Product category: Electronics Manufacturing Machinery and Materials
News Release from: Metcal | Subject: SmartHeat
Edited by the Electronicstalk Editorial Team on 16 October 2001

Smart route to lead-free hand soldering

Switching to lead-free hand soldering need not demand new soldering equipment or processes thanks to an established iron technology from Metcal

The likely transition to lead-free soldering continues to be one of the hottest topics in the global electronics manufacturing industry, with the impending ban on lead and other harmful substances expected to be enforced in 2008 by the EC's proposed Waste in Electronics and Electrical Equipment (WEEE) directive.

The successful campaign to eliminate leaded petrol has served to further raise awareness of the health risks and toxic environmental damage caused by lead.

While newspaper articles have effectively highlighted how dumping products containing lead into landfill sites can lead to contamination of ground water that may eventually be consumed by humans and animals, this negative publicity has encouraged 'greener' buying trends that may demand lead-free electronics long before the advent of legislation.

The problem for electronics manufacturers is that the transition to lead-free is not painless.

Practical alternative soldering alloys have melting points up to 40C higher than tin/lead, thus dictating several key changes to established steps in the manufacturing process.

Fortunately, thanks to long established soldering iron technology from assembly tools specialist Metcal, hand soldering may be the exception.

Metcal's technology - aptly named SmartHeat - was originally developed to ensure consistent soldering iron tip temperature under a wide variety of thermal loads.

In meeting this objective, Metcal also, quite coincidentally, future-proofed their soldering irons to be perfectly suited to lead-free soldering without need of any modification.

To illustrate why this is so, however, requires a basic explanation of the physics behind Metcal's SmartHeat technology.

"All conventional soldering irons tend to work in the same way, that is by supplying constant power, and varying the tip temperature", states Metcal's European Product Manager, Simon Hawkins.

This means that for a conventional soldering iron the amount of power supplied to the soldering tip is determined by the tip temperature se-point alone, and remains constant during the entire soldering operation.

"The problem with conventional irons", continues Hawkins, "is that the only way to speed up the soldering time, or to reduce the iron's recovery time, is to increase the tip temperature.

Operators often get frustrated, however, by the delay incurred when a correctly calibrated iron naturally cools during the soldering operation due to the thermal load demand of reflowing a joint.

So to speed things up they increase the tip temperature still further".

"As a result, it is an all too common occurrence to see irons set permanently at their maximum tip temperature setting in an attempt to increase soldering speed.

But the overriding problem is that soldering at higher tip temperatures dramatically increases the risk of causing overheating damage to components and boards".

Notwithstanding this operator intervention problem, the physical design of conventional irons is not conducive to maintaining stable tip temperatures in the first place.

This is because the thermal energy required for soldering is supplied from a remote heater and 'stored' by the heat capacity of the tip itself.

This arrangement excludes compensation for varying thermal demands created by differently sized solder joints.

Although a thermocouple is typically used to monitor an iron's heater temperature, the physical distance between the heater and soldering iron tip means that the temperature differential between the two can be large.

As a result, there is typically a delay between the tip cooling and this temperature drop being detected by the thermocouple, situated at the iron's heater.

Similarly, when power is applied to reheat the tip, the 'inertia' created by the distance from the thermocouple to the tip can cause a temperature overshoot.

In consequence, the iron's tip temperature is continually undershooting and overshooting the temperature setpoint, seriously compromising the soldering operation.

All in all, these factors combine to mean that the tip temperature of conventional soldering irons tends to run erratically and is regularly too hot.

This not only creates the risk of heating damage but can also dramatically increase operational costs due to power consumption and solder usage being increased, and average tip life being decreased.

In addition, any form of overheating will also result in a general decline in flux performance and, therefore, joint quality.

"All these drawbacks, however, pertain to conventional lead solders", says Hawkins.

"But when you add the higher temperature requirements of lead-free soldering, the risk of thermal damage to boards and components and the operational costs will further increase by an order of magnitude.

And for this reason, the vast majority of conventional soldering irons will simply be unable to cope with lead-free hand soldering in most real world production environments".

Metcal's SmartHeat soldering iron technology, however, addresses this major assembly problem by approaching manual soldering in a totally different way.

"In total contrast to normal soldering irons, with SmartHeat it is the tip temperature that is kept constant and not the power supplied to it", states Hawkins.

"As a result, the burden of maintaining the correct tip temperature is totally removed from the operator as the power varies automatically".

SmartHeat is based around the fundamental concept that in order to reflow solder during manual soldering, enough heat energy has to be transferred to the joint to cause it to melt.

Although the iron's tip temperature influences this process, ultimately it is the heat energy that the iron transfers that is the critical parameter.

"This means that if you are soldering a big joint with a large thermal capacity", explains Hawkins, "the amount of power required is greater than that required for a small joint".

SmartHeat technology detects the increase in thermal load demanded by the larger heat capacity of a bigger joint, and compensates by increasing power.

The tip temperature remains constant, but the increased power allows for greater transfer of thermal energy.

Unlike conventional irons, heat is not 'stored' in the soldering iron tip, but applied directly from the heater to the joint.

In application, SmartHeat exploits a Metcal tip cartridge manufactured to a proprietary design.

The tip comprises a non-heating copper core and an outer heating layer of magnetic alloy.

As alternating current is passed through the heater, inherent characteristics of the magnetic alloy confine the flow of current to the outer surface or 'skin'.

This confinement increases current density, decreases thermal inertia and dramatically accelerates heating.

In practical use the effect ensures the iron rapidly reaches soldering temperature, and decreases recovery time from joint to joint.

A second physical characteristic of the magnetic alloy ensures that the tip temperature is naturally regulated and consequently does not overshoot.

This characteristic is known as the Curie point and is the temperature at which the magnetic alloy loses its magnetic properties.

Loss of the magnetic properties eliminates the 'skin effect'.

Current is free to flow through the non-heating copper core, and the iron temperature drops slightly.

Upon cooling, the magnetic alloy passes back through the Curie point temperature and its magnetic properties are restored.

The skin effect immediately returns, and heating begins again, repeating the cycle.

Clever use of these physical characteristics ensures one of the key and unique benefits of SmartHeat technology: it self-regulates tip temperature to within +/-1.1degC of a set point, regardless of thermal load.

If the tip cools while transferring heat to a joint, the tip cartridge senses this change in thermal load, and instantly responds by increasing power to restore tip temperature.

However, the self-regulating SmartHeat technology ensures that there is minimal tip temperature overshoot, eliminating potential temperature-induced damage.

In the production environment, the combination of rapid tip heating with accurate regulation allied to increased power on demand, removes operator temptation to increase the iron tip temperature to 'compensate' for slow soldering.

In fact, SmartHeat irons have no operator adjustment controls.

One of the main advantages of SmartHeat is its productivity.

"If you attempt to solder 100 joints in sequence with a conventional iron, because the power is constant, the iron tip temperature will soon start to drop", explains Hawkins.

"The time to solder each joint will progressively increase.

With SmartHeat however, as the joint starts to drain thermal energy, power increases to compensate.

The time to complete each joint remains the same from the first to the 100th".

There are other advantages.

Reduced tip temperatures compared with conventional irons increases tip life, reduces solder consumption, decreases tip oxidation, and improves joint quality and integrity.

It is these properties of SmartHeat irons that make them a perfect 'drop-in' solution for lead-free hand soldering.

For while the higher melting temperatures of proposed lead-free alloys (typically in the 215-220C range) demand higher heat energy from the soldering iron, SmartHeat is able to cope with this without a dramatic increase in tip temperature.

"SmartHeat irons are in effect reliable, hassle-free production tools that can be used to solder lead-free boards immediately without needing to be continually recalibrated to meet the higher temperature requirements", comments Hawkins.

The most important technical challenge of switching to lead-free, however, is soldering heat sensitive components and boards at the increased temperatures demanded by lead-free without causing overheating damage to the components, or delaminating or scorching substrates.

This consideration is made particularly pertinent by the fact that most components are specified to a maximum temperature exposure of between 240 to 250C, leaving a very tight 20-30C margin for error between this and the alloy melting point during hand soldering.

But because the need or potential for operator intervention is completely removed from SmartHeat soldering irons, the risk of causing thermal damage is essentially eliminated and operators are left to focus on the actual mechanics of the soldering job at hand.

"As a result, most manufacturers find that even relatively inexperienced operators with only a minimal amount of training can produce better solder joints with a Metcal SmartHeat iron than experienced operators with a conventional one", reveals Hawkins.

"All in all, therefore, SmartHeat is an enabling technology that is especially relevant to lead-free.

It not only empowers operators to produce higher quality product more quickly, easily and safely, it means that manufacturers will not have to significantly increase the tip temperatures of their soldering irons when switching to lead-free soldering".

Metcal's SmartHeat technology therefore makes the transition to lead-free hand soldering surprisingly straightforward, and means that the higher temperature requirements of lead-free soldering can actually be met using standard existing equipment and processes without any cost penalty.

"By investing in Metcal's SmartHeat technology", summarises Hawkins, "manufacturers will be able to rest easy in the knowledge that they have the best soldering irons available on the market and are following the growing number of companies, from the smallest to the biggest blue chip giants, who are enjoying the cost savings and unparalleled process control benefits offered by SmartHeat.

More importantly, however, SmartHeat means that if and when lead-free becomes a reality, a manufacturer will be able to manually solder their boards without any significant process changes using the soldering iron equipment it already has in-house".

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