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Step 7 Soldering
December 31, 1969 |Estimated reading time: 11 minutes
By Bob Gilbert
As the efficiency and automation of production have improved, major savings now are found in equipment costs and associated overhead. Recent improvements in reflow soldering technology came in response to board and contract manufacturers' needs to solder smaller components, deliver faster throughput for high-volume production, use lead-free soldering alternatives and reduce common manufacturing defects.
Figure 1
Component miniaturization is the driving force behind the redesign of electronics products to be smaller and cheaper, offering more options and advantages than those of earlier designs. An increased number of small components can be incorporated into the same device, maximizing power and reducing the cost of the assembly. In fact, manufacturers today are dealing with many small components that previously were considered impossible to work with. They include chip scale packages (CSP), micro ball grid arrays (microBGA) and small capacitors such as 0402s and 0201s. Today, 0.02" pitch is considered standard, with fine-pitch starting at 0.016". The need to solder such small components has taxed the capabilities of placement equipment as well as soldering materials, and increased the incidence of defects such as tombstoning (Figure 1).Figure 1. Raised, or "tombstoned" components are defects caused by the parts' basic solderability and whether solder paste tackiness is sufficient to hold the component in place during reflow.
Reducing DefectsTombstoning typically is influenced by component solderability and solder paste tackiness. If the solderability of the two device ends varies, one side will wet sooner, causing it to pull away from the slower wetting end. Or, if paste tackiness is inadequate, the component can be dislodged from its position, causing one end to lose contact with the solder paste. Wetting then takes place on the opposite end only, creating a tombstone as the component aligns itself for maximum "relief" during soldering.
In response, new solder paste technology has been developed to address tombstoning defects in all capacitor sizes. New methods use physical (vs. chemical) paste characteristics to minimize the problem.
Tombstoning defects are rising among smaller capacitors because their weight can no longer keep them in place relative to paste adherence forces. New "phase reflow" solder pastes use noneutectic alloy blends, prolonging wetting sufficiently to ensure that the forces at both ends of the component equalize before full wetting occurs, and that maximum wetting force is obtained.
Phased Reflow ActionThese new paste formulations blend two different solder alloys Sn63 and Sn62 at different particle sizes to create a paste range that permits wetting to begin before the solder has melted completely. The alloy that melts at a lower temperature is present in a smaller particle size, which also slows wetting because of greater surface area. Figure 2 illustrates the distribution of the two alloys used and a typical phased reflow paste. In this graphic, the latter alloy consists of a broader mix of powder size diameters than that of typical IPC Type 3 or Type 4 powders.
Figure 2
Phased reflow alloy blends with smaller particle sizes have improved small-component placement printability. In particular, CSPs with openings as small as 0.010" benefit from improved print definition. The phased-reflow blend also gives better aperture release, which reduces incomplete pad fill defects while improving tack force, reducing component movement when using modern high-speed placement equipment.Figure 2.Melting pattern comparison of two solder alloys and a phased reflow paste, which features a broader mix of particle sizes than that of traditional powders.
Many soldering defects besides tombstoning can be traced to the printing process where variables such as environmental challenges and printing delays can have a profound effect on quality. Typical solder defects include incomplete pad fill, bridging of adjacent pads, flux bleed and poor print definition (Figure 3).
Figure 3. Typical solder defects include incomplete pad fill (a), bridging (b), flux bleed (c) and poor print definition (d).
Poor aperture release, paste drying and incorrect printer settings cause incomplete pad fill. Joints with incomplete pad fill will not pass visual inspection post-reflow. Solder pad slump, incorrect printer settings, or a dirty or damaged stencil can cause bridging defects in which the solder paste comes in contact between two adjacent pads. Bridging defects can cause shorts to occur during reflow.
Flux bleed typically caused by excess squeegee pressure, solder paste separation or environmental conditions can cause a flux system to flow away from the solder powder. This defect can reduce tackiness, increase solder balling or affect the solder's ability to coalesce in forming an acceptable joint. Poor aperture release, paste drying, incorrect printer settings and poor environmental conditions can cause the fourth defect unacceptable print definition which may lead to bridging, poor solder joint formation and tombstoning.
Role of RheologyUsing solders with the correct rheological, or flow, characteristics can eliminate such defects. Environmental variables such as temperature and humidity can affect a solder paste's rheological characteristics. Solder pastes must be formulated to print satisfactorily across a wide range of environmental conditions as maintaining a constant environment on the production floor is not always possible. Many manufacturers have multiple locations with varying conditions in their production environment.
Figure 4. A comparison of viscosity profiles for two solder pastes at 20° and 30°C. The effect on paste rheology of temperature changes must be considered for use under varying environments.
A comparison of viscosity profiles for representative solder pastes measured at 20° and 30°C is illustrated in Figure 4. The results confirm that temperature changes affect solder paste rheology, which must be considered when developing a solder paste for use in varying environmental conditions. Humidity changes can have a similar effect on the solder paste's viscosity. Reducing viscosity can increase the chance for bridging due to underside paste buildup on the stencil.
Solder paste's ability to print acceptably after a delay in production is critical to reducing defects on the modern production line. A paste's "abandon time" is defined as the maximum delay between two printing cycles that will sustain acceptable results without reconditioning or kneading the solder paste or cleaning the stencil. Printer downtime typically is due to placement problems or operator breaks. Solder pastes with tolerant abandonment times reduce printer maintenance and solder defects because of paste drying, and improve subsequent processes in the line. Evaluating abandonment time, however, is very subjective because the definition of acceptable printing can vary greatly.
Solder paste formulations can extend unavoidable delays by incorporating unique solvent systems that allow for slow changes to the solder paste composition in ambient conditions. Solder pastes that incorporate solvent systems with boiling points above 250°C offer slower solvent evaporation at room temperature, maximizing paste abandon time.
If solvent system specifications or abandonment time information are not listed in a solder's technical data, a simple test can be performed to verify such delays. Manufacturers may print boards with delays of 30 minutes, one hour, two hours and four hours to determine when printing with a paste formulation is no longer acceptable. Abandonment time will vary with the aperture size, printing environment and equipment used in a specific application.
A paste with long abandonment time also can improve open time, i.e., the paste will better maintain its tack. Should there be a delay after printing, pastes with extended open time will keep components in place before reflow and even can provide adequate tackiness for component placement to occur the next day. Slower evaporating solvent systems also help reduce solder powder oxidation before reflow and can permit the latter step to be delayed as long as 24 hours after printing.
Speeding Up the Soldering ProcessDemand is increasing for solder pastes able to print at higher speeds, thereby reducing printer cycle time. While just three years ago it was uncommon to find a production line running at printer speeds in excess of 50 mm per second, today's paces are exceeding twice that. On a high-volume production floor, increased speed allows for more in-line inspection and reduced production time.
Figure 5. In a test of viscosity vs. high shear (30 rpm), 17 solder pastes were compared for print speed.
Numerous solder paste formulations were evaluated to determine if a relationship exists between the laboratory results of the Malcom Viscosity Test at high shear (30 rpm) vs. actual printing trials that determine maximum print speeds for solder pastes. In the test, solder pastes were printed at 25 mm per second with print definition ascertained for acceptability. If the print passed, the print speed was increased in 25 mm per second increments until the paste failed. Failure was defined as the presence of "insufficients," bridging or missing paste on the sample. Results were compared graphically with the 30 rpm Malcom Viscosity data. The maximum print speeds achieved for 17 solder pastes and their high shear (30 rpm) viscosity results are outlined in Figure 5. Solder pastes with faster printing speeds will continue to increase in popularity as companies attempt to improve productivity. Fast-print solder pastes must be more "fluid" under the shearing conditions of the print process, (i.e., they require viscosities in the 600 to 1,000 cps range to obtain speeds above 250 mm per second).
Use of new gelling agents in solder paste formulations reduces paste viscosity at high shear, but still maintains low slumping characteristics after printing. Manufacturers can recognize new high-speed pastes by reviewing viscosity and slump test results for each solder paste formulation under consideration.
Lead-free SolderingBecause assembly processes traditionally have responded to the unique requirements of Sn/Pb alloys, conversion to lead-free solder will be very complex and will affect all facets of current production. The "ideal" lead-free solder would be an alloy with the same melting point and similar mechanical strengths of Sn63 and Sn62, which would permit the same fluxes to be used in the soldering process. Unfortunately, the alloys currently under development melt at higher temperatures than the lead-bearing alloys, which may affect the flux chemistry's ability to protect the solder powder before reflow while also reducing the flux's activity levels to a point where reflowed joints do not pass visual or mechanical inspections.
No-clean chemistry is the predominant flux type sold in the industry today. No-clean fluxes combine complex mixes of chemicals to accomplish myriad tasks in the soldering process, such as cleaning the parts to be soldered and keeping the solder powder from oxidizing during both storage and reflow heating. These fluxes produce a solder paste that can be applied as required, stay where applied, hold the components in place prior to reflow and be inert after the soldering process.
Usage condition changes limit the flux chemistries and place additional requirements on performance. The table lists the composition of a typical no-clean flux system by chemical group and provides the solder paste performance characteristics that each "ingredient" may affect. Complex interactions lead some outside observers to consider flux formulation to be "black magic." Current Sn/Pb flux formulas for lead-free-alloys will depend on how the flux behaves given the higher reflow requirements needed for lead-free alloys. As flux is subjected to higher temperatures, the activation systems become exhausted quicker and the inherent solution viscosities of the flux are reduced, potentially leading to a reduction in the protective layer of flux that surrounds the solder powder before reflow. While this is not a major concern in an inert atmosphere, it could cause poor results in an aerobic reflow process.
Visual solder joint quality for lead-free alloys noticeably improves when the resin content is high, a finding that is consistent with the theory that a higher resin content allows for more solder powder protection during the reflow process and keeps powder surface oxidation to a minimum. High resin contents permit the powder to coalesce without producing the "powder nodule" effect. The latter is where solder leaves an indention on the joint's surface in the shape of the powder particle, giving the appearance of a grainy, rough surface that may be viewed by inspection operators as a cold solder joint.
Lead-free Solder PerformanceCurrent lead-free solder paste formulations provide the same critical characteristics as those of leaded solders, including printing behavior, printer abandonment time, tackiness performance and life, no-clean residue electrical reliability, and residue appearance. New lead-free technologies are being formulated to address voids in reflow-soldered joints, deficiencies in reflowed joint appearance and solder spread.
By increasing resin content, lead-free solder joints exhibit reduced voids and flux volatiles. Additionally, more protection is provided to the reflowed joint, yielding an improved joint appearance. Other important factors found to reduce voiding are high solder surface tension and lower evaporation solvents. Factors that have less effect, if any, on voiding include reflow profile, choice of lead-free alloy and flux activity.
While increased lead-free reflow temperatures affect the flux system's ability to promote solder powder to reflow and coalesce into a smooth joint, increasing resin content has proven to improve the appearance of the reflowed joint. This is critical for quality control inspectors because the natural appearance of lead-free alloys already is slightly different than that of standard Sn/Pb alloys.
Figure 6. A reflow profile for lead-free solder that provides the best solder spread compared to those featuring preheat soak zones.
By modifying the solder reflow profile, improved wetting and spread can be accomplished. Major improvements in spread have resulted from increasing the resin content and choosing activator packages that have better high-temperature resistance. A slight improvement occurs if the reflow profile is kept as linear as possible, which maintains activity levels at the time of solder melting. The reflow profile shown in Figure 6 provides the best spread when compared to profiles containing preheat soak areas.
To keep defects in lead-free solder processing to a minimum, manufacturers should maintain several key factors:
- Peak reflow temperature is the most important variable in alloy selection.
- The best reflow results are found with linear profiles.
- Sn/Ag3.8/Cu0.5 alloy has the lowest peak reflow temperature without jeopardizing reliability.
- The practical minimum board temperature is 230°C for
- 20+ seconds.
- The correct flux selection, most likely a high resin content flux, will allow for air reflow to produce acceptable spreading and wetting.
- Small temperature differentials across the board and accurate temperature measurements are critical for lead-free processing.
ROBERT GILBERT may be contacted at Loctite/Multicore Solders, 1751 Jay Ell Drive, Richardson, TX 75081; (972) 238-1224; E-mail: Robert.gilbert@loctite.com