ reliability long after any issues involved in making the joint are forgotten. Primary considerations include the microstructural stability of the alloy over the required life of the product under the conditions to which it will be exposed.”
Mario Scalzo, senior technical support engineer at Indium Corporation, listed dispensing vs. printing, smallest needle diameter/aperture opening, metals to be soldered, operating temperature, maximum reflow temperature (component limitations), lead-free, halogen-free, voiding, issues such as head-in-pillow defects (Figure 2), and tensile strength requirements.
In general, you want a material that meets process needs and produces a product with the level of reliability required.
Brian Toleno, Ph.D., of Henkel adds that the two main questions are water-wash or no-clean, then tin/lead or lead-free. “After that – does the material meet the process needs and the final product requirements?” In printing, if the device is fine pitch, then excellent printability, little to no slump, and the correct particle size are critical (Figure 3). In reflow, if the PCB is a dense board with a large ΔT, the paste should have a wide reflow process window. After reflow, ask if pin probability is a concern. “What about voiding if you have area array components?” he asks. Every assembler has a different set of criteria that is important to them.
Figure 2. A head-in-pillow solder past reflow defect.
Jim Wertin of FCT Assembly agreed with this matchmaking of purpose to product. “Consider the application method,” he says. Will the paste be printed or dispensed? Seasonal environmental conditions should be factored in ... including humidity and temperature swings. Tac time, alloy selection, powder classification, and, of course, chemistry type should be at the top of the list. “The chosen solder paste should, at minimum, pass the required IPC-J-STD tests for slump, solder balling, and flux classification.”
Key considerations before choosing the right paste should be application-driven, agreed Gerjan Diepstraten, process support manager at Cobar, member of the Balver Zinn Group. RoHS regulated or exempted is a good place to start. Next, look at the end product: automotive, industrial, consumer, or aerospace. The issue of price expressed in good printing, long stencil life and wetting, and elimination of rework should be weighed.
Howard P. Stevens of Metallic Resources Inc. added that whether the boards will be washed or not determines the flux type, and at what temperature the paste should reflow will determine the solder alloy. “Whether or not the board need be RoHS compliant will determine the type of paste.”
Solder paste selection typically is driven by a need to reduce or eliminate nagging manufacturing defects. “Quality and manufacturing engineers collectively should focus on reducing the highest defect while increasing the manufacturing process window,” says Karl Seelig, VP technology of AIM. Most common requests are for fine-feature printing, less voiding, and elimination of head-in-pillow defects (Figure 2).
Dongkai Shangguan, Ph.D., VP and senior fellow, technology leadership group at Flextronics, concluded that the foremost consideration is reliability. The type of alloy used for the solder paste will have direct impact on the thrmomechanical reliability and dynamic mechanical reliability of the solder joint. The flux system will have direct impact on the electrochemical (electromigration, dendritic growth, corrosion, etc.), reliability, as well as RF performance. “Of course, solder paste directly impacts quality,” he says. The considerations include printability, slump, wettability, solder balling, thermal stability (i.e. robustness with different reflow profiles), voids, etc. Consistency in solder paste quality is of great importance to high-volume SMT production.
What percent of leaded paste do you sell as compared to lead-free alloy?
Most suppliers answered that the volume of leaded to lead-free depends on the local market. “In Asia, the market has become a clear majority of lead-free users with more than 80% of the market share,” says Smith of Kester. European consumption is more evenly split, roughly 50/50 between the two alloys. North America, according to Smith, is still the most heavily dependent on leaded alloys, with approximately 25% conversion to lead-free. This is to be expected with the high concentration of high-reliability electronics (automotive, medical, military, aerospace) produced in North America as compared to other regions.
Table 1. The most used lead-free alloys
Most suppliers came close to the same percentages depending on where they sell the most paste and for what application. Qualitek and FCT Assembly both sell 70% leaded. Metallic Resources sells approximately 50/50. Heraeus sells approximately 50/50 in Asia, and 60/40 WHAT TO WHAT? in Europe and the Americas, though there is a Februaryly change in these ratios toward lead-free. Cookson reported selling >65% lead free globally. Diepstraten of Cobar Europe BV says, “We have a large number of automotive customers that are still soldering with tin/lead alloys.”
AIM’s Seelig said that lead-free varies by region of the world and customer base. Consumer electronics use mostly lead-free materials. Military and medical products do not. Servers and industrial controllers are using mostly tin/lead. Some previous lead-free exemptions are due to expire at the end of 2014, which will drive greater use of lead-free materials in the years ahead.
What are the most used lead-free alloys and why?
Lathrup of Heraeus responded with a table listing Sn96.6Ag3.0Cu0.5 at 17% market share (Table 1). The most common lead-free alloy is SAC 305 (3.0%AG, 0.5%Cu, 96.5%Sn), according to Toleno of Henkel and the majority of suppliers. “This is primarily due to the industry-accepted reliability behavior of this alloy and the industry standardization onto this alloy following an IPC-sanctioned test report released a few years ago,” adds Kester’s Smith.
Other alloys had favorable indications as well. “We also see a growing interest for SN100C because of its reliability, voiding, shiny appearance and cost,” claims Cobar’s Diepstraten. SN100e alloy has comparable performance, though it has a higher melting temperature, says Wicker of Qualitek. Colin Longworth, managing director of DKL Metals, asserts that the current high price of silver will drive change in today’s assembly marketplace. “The price places silver-containing alloys at a disadvantage and is making consumers consider their choice of alloy.” Component manufacturers prefer Sn95Sb5 and other higher-melting temperature alloys, says EFD’s Vivari. “SAC305/405 is the most common at the moment due to its early adoption and approval from IPC and INEMI.,” says Wertin of FCT Assembly. He, too, adds that SN100C and copies of this alloy are growing rapidly and should surpass the SAC alloys in the near future. Scalzo of Indium says that other popular lead-free solder pastes are low-temperature alloys made with indium or bismuth, though the most common lead-free alloys are still the SAC compositions.
Figure 3. An example of solder paste printing
In addition to the SAC alloys, because the cost of silver is rising, interest levels in low- or no-silver alloys are growing adds Seelig. Stevens of Metallic Resources agrees, “As silver and tin increased in cost, contract manufacturers became more open to examining other lead-free alloys, most notably Sn/Cu/additive alloys. These made great inroads, since they solve all the problems associated with SAC alloys.”
Does no-clean mean no residue when it comes to paste?
No was the resounding answer from all suppliers. Then what does it mean? According to Stevens and Lathrop, it means that any residue left on the board will be benign and will not cause corrosion. Wertin adds that it is non conductive as well. Vivari says that the residue is dry after reflow without being tacky, and it passes IPC J-STD-006 SIR testing after being reflowed with the recommended profile. It’s safe to leave the residue on the board say Ligouri, Toleno, and Seelig. There will always residues, since flux is needed for spreading, stencil life, and other paste requirements adds Diepstraten. Holtzer identifies the amount of flux residue as 5–10% post reflow. Smith identifies the residues as rosin-based with other materials trapped in the solidified rosin, which tends to be an insulator and beneficial in preventing both corrosion and moisture absorption.
Scalzo says, “It is the responsibility of the finished good manufacturer to confirm that the solder paste is an acceptable no-clean product for their application. Conditions that may affect a product’s no-clean status include: reliability requirements, the component’s sensitivity to residue, service temperature, service and storage conditions (humidity, temperature, etc.), guaranteed product life, etc.”
Conclusion
As RoHS and other regulatory initiatives, such as the Montreal Protocol, change requirements for solder paste, a prodigious amount of testing has been done by paste suppliers. This industry report is a portion of the overall survey on solder paste basics in which suppliers share their knowledge and experience with users. The rest of the survey will appear in future articles. SMT
Gail Flower, editor-at-large, SMT, may be contacted at gailflower@comcast.net.