No, you do not need to buy a large oven. However, you must ensure that the oven you purchase has been specifically designed to support the revised process parameters required by lead-free solder pastes and assemblies. These new parameters can be translated easily into an oven design “wish list.”

Oven design criteria “wish list”:
1) Higher process temperature capability — up to 280°C at process level.
2) Faster conveyer transit speeds to support shorter lead-free profile times.
3) Improved speed regulation. This is critical to consistency between boards due to the requirement for shorter peak temperature times. Correct peak temperature time and minimal variation is key to ensuring components are not processed outside their recommended time/temperature envelope, thereby increasing statistical in service failures.
4) High-accuracy process temperature control. The safety margin of the process window is much smaller for the lead-free process, typically 10°C or less. Further, less overshoot and time above liquidus (TAL), reduces oxidation and intermettalic growth, which in turn reduces the brightness of the soldered joints.
5) Improved across-conveyer temperature linearity. There is little point in improving the overall process hysteresis if the temperature differential from edge to edge is excessive.
6) Uprated rates of convection energy transfer so that the delta Ts across the board and between large and small thermal mass components are minimized. This must be combined with lower levels of convection velocity at the point of board contact to compensate for the reduced wetting and meniscus energy of lead-free reflow. The result should be to eliminate the risk of displacement of small SMD components.
7) Uprated work cooling module with top and bottom convection cooling to cope with the increased process exit temperatures of lead-free soldered assemblies. The system should be equipped with variable cooling fan speed control to optimize the cooling rates to each processed assembly type.
8) It should be affordable, rugged, low maintenance and feature low operating costs.
9) It should be simple for the operator to set up and use, and capable of continuous use with no restrictions on the duty cycle or operating conditions.

Asking for too much? Surface Mount Technology has distilled the experience earned from 17 years of reflow oven design and manufacture and over 1,600 systems shipped to produce its new LFR Tornado range of Reflow Ovens.

The result of a two-year development program, the LFR Tornado systems have been designed from the start to fulfil the requirements of lead-free reflow, but packaged to suit the lower-volume user.

Key to the exceptional performance is the highly developed and unique SMT HAWK Oven control software, which has extensive management facilities coupled with clear intuitive point-and-click graphics to allow maximum process flexibility and give the ability to evolve optimum profiles.

Dynamic zone temperature balance is shown in real-time histogram graphics and on board multi-channel profile logging is built in as standard. All SMT LFR Tornado reflow Ovens use exclusive ADTEC (adaptive dynamic temperature control) technology. This samples the air temperature 30 times a second to detect the thermal loading from the PCB assembly as it moves into the reflow zone. The system calculates not just real-time loading, but predicts the thermal inertia and resulting temperature drop curve. It responds proportionally, controlling the power to the convection heaters, boosting energy for heavy loading and reducing power when idling. Benefits include less need for profiling, improved accuracy and a lower peak temperature time for lead-free soldering. With lower safety margins for lead-free soldering, ADTEC provides the ultimate in safe accurate process temperature control. Other LFR features designed to specifically support lead-free processing include extra high flow rate recirculating forced air convection heating zones for low delta Ts on the board. Low-air velocity at PCB level means even the smallest SMD components can be processed without displacement. The balanced mesh conveyer is smooth and vibration-free. Speed control and calibration is in 1 mm per minute increments for precise control. High-power top and bottom work after-coolers with adjustable speed control are fitted as standard.

Still not convinced? The SMT LFR400 Tornado Oven has been the selected convection reflow demonstrator for the UK SMART Group NEPCON “Lead Free Experience” for the last two years. This involves two days of live production processing demanding high delta T lead-free demonstration PCBs. The component technology used ranges from large BGAs down to the smallest passive SMDs and includes large SM connectors, and even larger through-hole connectors soldered using intrusive pin reflow techniques. Processed boards are independently subjected to rigorous inspection right through to X-ray and destructive testing by the UK’s National Physical Laboratory (NPL) and the results made available to everyone. The data proved conclusively that the LFR400 makes it possible to achieve first class soldering of lead-free high delta T assemblies with a smaller reflow oven, providing it is correctly designed for the process. Despite the large component thermal mass differential, the maximum measured delta T was less than 12°C.

The LFR ovens have also proven themselves in the field with customers. Since launch in 2004, unit sales are already well into double figures in six countries SMT LFR Tornado ovens are helping manufacturing companies daily to improve quality and yield on their lead-free assembly reflow processes.