|A publication of the National Electronics Manufacturing Center of Excellence||January 2005|
Michael D. Frederickson
When performing a process audit, heating capacity is the first aspect to be investigated. The first major difference between tin lead (SnPb) and lead free soldering is the melting temperature. The majority of lead free solders require higher melting temperatures (Table 4-1); therefore, the soldering processes must have sufficient heating capacity to solder the assemblies without damaging the hardware.
The second major difference between tin lead and lead free soldering is wetting. Lead free solders do not wet as well as tin lead. To improve wetting, nitrogen or more active solder fluxes can be used. If a more active solder flux is used, the manufacturer must employ aggressive cleaning processes to remove residues from the hardware.
Hand soldering processes
Because of the higher processing temperatures, components and boards must be baked out prior to soldering. Studies show that components increase moisture sensitivity by 2 levels, based on IPC J-STD-020 guidelines. Bake out insures that moisture is driven out prior to processing. Entrapped moisture is the root cause of component and board delamination and popcorning. The EMPF determined that boards should be preheated between 100°C and 125°C in order to reduce thermal shock and prevent pad lifting.
Operators must be re-trained to support lead free solders. EMPF staff have trained operators in the nuances associated with lead free hand soldering. Lead free solders require a longer dwell time – the time the soldering iron is in contact with the hardware. The longer dwell time promotes adequate heat transfer.
Due to the higher soldering temperature, the soldering iron needs to be removed more quickly for lead free solders, otherwise icicles will form. The size and frequency of solder icicles is dependent upon the alloy used and the temperature setting of the soldering iron.
The higher soldering temperature also requires that the soldering iron must be kept clean and coated with the solder alloy. Lead free solders are more sensitive to the effects of a dirty soldering iron. The higher soldering temperatures can cause oxidation of the soldering iron tip if not cleaned and coated. In addition, the EMPF recommends that different soldering irons be reserved for lead free and tin lead soldering. If the same soldering iron must be used for both, the EMPF recommends that different soldering tips be used for each type. Separate tips will prevent lead contamination of lead free components and solders.
A completed lead free solder joint has a grainy, dull finish. The bright surface associated with tin lead solders is not a material characteristic of lead free solders. Because of this difference, operators and inspectors need to adjust their visual inspection criteria. The IPC has changed the visual inspection criteria of J-STD-001 and IPC-A-610 to take this into account.
Once these adjustments were implemented, the electronics manufacturers audited by the EMPF were determined to have the capabilities to perform lead free hand soldering.
Reflow soldering processes
The EMPF audited an electronics manufacturer that used a 10-year-old 5 zone reflow oven (without nitrogen). The EMPF and the manufacturer developed a tin silver copper thermal profile. To compensate for the fewer zones, the belt speed was reduced from 20 inches per minute to 15. The profile slope averaged 2.495°C per second. The profile produced its average 232°C peak temperature in zone 4 instead of zone 5, which was 15 degrees above the melting temperature for tin silver copper.
In theory, this appeared to be a capable process for lead free soldering. However, there were several problems. In reviewing the profile with several solder paste manufacturers, it was determined that the solder paste activators and fluxes would be exhausted prematurely. The exhaustion of the activators and fluxes would reduce the solder paste wetting performance. The electronics manufacturer’s 5 zone reflow oven did not offer the necessary level of control.
It is well documented that lead free solder alloys do not wet as well as tin lead. The use of nitrogen is recommended to improve solderability, but it is not required. For example, the Joint Group of Pollution Prevention (JG-PP) Lead Free Soldering Program has shown that it is feasible to manufacture hardware which meets IPC-A-610 Class 3 specifications. However, the JG-PP Lead Free Soldering Program soldering process required a higher level of optimization to produce acceptable solder joints for a wide range of components. Using nitrogen in the reflow soldering process would open the process window to take into account components and boards with questionable solderability.
At another manufacturer, the EMPF adjusted the thermal profile from the tin lead peak temperature of 210°C ± 10°C to the tin silver copper peak temperature of 240°C ± 10°C. To ensure that the cycle time was not affected, the conveyor speed was not altered. For the majority of assemblies, the final zone was set at 280°C, and this produced good results with the tin silver copper alloy.
However, for ceramic assemblies in a pallet, zone 7 was adjusted to its maximum temperature of 300°C, in order to compensate for the increase in the assembly’s thermal mass. This produced acceptable soldering results, but it used the maximum thermal capabilities of the reflow oven. This caused zone temperature alarms to activate, and, in some instances, caused the oven to shut down during soldering.
For the short term, the EMPF determined that the reflow soldering equipment was capable, but not optimal, for lead free soldering. The oven could support process development activities and small prototype production runs. The oven could be operated at a slower belt speed, but this would reduce productivity. Operating at the oven’s maximum thermal limits could reduce the operating life of the oven’s heater and fan.
These issues are consistent with the EMPF’s experiences when manufacturing hardware with lead free solders alloys in its Demonstration Factory. In one instance, a reflow oven capable of meeting the higher lead free reflow soldering temperatures was used in a simulated production run. However, because the oven used chilled water in its cooling zones, the heat generated by the higher soldering temperatures exceeded the capacity of the chiller to support the reflow oven and several thermal cycling chambers. The EMPF had to acquire a larger chiller with sufficient capacity to support cooling of the higher temperature profiles.
Rework and repair processes
One manufacturer increased the hot air reflow temperature from 350°C / 662°F to 400°C / 752°F. The hot air bottom side preheat temperature was increased from 125°C / 257°F to 175°C / 347°F. The station’s maximum heater capacity was 216°C / 420°F for the top and bottom sides. The process time was increased to reach the proper preheat and reflow temperatures. It was determined that no equipment upgrade or replacement was necessary.
Another manufacturer’s station was capable of meeting the temperature requirements of lead free soldering. The component’s peak temperature was 311°C / 591°F and 301°C / 574°C, well over the tin silver copper lead free solder alloy’s 217°C / 423°F liquidus temperature. The board temperature under the component was monitored to be 298°C / 568°F.
The EMPF found some rework and repair stations use a combination of conduction and convection heating to preheat the hardware prior to soldering. While this is acceptable in the short term for experiments, it is recommended that rework and repair stations using only hot air convection be used. Hot air convection heating is more efficient and easier to control in a production environment.
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