Hand soldering in the 21st century: state-of-the-art hand soldering systems offer improved temperature control, increased process speeds and repeatable process control.
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A solder joint has two functions: it conducts heat between a component and a PCB. and it provides long-term mechanical support to the component. Traditional tin/lead solder with its lower melt temperature is giving way, under government mandate, to lead-free alloys with their higher melt temperatures. With hand soldering of lead-free components, the higher melt temperatures must be considered in process control capabilities, maintaining tip temperatures and extending tip life.
The hand soldering process is simple, but two important factors must be considered: the quality of the soldering iron and the skill of the technician.
Soldering Iron Considerations
The soldering iron must provide the correct amount of thermal energy to heat a joint and melt the solder. The iron must also provide sufficient power and the correct temperature to meet differing thermal demands dictated by each joint on a board.
The soldering process is critically reliant on heat and tip geometry. With too little heat, the flux will not perform properly, joints will form poorly and defects such as dry joints and voids may occur. With too much heat, thermal damage may occur, charring and delaminating the PCB or overheating the components. The right tip geometry will also affect the results.
Larger joints require more heat and, therefore, expose the substrate and component to the risk of thermal damage. To avoid damage, technicians normally set the iron temperature to quickly heat the joint to soldering temperature, but not overheat it so that thermal shock results. Other methods involve preheating the site, which reduces the [DELTA]T and the need for hotter operating tips.
However, some compromise always occurs because of the difference in joint sizes. One solution is to use different sized tips. Larger tips suit larger joints, as the greater surface area of the tip increases the rate of heat transfer to the joint.
Repetitive soldering can drain heat from the iron and slow production. This condition happens because the iron has a thermocouple in the tip that responds as the tip temperature drops. Consequently, the iron temperature can lag behind demand, thus extending the time needed to raise the joint to soldering temperatures. The problem is exacerbated if the PCB includes large components.
Modern rework requirements, driven by demands for smaller, faster and cheaper products. have created the need for innovative materials and tools including newly designed soldering systems. Densely populated, multi-layer boards and miniaturized, high pin-count, fine-pitch devices cannot be efficiently repaired using old technology.
New variable power, constant temperature systems offer an alternative to the traditional electric soldering iron. These systems enable tips to remain at a fixed temperature regardless of the thermal load. As the need for more thermal energy is sensed, the power to the iron automatically increases to maintain a constant temperature. As a result, production rates can be maintained because the tip never cools below its predetermined operating temperature.
The Importance of Flux
When solder is used to join two metal surfaces, the weak points are the interfaces between the surfaces and the solder. The interface strength is determined by how well the solder wets the pads. In turn, the wetting efficiency is determined by the cleanliness of the metal surfaces.
Pads can be easily contaminated by dirt, grease or oxides, with the latter being the hardest to remove. Long-term storage can also encourage oxide growth, so PCB stock must be rotated. Ultimately, however, the flux in the solder wire must clean the preheated component lead and pad for soldering by eliminating oxides and encouraging wetting.
Highly active fluxes are the best for this purpose. Unfortunately, they can leave post-solder residues that must be cleaned from the PCB to prevent long-term deterioration. Recently, the emphasis has shifted toward no-clean fluxes that are safe to leave on the board after soldering.
No-clean fluxes have less active contents, and, as a result, the flux's effectiveness as a preparation agent is reduced. Accordingly, PCBs and components must be kept as free from dirt and oxidation as possible if no-clean soldering is contemplated. Even then, the soldering will be more difficult than with a highly active flux.
The tip of the iron determines the rate of heat transfer. A poorly maintained tip will inevitably restrict heat transfer, extending heating times, decreasing productivity and exposing the assembly to thermal damage. Reduced tip life can also occur. The tip's plating tends to oxidize upon exposure to air. Oxidation can damage the tip over time and introduce impurities to the solder joint that undermine its strength and conductivity.
A light coating of solder (tinning) helps protect the tip. The tip should be cleaned on a sponge lightly damped with distilled water at frequent intervals during use. Distilled water is preferred because tap water contains additives that can damage a tip's plating and reduce its life.
The tip should be briefly inspected for signs of damage or pitting, and should be replaced as needed. Before use, sufficient solder should be applied to cover the tip in a film of solder as this dramatically aids heat transfer. Excessive solder should not be added as it can form globules that can fall onto the PCB.
Essential Operator Training
Producing good solder joints at production speed requires skill and training. No matter how good the equipment, an unskilled operator can easily damage an assembly beyond repair by applying too much heat. Worse, untrained operators do not know how much solder to add to a joint and whether or not a completed joint is satisfactory. They will also have difficulty adapting processes to various joint sizes and package technologies.
This last point is critical. With a high turnover of rework staff, Finding an operator with the experience necessary to ensure a consistent level of throughput and quality is often difficult. An inexperienced operator may not have the necessary skill to adjust tip temperature. However, such an uncertainty can be removed. The soldering iron systems help control the process and remove, as much as possible, human variability.
Still, even with skillful operators, good materials and equipment, hand soldering is a manual process and, as such, is subject to human error and unreliability. Therefore, soldering quality must be continuously monitored. The best way to do this is to visually inspect the joints against known good examples.
However, with variable power, constant temperature soldering systems, the need for operator intervention is reduced and the risk of causing thermal damage is eliminated because operators of these systems must only focus on the actual mechanics of the soldering job. As a result, even relatively inexperienced operators with minimal training can produce better solder joints than experienced operators with standard electric soldering irons.
The Need for Process Control
Because hand soldering is a manual process, strict process control procedures must be adopted. These procedures should include: operator training in the art of soldering and visual inspection of completed joints; continuous training updates to cover new materials, substrates and components; recommended soldering temperatures, maintenance and inspection of equipment; and regular temperature calibration.
Operators often change temperature settings from day to day and job to job, leading to inconsistent solder results. With conventional soldering irons, the power supplied to the tip is kept constant, and the tip temperature varies. As a result, the tip temperature of even a correctly calibrated iron will naturally fall as it supplies thermal energy to the increased thermal load of a colder joint to bring it up to reflow temperature.
Operators often get frustrated by the delay incurred before the iron heats up again. So, to speed up the soldering time or reduce the iron's recovery time, they increase its tip temperature, often up to its maximum setting. Of course, this action dramatically increases the risk of overheating damage.
As a further complication, the irons must be continually recalibrated to ensure that the 200[degrees]C tip, for example, is indeed reaching and not exceeding that temperature. This procedure is vital. If production engineering has specified soldering temperatures, irons need frequent calibration to ensure that a control setting actually corresponds to a tip temperature.
However, not all systems require calibration. Variable power, constant temperature systems, for example, require no calibration by production engineering staff.
Faulty solder joints remain a major cause of PCB failure. Thus, the importance of high-standard soldering workmanship cannot be overemphasized.
In relation to repair and rework, the main cause of solder joint defects is poor wetting, which is caused by an overheated solder tip that burns away the flux before it can do its job. This condition increases the soldering time and hampers joint formation, by preventing solder from sticking to the iron's surface.
When a solder tip does not reach the required temperature, the cause is usually a build-up of oxide acting as a thermal insulator and impairing heat transfer. When using standard electric irons, operators will often boost the power input in an attempt to increase tip temperature. The elevated thermal demands placed on the tip to perform quickly add to overall processing costs, as tip life is greatly reduced.
With lead-free processes, which reflow at temperatures as much as 40[degrees]C above eutectic solder, the tip life of standard electric irons is even further reduced. Lead-free also presents other challenges. The most critical challenge is soldering heat-sensitive boards and components at the 215[degrees]C to 220[degrees]C melting range of lead-free alloys without causing damage. This consideration is particularly pertinent because most components are specified to a maximum temperature exposure of 240[degrees]C to 250[degrees]C.
With such a tight margin for error (only 20[degrees]C to 30[degrees]C), operators must use soldering irons that offer fixed temperature control. Irons without such control will almost certainly cause thermal damage to components and delamination or scorching of substrates.
Process control allows operators to produce higher quality product more quickly, easily and safely. Also, manufacturers will not have to significantly increase the tip temperatures of their soldering irons when switching to lead-free soldering.
Besides lead-free solders, electronics manufacturers face the challenge of decreasing array package sizes. For example, 0201 components are increasingly becoming mainstream. With their onset comes a host of process issues, not least of which is rework. Is 0201 rework possible? If so, can it be done cost-effectively?
Although reworking 0201s is a challenge, it is well worth the effort. As their use continues to increase, the ability to rework 0201s will grow in importance. How well manufacturers can reduce potential scrap will play a significant role in achieving long-term profitability.
However, reworking micro-components can be extremely difficult using standard tools. Specialist tools are required that have the process control and flexibility to:
* solder and touch-up micro-components
* provide topside angle views of the component when reworking or removing micro-chip caps or components
* fine drag and point-to-point solder, allowing lead-to-lead or solder bridge clean-up and access between components
* provide a long-reach for drag soldering components in tight spaces.
Up-Front Versus Real Costs
As PCBs become more sophisticated and valuable, soldering systems must be able to solder and re-solder components with speed and safety. If not, the scrap rate increases and profitability decreases.
When a PCB arrives at the rework department, it is worth more than at any other stage of the assembly process. However, it is often greeted by a standard soldering electric iron that offers no process control. A quick, high quality repair means the board will not end up as expensive scrap. A poor repair job means the entire cost of manufacturing the PCB is lost.
By initial cost comparison, a next-generation soldering system may appear to be a luxury option reserved for high-end, rather than mainstream manufacturing purposes. But in today's competitive marketplace, all PCBs are high-end and costly to scrap. Real cost-effectiveness can only be judged by considering all costs. Manufacturers must look at both the up-front price of the system and the cost of running it. Productivity rates, potential downtimes due to recalibration and maintenance, operator training and damaged assembly costs must all be considered.
With PCBs becoming more densely populated and components increasingly expensive, hand soldering and production line rework and repair have never been so important to throughput, quality and yield. Newer soldering systems provide better control temperature, speed the hand soldering process, and remove the need for operator intervention by establishing a high level of repeatable process control.
For electronics manufacturers, savings by using the correct soldering tool are both qualitative, in terms of reduced scrap and greater throughput, as well as quantitative, limiting thermal stress to assemblies, increasing uptime and raising productivity.
The key criteria for selecting any soldering system are the protection of components, the ability to easily and automatically control tip temperature, the avoidance of increased power consumption, the inherent repeatability of the process, and ultimately, the speed and quality of the repair.
Leo Huerta is training and application engineering manager with Metcal Inc., Menlo Park. CA; e-mail: email@example.com.
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|Date:||Sep 1, 2002|
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