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Advancements in Water-soluble Solder Pastes
December 31, 1969 |Estimated reading time: 11 minutes
The more successful formulations have provided several benefits over traditional products, including robustness, environmental variation resistance, compatibility with modern printing technology and void reductions under ball grid arrays (BGA).
By Brian Smith and Heather Wuttke
Controlling the effects of humidity on the production floor is a concern when using traditional water-soluble solder pastes. These materials are hygroscopic (readily absorbing water from the atmosphere) to some extent such that humidity in the factory and in the screen printer could affect performance dramatically. Water-soluble pastes tend to absorb excessive moisture under conditions of high humidity, first leading to cold slumping, increased viscosity and finally to reflow difficulties. Other negative effects include those on stencil life, slumping characteristics and solder balling.
For its part, low-humidity environments typically have been the source of dried-out water-soluble pastes, producing poor print quality and stencil life reductions. In either case, humidity variations on the factory floor have resulted in variable throughput and defect rates in the end product, forcing users to control the humidity to a tight tolerance of 40 to 60 percent relative humidity (RH) just to obtain acceptable results.
Some companies have gone to great expense to control humidity in the plant and the stencil printer but few have the means for perfectly controlling the environment. The result is that the majority of companies using water-soluble solder paste chemistry are resigned to defect rates that will vary with the daily or seasonal changes in humidity conditions.
New Systems Boost ResistanceFortunately, humidity effects on the newer water-soluble paste formulations have been decreased and, in some cases, have eliminated concerns over humidity control. New solvent systems have been optimized to sustain low-humidity print environments by using lower-vapor-pressure chemicals. Also, reducing the hygroscopic character of the solvent and that of other paste constituents permits the materials to resist high-humidity environments more easily.
These modifications to the newest paste formulations also allow for a printing window of 10 to 75 percent RH instead of the traditional 40 to 60 percent. With the wider window, the new solder pastes can be expected to perform equally well regardless of the environment an immense benefit and potential cost savings for assembly operations.
With the changes to the solvent systems, a significant improvement in stencil life has accrued to users of the newer pastes. Because solder pastes are now drying out much more slowly and absorbing much less water, their chemistry remains more stable with respect to stencil life. This, in turn, extends the paste's stencil life from about four to more than 10 hours, an increase that becomes more dramatic in high-humidity environments, i.e., greater than 65 percent RH. (Traditional pastes typically perform well for only one to two hours vs. the newer pastes' six to eight hours in higher-humidity environments.)
Many companies have attempted to control the environment within the stencil printer by using an environmental control unit (ECU), essentially an air conditioner for the printer. But results actually can reflect stencil life reduction. This is because the ECU produces an intense air flow within the printer similar to that of an air-conditioning vent that can dry out the paste rapidly. Further, if an ECU is used to avoid general environmental control within the plant, other paste problems can occur. For example, the paste will be in a cool dry environment while in the printer, but as soon as it is printed and the board goes to parts placement, the cold, dry paste suddenly will be thrust into warm humid conditions, which can cause severe paste slump. While ECUs can be the best option for some operations (especially those with minimal factory environment control), the newer pastes can eliminate their need entirely.
Printing CharacteristicsBecause more than 50 percent of all surface mount assembly defects can be attributed to stencil printing variabilities, defect minimization can best be achieved by placing greater control on the printing process and its parameters. Print-related defects primarily are a function of varying paste volumes from pad to pad and board to board. Reducing such defects must begin by optimizing the entire process to produce consistent print volumes on pads and prints.
The solder paste's stability through various printing conditions becomes the key to reducing the defect rate. The newest paste formulations are capable of consistent print volume independent of idle time, print speed, humidity effects and line throughput (Table 1). All these issues contribute to paste volume variability in traditional paste formulations, producing increased defects as well as line speed limitations and paste scrap.
Idle TimeIdle time is defined as the period in which a stencil printer remains inoperative yet is deficiency-free on the first print after the downtime. Traditional solder paste formulations can tolerate 10 to 20 minutes of downtime and follow with an effective first print. In contrast, the newest water solubles can sit idle for 60 to 90 minutes without drying or creating printing problems.
The mechanism for a short idle time is the small amount of paste that remains in each stencil aperture after each print stroke. The paste left in the apertures has the tendency to dry out and prevent a clean release of the paste from the stencil on the first print after the downtime. Traditional paste products, after 60 minutes downtime, will require five to 10 strokes before an effective print is achieved. In some cases, an hour's idle time will force the user to completely change out the solder paste and start over with fresh material. However, the more innovative products can resist the drying effects of idle time and produce acceptable prints on the first stroke after much longer downtimes.
Idle time advancements are due to a modification of the solvent systems in the new pastes' chemistries. Solvents with lower evaporation rates at room temperature will reduce the dry-out phenomenon. This widens the process window in the screen printer by allowing operators to resume production immediately following downtime for maintenance, shift changes, line changes or operator breaks. Additionally, the newer solder pastes are releasing from apertures more cleanly, leaving less paste behind that may reduce the quality of subsequent prints.
Print SpeedWater-soluble solder pastes traditionally were designed to print at a speed of one or two inches per second. Faster speeds were seldom required as SMT engineers realized that overall line speeds rarely depended on the stencil printer's pulse. Thus, squeegee speeds typically are set low (~1 ips) to optimize paste rolling behavior, aperture fill and print definition. However, as the development of faster component placement equipment has served to increase the typical process pulse rate, the bottleneck more frequently is directed toward the stencil printing process. This has resulted in assemblers requiring new solder paste formulations capable of printing 4 to 6 ips without print quality changes.
If traditional formulations were evaluated for high print-speed characteristics, the result would be poor rolling and insufficient deposits in the fine-pitch areas. But the newer products have reduced viscosity and tackiness, which enables paste to flow into fine-pitch apertures even at increased print speeds. The most advanced of the paste chemistries can deposit a comparable paste volume at any print speed from 1 to 6 ips. Volume changes as print speed increases should be negligible.
Enclosed-head PrintingA recent shift in stencil printing equipment usage has seen fewer companies using traditional methods while more employ the enclosed-head printing systems. The latter units protect the solder paste from the elements by enclosing the material within a "pump head" instead of exposing it to the elements as with standard stenciling methods. This has been an advantage to those that have used this technology because the pastes remain more consistent with respect to time. However, the biggest drawback is that several traditional paste formulations are incompatible with enclosed-head equipment. The materials tend either to separate or demonstrate compaction (a fusing of the individual powder particles), a direct result of the constant pressure under which the paste is contained.
Because the properties and desirable characteristics are different, installing an enclosed-head printing system almost invariably will force the engineer to re-evaluate his solder choice. While traditional squeegee printing places requirements on stencil life, dry-out phenomena and shear thinning characteristics, an enclosed-head system's needs include paste separation and compaction concerns within the head. These performance differences will result in new solder paste evaluations when an enclosed head printer is installed. Though enclosed-head printing is a new application, there are few new solder paste formulations that perform adequately with both methods.
Reduced-viscosity IssuesMany of the requirements placed on the newer water-soluble pastes are related to enhanced and more robust printing characteristics. Such requirements include a finer pitch capability, high-speed printing, longer stencil life, greater resistance to humidity changes and compatibility with enclosed-head printing. These advancements do not come without a price paste viscosity must be reduced to meet these needs. However, when the viscosity is reduced, many engineers feel that slumping and bridging are imminent. The reduced viscosity of these newer products should not be a warning signal to greater defects, however. Rather, they should be the reason that the paste has a larger process window.
Reduced-viscosity solder pastes will tend to produce marginal results in industry-accepted slump test procedures. However, it is important to note that the IPC Association Connecting Electronics Industries has described the slump test as a pass/fail procedure, not a test in which a quantitative analysis is required. If a paste passes the test, it can be assumed that it will not produce slump-induced defects in a normal application.
The change in slump-test characteristics can be attributed to the lowered initial viscosity together with the change in solvent systems of water-soluble pastes. The newer solvent systems do not dry out as quickly as the traditional solvents, which is good for printing, but can be a negative attribute if a hot slump test is performed.
Another byproduct of reduced viscosity can be increased paste bleed-out under the stencil during printing. A lower-viscosity paste will have a greater propensity for this problem, requiring the stencil's underside to be cleaned more often. Improving the gasketing between the stencil and the printed circuit board (PCB), as well as reducing the squeegee pressure during print stroke, can alleviate this problem.
BGA Solder Joint VoidsAlthough the amount of solder voids permitted and their influence on reliability has been debated greatly, most engineers prefer minimal voiding in their BGA solder joints. The IPC recommends less than 25 percent voids in BGA joints. With traditional pastes exhibiting voids of at least 20 percent, this presents a common dilemma.
Numerous theories exist on the exact cause of voids, which essentially are entrapped volatiles. They form either from reaction byproducts produced as the flux attacks the oxides or from outgassing by the flux itself. One approach to counteract this inherent hindrance is to modify the reflow profile, extending the soak-time zone and maintaining a very cool peak temperature (sometimes as low as 203°C). The theory is that the long soak allows for complete flux solvent volatilization. Unfortunately, however, a longer soak zone can over-oxidize metallized leads. For most components, this will not be a problem. Unfortunately many parts feature marginal solderability, and because a main reason water-soluble paste is used is related to soldering these suspect parts, a soak zone extension may produce an unacceptable soldering result.
Additionally, some metallizations will not solder well at a lower peak temperature. For instance, nickel and palladium form intermetallics with tin at a rate of only about one-twentieth of that of copper, which means that higher peak temperatures and longer times above liquidus are required to form good solder joints. For nickel/gold boards or palladium parts, a peak temperature of 220° to 225°C is mandatory for proper intermetallic formation. With traditional water-soluble pastes, however, these profile requirements to avoid BGA voids have tended to limit the engineer's control of the process. In response, the new pastes are formulated for reduced solder joint voids whether the profile is quick and cool or long and hot. Additionally, they produce less than 5 percent voids consistently in a BGA joint regardless of profile used, permitting the process engineer maximum flexibility in reflow profiling parameters with BGA assemblies.
CleanabilityCleanability vs. Time: All water-soluble solder paste products have post-reflow residues that require complete cleaning after soldering. This is because the residues have active organic acids present that are conductive and corrosive. Generally, when the flux residues emerge from the oven, they are fairly inert. Due to the hygroscopic nature of water-soluble products, however, the residues absorb moisture from the air, resulting in an ionic mobility increase, i.e., the active acids on the PCB now are capable of movement and corroding the metals of the soldering process.
Similarly, for example, if salt is placed on a circuit board, nothing happens until water is added. Because the hygroscopic nature of pastes has been reduced, however, the flux residues can remain on the board after soldering for a longer time before cleaning is necessary. While the newer formulations still require complete cleaning, the added advantage is that the cleaning window will have been extended from a typical eight to 96 hours. This means that the cleaner will not have to run immediately after soldering and the residues can be removed safely even days after soldering occurs.
The increased cleaning window is an advantage for low- to medium-size users that typically clean PCBs in a batch cleaner. The newer pastes can be cleaned effectively even days after soldering, which may reduce the number of times per week that a batch cleaner must be used.
When Mixing Flux Chemistries: Because all organic-acid flux chemistries must be washed completely, it has been commonplace for assemblers to clean boards after each process step because of flux interactions (e.g., between a paste and a liquid flux) that result in difficult-to-clean byproducts. This concern has been alleviated through flux compatibility testing advancements among soldering materials suppliers. The latter now should be able to offer a materials water-soluble system that will be intercompatible even without cleaning after each step of the process. Because flux materials have been designed to work well with each other not just independently this could eliminate one or more cleaning steps.
BRIAN SMITH and HEATHER WUTTKE may be contacted at Litton, Kester Solder, 515 E. Touhy Ave., Des Plaines, IL 60018-2675; (847) 297-1600; Fax: (847) 699-4980; E-mail: bsmith@kester.com and hwuttke@kester.com; Web site: www.kester.com.