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Optimizing Lead-free Reflow Processes
December 31, 1969 |Estimated reading time: 8 minutes
Lead-free SMT assembly can be achieved reliably, however, several variables should be taken into consideration.
By Peter Biocca
The most common alloys used in lead-free SMT, tin-silver-copper (SAC) alloys, have a melting temperature range between 217°-220°C. This is a higher melt temperature than traditional leaded solders such as the 63/37, which has a melting point of 183°C. Because the melting point of lead-free alloys is higher, the thermal profile also requires optimization to avoid excessive temperatures on components and boards. Typically, peak temperature ranges for SAC alloys will be between 235° and 245°C. However, if the boards have small thermal masses and the oven has sufficient heating zones, peak temperatures down to 229°C have been used (Figure 1). With hotter preheats and higher peak temperatures, the classification of non-hermetic solid-state SMD components may change. Moisture pick-up can further aggravate component reliability with lead-free reflow soldering. Internal delamination, cracks, bond lifting, die lifting and, in extreme cases, popcorning effects can occur. The IPC/JEDEC J-STD-020 specifies the new reflow requirements for small to very large-bodied components (Table 1).
The flux chemistry used in lead-free pastes is designed to minimize issues associated with higher reflow temperatures, such as increased paste slump and residue charring. Solder paste manufacturers are using resins and gelling agents, which offer hot-slump resistance and activators that are stable at higher preheats, as well as at higher peak reflow temperatures.
Figure 1. Lead-free reflow profile.
Figure 2 shows two lead-free solder pastes after hot slump testing at 185°C. Paste A has gelling agents incorporated within its flux system that do not prevent slumping. Paste B contains gelling agents, which prevent slumping at higher preheat temperatures; this paste will reduce the incidence of bridging, solder balls and mid-chip balling. Paste B has better hot slump behavior.
Figure 2. Solder pastes printed onto inert ceramic and heated at 185°C in air.
Due to the lower wetting speeds associated with alternative lead-free solders, flux activation will be a critical factor in paste performance. No-clean and water-washable solder pastes are being designed so they do not require nitrogen reflow and can produce reliable solder connections with good wetting in air. Water-washable solder pastes with their higher concentration of activators will solder most metal finishes adequately. No-clean solder pastes require careful finish and paste attribute selection.
Figure 3. Impact of nitrogen on tin/silver/copper solders.
Nitrogen will impact solder joint cosmetics (Figure 3) by making solder joint surfaces brighter and more uniform. Nitrogen reflow also will enhance wetting with lead-free solders, especially on bare copper OSP surfaces. The test in Figure 3 was achieved by printing lead-free solder paste on a white ceramic substrate and reflowing one in air and the other in nitrogen. Although the surfaces look different, this is only a surface reaction.
Lead-free Defects
Common defects can be avoided in a properly optimized process with a lead-free solder paste designed to give adequate wetting, low slumping and low voiding with lead-free alloys. Good activity at higher reflow temperatures will be achieved by choosing solder pastes with activator packages that do not decompose before 217°C. The common defects associated with lead-free include off-pad solder balling, mid-chip solder balling, tombstoning, bridging (shorts) on fine-pitch QFP leads, open joints, non-wetting, de-wetting, cold solder joints, voids and excessive dullness or surface cracks.
When soldered, SAC alloys will wet the metallization at substantially reduced rates when compared to 63/37. Solderability is impacted by the speed of wetting and the degree of spread. Pure tin finishes are the most suitable to solder with SAC alloys. Bare copper OSP usually gives the poorest results, especially on assemblies that have seen a previous thermal process. Silver immersion and Ni-Au finishes give values between tin and copper, and are common lead-free choices.
Solder joints containing bismuth, such as those using SnAgBiCu lead-free paste to solder lead-bearing terminations are not recommended. High-temperature storage of 1,000 hours at 150°C resulted in more solder joint cracks (Figure 4).
Figure 4. Close-up of fracture.
Lead-free solder pastes designed for both lead-free alloys and alloy specific will function best and help prevent solder defects. However, there are defects associated with lead-free soldering, including bridging, solder balls and mid-chip balling. These can arise from the solder selection process. Because preheats are higher with lead-free, the hot-slump character of the paste is critical; solder pastes with good hot slump at higher temperatures such as 185°C are needed.
Termination and Pad Wetting
Non-wetting or insufficient wetting also can be encountered. Different metallizations will exhibit differing spread and wicking characteristics. Flux activity also plays a key role. During solderability testing using wetting balance instruments, lead-free SAC alloys demonstrated the best wetting when water-washable flux systems were used. No-clean flux systems containing less activator and/or halide-free demonstrated lower wetting speeds and lower maximum force readings.
Poor solderability, insufficient wetting, poor wicking of solder and large contact angles also can result from an inadequate thermal profile. Good thermal equilibrium across the whole board is paramount. This becomes more important with lead-free because the peak temperature window is narrower. SAC alloys melt at 217°C, while the peak temperature needs to be in 235°-245°C range.
Figure 5. (a) No wetting due to low heat (b) Excessive temperature (c) Good thermal profile
Balls will not reach reflow when there is insufficient heat (Figures 5a, b, c). Measuring temperature accurately at the ball site alleviates this. Excessive temperatures should also be avoided. When the thermal profile is set properly, proper collapse of lead-free balls is reached. The standoff distance may be higher with lead-free SAC due to its higher surface tension.
Lead-free Joint and BGA Voids
Excessive solder voids can create reliability issues, especially in applications where the lead-free assembly is exposed to thermal cycling conditions, or in applications where the assembly is exposed to vibration or flexing during box builds. Voids also can reduce thermal performance and electrical integrity.
In some cases, smaller voids can increase reliability by changing the crack pattern. Studies have shown that there is no reliability reduction when voids are present to up to 25% by volume in the joint. Voids can act as stress relievers, due in part to the compressive nature of air pockets.1
Lead-free alloys such as SAC alloys have slightly higher surface tensions when compared to 63/37. It is important to select a solder paste that has a flux chemistry designed for the higher preheats and peak temperatures. Choosing a solder paste that does not contain resins and activators and that decomposes at these higher temperatures is a factor in void reduction. Optimizing the reflow profile to remove any volatiles by extending preheat times and increasing time above liquidus can reduce void entrapment. Ensuring components and boards are free of moisture and plating contaminants also reduces voids.
Tombstoning Defects
Lead-free may increase the uplifting of smaller components. This is due, in part, to the reduced wetting behavior of lead-free alloys. Component placement is more important with lead-free alloys because less centering will occur during reflow, increasing the incidence of tombstones.
SAC305 tends to reduce tombstones. This alloy has a concentration of 96.5Sn-3.0 Au-0.5 Cu, and a melting range of 217°-220°C. Because of the small pasty range, the component prone to tombstoning is tacked by the initial melting phase of the alloy. On the other hand, a solder paste that exhibits excessive out-gassing during initial solder-powder melting stages will increase tombstone defects.
De-wetting with Lead-free
De-wetting often is due to a lack of flux activity. This rarely occurs with water-washable type pastes because these pastes are highly activated. Lower-activity solder pastes tend to create this on more difficult finishes, such as bare copper OSP or Ni-Au where the nickel-based metal may have experienced oxidation or plating contamination.
SAC no-clean paste was applied to two surfaces of test coupons. The test coupons then were reflowed in air using the manufacturer’s recommended thermal profile. The one on the left shows de-wetting, while the one on the right exhibits good wetting. Solder pooling was due to the base metal being difficult to solder. The molten solder initially spread across the surface, without a stable intermetallic bond, resulting in surface tension pulling the solder away.
Excessive Dullness and Surface Effects
SAC alloys offer solder joints that are less reflective than 63/37; the contact angles tend to also be higher and spread less. These are not considered defects, but cosmetic attributes. If air reflow is used, SAC joints will be less bright and will show surface effects such as crazing. These are due to the intermetallics within the solder and oxidation effects. If nitrogen reflow is used, the joints will be more reflective with enhanced spread. Lower peak temperatures and lower times above liquidus reduce intermetallic growth, but increase the overall solder-joint brightness.
Conclusion
Developing a lead-free SMT process requires planning and a close-working relationship with all suppliers. Understanding component and board compatibility issues with the use of higher temperatures is essential. Avoiding certain elements, such as bismuth and lead that may impact solder joint reliability also is important.
First, a solder paste alloy must be selected, then a compatible flux system is needed to meet the solderability requirements of the finishes to be soldered. The reflow process needs optimization to warrant good intermetallic bonding while avoiding temperature excesses to prevent board delamination, component damage, excessive intermetallic growth, solder-surface and flux-residue effects.
References
- Martin Wickham, National Physical Laboratory, “Voiding: Occurrence and Reliability Issues with Lead-free.”
Peter Biocca, senior development engineer, Kester, may be contacted at (972) 390-1197; e-mail: pbiocca@kester.com.
Variables in SMT
- Melting temperature of alloy
- Flux chemistry activation, temperature effects
- Alloy wetting and surface tension properties
- Solder balling and bridging potential increases
- Component/board reliability
- Compatible rework/repair
- Compatible wave, selective soldering processes
- Quality inspection criteria modifications
- Cosmetic effects of flux at higher reflow temperatures
- Nitrogen vs. air reflow
- Pin-testability of flux residues
- Solder voids impact
- Residue cleaning/removal process changes
- Conformal coating and underfill compatibility
- Oven maintenance, flux decomposition volumes