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No-clean Solder Paste Innovations
December 31, 1969 |Estimated reading time: 12 minutes
By Brian Smith
In the past few years, several innovations in the performance of no-clean solder pastes have been introduced.While the majority of electronics assembly companies are still using "traditional" no-clean solder pastes, more innovative products are quickly making their way onto production floors all over the world. These pastes offer more robust printing characteristics, wider reflow profile windows and enhanced probeability. These developments provide users with several reasons to perform qualification testing on new paste products.
Recent Developments in No-clean Solder PasteThe no-clean solder paste formulations that have been successful in the last few years have provided several benefits over the more traditional products from the early and mid-1990s. These new products are more robust than their predecessors in three main areas of electronics assembly: printing, reflow profiling and pin testing. They also provide the industry with a stable, consistent product with respect to shelf life and batch variability. This article outlines the major areas of improvement in no-clean solder pastes (see Table below), giving the SMT engineer several attributes to examine during solder paste evaluation.
Printing CharacteristicsTraditional pastes were only adequate in terms of printing; they never offered end users a wide process window in terms of idle time (printer downtime), print speed and the effects of shear thinning. All of these issues contribute to defects, line speed limitations and paste scrap. The best of the new paste formulations can open the process window in all three of these critical areas.
It has been estimated that more than 50 percent of all SMT defects can be traced back to the stencil printing process. These defects are primarily a function of varying paste volume from board to board. To reduce print-related defects, the goal of the printing process should be to produce consistent print volume on every pad and every print. Therefore, the stability of the solder paste under these varying printing conditions becomes the key to reducing the defect rate. The most innovative paste formulations are capable of consistent print volume, independent of idle time, print speed and shear thinning effects.
Idle TimeIdle time is defined as the amount of time that a stencil printer can remain idle without any printing deficiencies occurring on the first print after downtime. Traditional formulations have the capability of 10 to 20 minutes of downtime with an effective first print. The newest formulations are able to sit idle for 60 to 120 minutes without drying out or creating printing problems.
Following each print stroke, a small amount of paste remains in each stencil aperture. This paste has the tendency to dry out in the apertures and prevent a clean release from the stencil on the first print after downtime. If left idle, older paste products will require 5 to 10 print strokes before an effective print is achieved after a 60-minute downtime. More innovative products are able to produce acceptable prints on the first stroke after a longer downtime, which reduces waste and improves line efficiency.
The improvement in this area is the result of modifications to the solvent systems used in paste chemistries. Solvents with lower evaporation rates at room temperature will reduce the dry-out phenomenon. This greatly opens the screen printer process window by allowing operators to immediately resume production following downtime for maintenance, shift changes, line changes or operator breaks.
Print SpeedIn the early to mid-'90s, most pastes were designed to print with a maximum speed of 1 or 2" per second. Faster speeds were not required as SMT engineers realized that overall line speeds were rarely dependent on the speed of the stencil printer. Therefore, print speeds were set fairly low in an effort to optimize paste rolling behavior and print definition. However, as development of faster placement equipment is increasing the typical SMT line speed, the stencil printer process is often the process bottleneck. This has resulted in SMT engineers requesting solder paste formulations that are capable of 4 to 8" per second without any substantial degradation in print quality.
If traditional formulations were evaluated in high-speed print applications, the typical result would be poor rolling and insufficient deposits on the fine-pitch areas. 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 will be capable of depositing a comparable volume of solder paste at 1 or 8" per second. As print speed increases, changes in volume should be negligible.
While reduced viscosity assists with the print speed and idle time issues of older pastes, simply reducing the viscosity alone will not solve these problems. Reducing viscosity can present other issues in solder paste development, including cold slump, poor brick definition (a potential cause of bridging), and poor resistance to temperature and humidity conditions. It is of particular importance to reduce viscosity without increasing the paste's propensity to slump. Only the most innovative paste formulations can accomplish increased idle time and print speed without creating more slump-related defects. Viscosity and slump are not necessarily related phenomena. While a reduced-viscosity solder paste may seem more likely to slump, this is not necessarily the case. A paste can have a reduced viscosity without any change in slump properties, because the anti-slump characteristics can be built into paste chemistries.
High print speeds are generally only achieved when the squeegee pressure is increased to above 2.0 lb per linear inch of squeegee. The combination of high print speed and increased squeegee pressure can cause a different problem associated with high-speed printer processes: shear thinning.
Shear ThinningShear thinning is the breakdown in viscosity characteristics as the solder paste is placed under stress. Greater stress will tend to cause greater viscosity losses, which will create a "soupy" paste on the stencil; this can promote bridging defects. Therefore, it is a challenge for the paste formulators to develop products that can sustain high stresses (from the high-speed and high-squeegee-pressure printing) without losing a substantial amount of viscosity character. Older pastes break down very quickly when placed in a high-speed print environment, while new formulations are capable of increased print speed without significant shear thinning. Because the amount of shear thinning directly impacts the viscosity and ability to print, shear thinning minimization must be a goal of the paste formulator.
Consistent print quality can only be achieved if the paste remains relatively constant during the print operation, regardless of the print speed or idle time. The most advanced solder paste chemistries are capable of long-term print stability, regardless of the print speed, squeegee pressure, amount of idle time or length of time that the paste has been in use.
Reflow RobustnessEvery solder paste comes with a technical bulletin that describes that particular paste's optimal or recommended profile. However, many SMT processes cannot accommodate these specific profiles. As the variety in board style (size, thickness and density) and component style (including more large area-array packages and alternative finishes) has increased, the need for enhanced reflow profile robustness has become more evident. Contract manufacturers, in particular, tend to want one solder paste that will perform equally on any SMT line, regardless of line speed, profile, metallization or component concerns.
Older solder paste formulations have activator packages that are relatively profile-dependent, meaning that severe deviation from the recommended profile may create problems with non-wetting or dewetting of solder. For example, one paste may successfully accommodate a three-minute profile (room temperature to peak temperature) and peak temperature of 210°C, but may not accommodate a five-minute profile with higher peak temperatures. This paste will begin to dewet as the profile extends beyond the recommended profile requirements. Such pastes experience a quick consumption of flux activity during the soak zone of the profile, limiting them to shorter reflow profiles. Conversely, other solder paste formulations may have good wetting characteristics when subjected to long, hot profiles, (five minutes, 225°C) but may not perform as well with quick, cool profiles (three minutes, 210°C). Such products will exhibit incomplete wetting when subjected to the shorter reflow profiles. These pastes require a full soak zone to chemically activate the flux, which is why they do not perform well with quick profiles.
The narrow reflow windows of standard no-clean solder pastes are starting to cause a greater number of wetting issues, especially with the constantly increasing variation in thermal requirements that SMT processes incorporate. Some pastes cannot sustain the profiles required by large ball grid array (BGA) devices, leaving the SMT engineer in a quandary. Should the reflow profile match that of the paste supplier's recommendations or that of the component's thermal requirements? Which is less critical to the overall application?
Fortunately, the advances in paste technology are helping to make this a moot point. The newest no-clean solder pastes are capable of consistent wetting behavior, almost completely independent of the time and temperature before reaching the reflow temperature. This represents a huge benefit to the process engineer that may have a large variety of board sizes, board thicknesses, component finishes and package styles. It allows the SMT engineer to set the profile optimally for the process and for defect minimization, not the solder paste. The process engineer now can think in terms of optimizing the entire process.
The Lead-free IssueAnother profiling concern is the potential shift to lead-free alloys. The majority of reflow profiles reach peak temperatures in the range of 203° to 230°C for standard Sn63 and Sn62 alloys. However, most lead-free alloys have melting points that are 30° to 45°C greater than that of Sn63, resulting in peak temperatures that may reach 255°C or higher. The majority of traditional no-clean solder pastes have difficulty performing effectively at such high peak temperatures they typically cause problems with dewetting and residue color, as many types of rosin become darker with increased temperatures. These issues may prevent successful implementation of traditional solder paste formulations in lead-free alloy systems, forcing engineers to search for a paste that is more profile-robust. Recent solder paste formulations tend to have activator and rosin systems that are capable of similar wetting behavior and residue color even at peak temperatures as high as 255°C. Engineers currently performing solder paste evaluations should be conscientious of the paste's ability to sustain reflow profiles with greater peak temperatures, which will be required by lead-free alloys. Finding a solder paste flux that is flexible enough to provide similar wetting characteristics, regardless of the alloy's thermal requirements, could save engineers time when (and if) the conversion to lead-free alloys takes place (Figure 1).Figure 1. Reflow profile robustness.
Residues and Pin TestabilityFalse failures in in-circuit test (ICT) equipment are another reason to consider a change from a traditional product to a more innovative formulation. Traditional products are known to have a residue character that is either too hard or too sticky (or tacky) for an optimized ICT process. Hard residues will create non-contact failures because of non-penetration; sticky residues will eventually clog test probes, ultimately resulting in non-contact failures. Beyond these concerns, traditional solder paste residues typically get harder with respect to time; the ICT window following the reflow process may be as short as two or four hours. The goal of the solder paste formulator is to soften residues without making them too gummy. Furthermore, the residues should remain relatively probeable over a period of weeks instead of hours.
Recently developed no-clean solder pastes are solving these issues. Residues of these products tend to have soft, nonsticky residues that get out of the way of the test probe without sticking to the probe itself. Such paste residues can be subjected to several thousand ICT hits without false failures or clogged pins.
Although probeability of no-clean solder pastes has improved dramatically in recent years, some of the products that are solving these issues also have trade-offs that must be considered. While soft residues may seem inherently good to the test engineer, these residues can create long-term reliability concerns. The reasoning behind this is related to the required stability of rosin residues at board-operating temperatures. Ultra-soft residues can become liquidus at lower board-operating temperatures, which can liberate entrapped activators and ultimately cause current leakage and reliability concerns. Such formulations will have residue that is very reactive toward copper during surface insulation resistance (SIR) testing; the green discoloration of the copper will be evidence of this reaction.
The solder paste formulator must carefully consider the balance between reliability and probeability. In most cases, a completely solid residue will present probe penetration issues. However, an overly soft residue will commonly fail standard industry reliability tests. Therefore, the newer solder pastes are required to find a comfortable middle ground that demonstrates acceptable probeability and residue stability. Assuming that the softest residues will create the greatest reliability concerns, reliability failures can only be avoided by making the residue more solidus (stable) or by making the entire flux system less active. In the end, the ultra-soft residues will be problematic in either the reliability or the activity area. The optimal solution for balancing probeability and activity must incorporate a standard level of activity and a residue that remains semi-solid at SIR test and board-operating temperatures. The most recent solder paste formulations are capable of ideal probeability characteristics, without compromising the reliability of the residue or the activity of the paste.
Another drawback to ultra-soft residues is the compatibility with conformal coatings. Conformal coatings tend to be slightly reactive to some no-clean solder paste residues; this becomes critical as residues become softer and more mobile. When qualifying a new solder paste, it is crucial to examine the compatibility properties between paste residues and conformal-coating products. Many products that are designed with ultra-soft residues tend to have adhesion problems with some conformal-coating formulations.
Other InnovationsShelf life. No-clean solder pastes are reactive mixtures, and the reactions taking place are altering the overall chemistry and performance of the product. Storing the solder paste at refrigeration temperatures will reduce the rate of these reactions, but not prevent them from happening. Some degradation of the solder paste properties is always occurring in the container. Traditional formulations would typically see substantial performance degradation within the first three to four months of refrigerated shelf life; recent formulations have extended this to six to 12 months. This means that the reactions within the container are slowing down substantially, and paste delivered to the factory floor ought to be more consistent with respect to time.
Lot-to-lot consistency. All standard no-clean solder pastes contain some ingredients that are found only in nature (not manufactured), meaning that some variation should be expected from lot to lot. The majority of this variation is recognized in the stencil printer, where substantial fluctuations in viscosity can wreak havoc on print repeatability. While traditional paste products were highly vulnerable to this lot variability problem, it has been significantly reduced in recent for-mulations. Innovative products contain a lowered percentage of naturally occurring ingredients, leaving the overall flux chemistry more controllable. The lot consistency of the newest paste products results in a solder paste that can be counted on to be repeatable and reliable in terms of performance characteristics.
SummaryNo-clean solder pastes have improved greatly in the printing, profiling and ICT areas. Paste products have become more robust and operator-friendly. The innovations of no-clean solder pastes have helped to increase line speeds, decrease defects, reduce waste, maximize oven flexibility and eliminate false test failures. All of these reasons give SMT engineers plenty of potential improvement areas as they consider new no-clean solder pastes.
BRIAN SMITH may be contacted at Kester Solder, 515 East Touhy Ave., Des Plaines, IL 60018-2675; (847) 699-5515; Fax: (847) 699-4980; E-mail: bsmith@kester.com