Since July 1, 2006, the European WEEE (Electrical and Electronic Equipment Waste) Guide clearly stipulates that the import of lead-containing electronic products is prohibited, and the product with the highest lead content (by weight) not exceeding 1000 ppm is marked as “green”. Environmentally friendly products. The industry must develop the ideal solder and corresponding processing equipment to meet the current requirements of lead-free production.

Renewing equipment is costly, and tin-silver-copper (Sn-Ag-Cu) alloys are also expected to be a lead-free solder. This change has also changed the traditional welding process, so the purchase of lead-free reflow must have a deep understanding of the new features of the equipment and process, while not only considering the initial investment costs, but also the long-term cost of the equipment. In addition, the ease of maintenance of lead-free reflow reduces production time and maintenance costs, so the ease of maintenance of lead-free reflow should be taken into account when purchasing.

High melting point of lead-free solder narrows process window

260 ° C is the dangerous temperature of the reflow oven, the test temperature of most components will not exceed this point, so it can be considered as the upper temperature limit of the reflow process. For proper reflow soldering, the peak temperature of the soldering furnace should be at least 15 °C higher than the melting point of the alloy. The tin-lead (Sn-Pb) solder has a peak eutectic temperature curve of 225 ° C, which is higher than the melting point of 42 ° C and lower than the dangerous temperature of 35 ° C. Therefore, the furnace temperature should be set between 230 ° C and 280 ° C. The maximum temperature rating of the selected lead-free reflow soldering meets the peak temperature requirement as long as it reaches 300 °C.

The peak temperature of the tin-silver-copper alloy is 245 ° C, which is higher than the melting point of 28 ° C and lower than the dangerous temperature of 15 ° C. This allows the process window to be reduced by almost 50%, so temperature control is strict. The tin-silver-copper alloy has a peak temperature of 245 ° C. The furnace temperature must be set to about 280 ° C to 325 ° C according to the temperature profile of the lead-free process. Most lead-free reflow soldering equipment has a maximum temperature rating of 350 ° C and is also suitable for purchase.

The effect of heating time on reflow is also great, as determined by the total length of the heater. The product is heated for a long time and its temperature is higher. For large parts, reducing the speed of the belt by 10% to 15% results in a better lead-free reflow profile, but lowers productivity. Even furnaces rated at 325 ° C (or even 300 ° C) can be used for lead-free reflow without any modification, depending on the size of the product. If the product is large in size and the radiator area is large, a furnace with a slower speed, a longer furnace chamber and a rated temperature of up to 350 °C is required.

Closed-loop static pressure control is required to control convection

Convection is a very important factor in the heat transfer process, and static pressure is a measure of the convection of a reflow oven. The generation of static pressure is directly related to the heat transfer capacity of the furnace heating chamber. Tested by a 15" x 11" x 1/16 board with a large quad flat pack (QFP), plastic lead chip carrier (PLCC) and small size small pin IC (SOIC). We have four thermocouples in different sizes to detect how static pressure affects the peak temperature of different size components. When the static pressure was reduced, the peak temperature of this test board with various components was changed by about 7 °C. Closed-loop control of the static pressure sets the convection to the desired value. In addition, after using the static pressure controller, the static pressure is the same regardless of whether the furnace is operated at 50 Hz or 60 Hz.

Thermal performance is critical for lead-free reflow

The convection of the reflow oven is related to its nozzle configuration, gas pressure and the gases used. When the convection is strong, the temperature of the welding furnace is required to be low, so that the heat conduction is uniform. Therefore, it is important to choose a lead-free reflow oven with good thermal performance.

Typical cyclic convection has three basic steps, namely inhalation, pressurization, and delivery. Heating can be performed by placing the heater in either step. Heating in different steps, the heating characteristics of the corresponding parts of the furnace are also different. The suction step is heated, the gas density is relatively low, and the heating is slow. When heated during the pressurization process, the density of the gas is high, the collision with the gas molecules is intensified, and the heating rate is the fastest. Heating in the transfer step requires a plate heater, which requires the gas flow to be very slow so that it can be heated evenly. However, due to the large heater area, the heating system will react slowly when sudden heating is required.

Long-term cost of ownership is more important than initial cost

The cost of ownership can be considered in many ways. However, we should focus on the long-term benefits of equipment, not the cost of depreciation. In contrast, long-term cost of ownership is more important, and it determines the associated operating costs and maintenance costs. The cost of ownership mainly includes three aspects: heating efficiency and heat loss, maintenance cost and resource loss. It is natural to generate heat loss, and the amount of loss is related to the insulation quality and design use of the furnace. Insulated lead-free reflow ovens help maintain heat and reduce the workload of the furnace. The heat transfer efficiency is high and the heater requires only a small amount of work to provide the required heat. A good quality insulation furnace can also reduce the amount of heat radiated into the environment and reduce the associated costs, because if the ambient temperature rises, the heat consumption of the air conditioner will increase due to heat dissipation.

Furnace maintenance is another part of the cost of ownership. Frequent maintenance increases labor costs and also takes up production time. The only by-product of the reflow process is the accumulation of solder in the furnace, which requires frequent cleaning.

The third cost is resource loss, including electricity consumption and nitrogen use. The amount of electricity used depends on the heating efficiency. For a product, it is cooled and heated as needed for heating. Nitrogen is a protective gas that prevents excessive oxidation of metals during reflow. In addition, nitrogen has better thermal conductivity than air. The use of nitrogen allows for a larger process window while maintaining optimum weldability, so we hope that the furnace can control oxygen without fluctuations with small precision. By selecting a reflow soldering device that controls the exhaust gas in the furnace, optimizes or reduces the size of the furnace chamber, expands the furnace chamber inlet, and welds the product at acceptable oxygen concentrations, the amount of nitrogen used in the furnace can be reduced.

Equipment maintainability is also the focus of procurement

At the high temperature of reflow soldering, about 50% or more of the solder will vaporize into the furnace. Due to the high productivity of the furnace, if its design is not conducive to cleaning, maintenance is very troublesome.

The temperature required for lead-free processes is increased, and more solder is deposited, requiring control or simplified maintenance. Therefore, it is important to choose equipment that is designed to be easily replaced for maintenance. If it is convenient to replace contaminated parts with clean parts, the production time can be as short as possible.