Extending tip life in Pb-free hand soldering: improved heater control and tip profiles eliminate overshoot, improve thermal transfer to the joint and reduce idle temperature.

Soldering iron tips continually erode during normal use. Pb-free solder alloys--and their associated fluxes--accelerate these mechanisms, leading to higher replacement rates. This increases the cost of ownership of a soldering iron, through higher purchasing costs and lower equipment utilization. The key lies in ensuring effective soldering performance, while reducing both the maximum and average temperatures attained at the soldering iron tip.

There are two main erosion mechanisms for soldering iron tips. The first is tin, which is a more reactive metal than iron and will naturally tend to attack the hand tool tips iron plating. The other is Pb-free fluxes, which are more aggressive than those used with ordinary SnPb alloys. Tin, of course, is now the major constituent of SAC solder alloys. Pb-free solders, as such, accelerate this erosion.

Because SAC alloys melt at higher temperatures than SnPb alloys, the temptation is to increase the soldering iron temperature to speed joint formation. The belief is that the rise in solder melting temperature, from 180[degrees] to 217[degrees]C, requires a corresponding increase in soldering iron tip temperature. This is not necessarily true.

Figure 1 compares the soldering process window of SnPb and SAC alloys. The optimal temperature range for a Pb-free joint formation is much narrower, but the upper limit of that range is the same. Heating the joint beyond that upper limit drives the process into potential failure and can lead to pad delamination. As far as tip life is concerned, increasing the tip temperature provides a powerful catalyst for the chemical reactions that erode the tip.

Hence, soldering at an elevated temperature when using SAC alloys, while unnecessary from a process point of view, may damage the assembly and will certainly lead to higher soldering iron tip replacement rates.

Raising the soldering iron temperature also tends to burn the flux present on the tip, which impairs thermal transfer. This burning has two consequences that also act to reduce tip life. First, operators tend to apply excessive force to the soldering iron to make up for the reduced heat transfer, increasing the likelihood of tip damage. Second, there is a tendency toward more aggressive and frequent cleaning of the tip, which also promotes erosion of the iron plating and leads to more frequent replacement.

[FIGURE 1 OMITTED]

Basic housekeeping, including maintaining the tip in a well-tinned condition, should always be observed. Although this will slow erosion caused by the flux action, it does nothing to protect the tip against the effects of the high tin SAC alloys.

Applying thicker plating to the copper tip will, naturally, extend its life. However, undesirable effects include reduced thermal conductivity of the path from the copper core to the soldering site, causing a reduction in productivity. A thicker plating will also tend to increase the size of the tip, which is not compatible with modern fine-pitch interconnects, tight component spacing and small pad dimensions. The challenge must be met at a more fundamental level.

Improved Temperature Regulation

Improved tip temperature regulation is critical, both during soldering and when idle, to prevent the tip attaining excessive temperatures. In fact, reliable Pb-free solder joints can be formed, at commercially acceptable wetting speed, without increasing the tip temperature above the levels established for SnPb soldering.

The key is to combine improved temperature regulation with an optimized tip profile, designed to maximize the thermal energy transfer from heater to soldering site. The nominal tip temperature is 382[degrees]C, the same temperature recommended for hand soldering with SnPb alloys. The Pb-free solder joint is formed at 257[degrees]C, and the pad temperature rises quickly as thermal energy is transferred from the soldering iron tip.

Most soldering irons have a ceramic heater embedded in the main body of the hand-piece, with a temperature sensor located between the heater and tip. Sensor output is detected to control the ceramic heater temperature and thereby maintain constant tip temperature. However, it is not practical to locate the temperature sensor at the point where the soldering iron makes contact with the work-piece. As a result, the heater controller does not respond to the actual tip temperature, and this introduces an unavoidable lag in the system. When the controller receives the signal to turn on the heater, the tip temperature will have already fallen below the optimum soldering temperature. Then, when the heater is turned on, the tip temperature will overshoot the desired setpoint before the sensor indicates the heater should be turned off. This overshoot accelerates the aggressive tip erosion mechanisms and also has the potential to exceed the soldering process window, thereby incurring damage to the component or pad.

An inductive heating technology, tailored for soldering irons, naturally limits the maximum temperature of the tip, without requiring temperature sensing or control circuitry. It also responds automatically and quickly to maintain the tip temperature when the thermal load is applied.

[FIGURE 2 OMITTED]

Sensorless Thermal Control

Heating by induction is the result of passing a current through a coil wound around a magnetic core. An inductive heater suitable for use in a soldering iron comprises a copper slug, which is coated with magnetic material and then wound with a current-carrying coil. The properties of the magnetic coating can be adjusted so that the temperature of the copper slug reaches a preset maximum temperature for the soldering iron at the point when the magnetic material reaches its Curie temperature. At the Curie temperature, the magnetic material ceases to display magnetic properties. As a result, the inductive heating action ceases naturally at this point. As the temperature of the slug falls below the Curie temperature--for example, when soldering begins--the coating regains its magnetic properties and heating recommences. With the heater element embedded deep within the iron, close to the tip, tight regulation of the actual tip temperature can be achieved.

The major advantage of this heater technology is that the soldering iron can never exceed the maximum temperature determined by the thickness and composition of the magnetic coating. This also eliminates any opportunity for unauthorized tampering with the tip temperature, for example, if an operator seeks to increase the tip temperature for Pb-free soldering.

It is known that a tip temperature of around 380[degrees]C is sufficient to enable satisfactory Pb-free alloy and flux soldering, resulting in good wetting speed and high-quality joint formation. The ability to preset this temperature with no overshoot greatly retards erosion of the tip by the large proportion of tin. It also reduces flux charring, leading to a corresponding reduction in the soldering force applied by operators and less frequent tip cleaning.

Idle Temperature

Measures to reduce tip temperature when the soldering iron is idle also have a large role to play in slowing tip erosion caused by Pb-free solders and fluxes. A soldering iron spends a significant portion of its useful life idle. By significantly reducing the tip temperature during this time, mechanisms that reduce tip life can be further slowed.

For an inductively heated soldering iron, this can be achieved by the use of a sleeper stand to park the soldering iron when idle (Figure 2). A sleeper stand uses passive techniques, transparent to the operator, to partially reduce the inductive heating effect and bring down the temperature to around 149[degrees]C during idling. As Figure 3 shows, this reduced idle temperature is below the active temperature range of the Pb-free flux, which prevents the flux from damaging the tip coating. Flux charring, which occurs during idle, is also eradicated, eliminating the need to clean the tip directly after withdrawing the iron from the stand.

[FIGURE 3 OMITTED]

The net effect: Tip coating erosion that occurs during idle is dramatically slowed. The sleeper stand permits operational temperatures to be restored quickly, as soon as the soldering iron is withdrawn from the stand, without overshooting the setpoint. In practice, tip life can be extended by as much as 40% simply by reducing the idle temperature to 150[degrees]C.

In addition to reduced tip replacement rates, further benefits include lower power consumption and reduced fume generation. These savings can be appreciable if many soldering stations are operated within an assembly area.

Operators soldering with Pb-free materials should also pay close attention to matching tip and solder joint geometries to optimize wetting speed without increasing the tip temperature setpoint. Using the correct size tip maximizes the contact area between the tip and joint, improving heat transfer efficiency. Ideally, tip and target dimensions should be the same.

Although Pb-free solder joints are formed at a higher temperature than is required for SnPb soldering, it is not necessary to increase soldering iron temperature to produce effective Pb-free joints at a commercial rate of throughput. Improved heater control techniques are required, however, to eliminate temperature overshoot, improve thermal transfer to the solder joint, and reduce idle temperature. This must be combined with a more optimal tip profile to avoid excessive temperatures, which will otherwise promote tip erosion, leading to high replacement costs and soldering iron downtime.

Ed Zamborsky is eastern regional manager at OK International (okinternational.com); [e-mail].