If component packages are not taught in the correct way it can cause the machines vision system to not see each part in the same way and so variation in placement will be observed. The placement of surface mount components must be repeatable and so a reliable, well maintained pick and place machine is necessary. Heating too quickly can cause defects such as components to crack and the solder paste to splatter causing solder balls during reflow. Preheat – During this phase the components, PCB and solder are all heated to a specified soak or dwell temperature being careful not to heat too quickly (usually no more than 2✬/second - check solder paste datasheet). The main aim being to transfer enough heat into the assembly to melt the solder and form the solder joints without causing any damage to components or PCB. Within a typical reflow soldering profile there are usually four stages – Preheat, soak, reflow and cooling. Ideally more thermocouples should be used to measure the full range of temperatures across the PCB – known as ‘Delta T’. It is recommended to have at least one thermocouple located on a pad towards the edge of the PCB and one thermocouple located on a pad towards the middle of the PCB. In order to create a reflow profile thermocouples are connected to a sample assembly (usually with high temperature solder) in a number of locations to measure the range of temperatures across the PCB. The calculation is for HEATED LENGTH – NOT OVERALL OVEN LENGTH. Note that the overall length of the oven will exceed 10 feet including the cooling section and conveyor loading sections. Oven heated length = 120 inches (10 feet) Therefore, the reflow oven must have a process speed of at least 30 inches per minute.ĭetermine oven chamber heated length with process speed equation: Load Factor = 0.8 (2-inch space between boards)Ĭalculate Line Speed: (3 boards/min) x (8 inches/board) How long an oven do I need to process boards at this throughput? The solder paste manufacturer recommends a 4 minute, three step profile. Process speed = Oven chamber heated lengthīelow is an example of calculation to establish the correct oven size:-Īn SMT assembler wants to produce 8-inch boards at a rate of 180 per hour. Conveyor speed is a critical affecting parameter of the reflow profile speeding up or slowing the conveyor results in altered temperature profiles.To be able to select the correct size reflow oven the process speed (defined below) must be greater than the minimum calculated line speed. The solderability will be maintained for second side reflow, wave or selective soldering. Below 130 ☌ the cooling rate is not critical to the solder joint quality however fast cooling is beneficial for copper OSP board rather than a very slow cooling rate. The cooling rate should therefore be 3–4 ☌/s down to around 130 ☌. On the other hand, the components can crack if the temperature drops too rapidly. A lower rate may increase the grain size of the intermetallic compounds making the solder joint brittle and weak. To obtain a strong joint between the solder pad and the components' terminal, the cooling should be fast. In the cool down phase, a reliable solder joint is formed. It is important to monitor the time at peak reflow temperature in several areas of the PCB. The typical peak temperature is 30–40 ☌ higher than the solder alloy melting point. This soak time should be long enough to allow the flux to clean the surfaces, but not so long that the flux is evaporated prematurely.ĭuring the reflow phase, the temperature increases further to melt the solder alloy and subsequently, intermetallic layer will form at the boundary surfaces. The pre-flow or soak and the ramp-up section, also called the flux activation stage, brings the entire assembly up to the temperature at which the flux activating and the alloy in the paste changes its phase from solid to liquid. A typical preheat phase has a slope of 0.5–1.0 ☌/s. Second, the solder paste can slump, because of a too slow temperature rise. First, solder particles can be spread when the solvents burst through the flux surface membrane. If the temperature rises during the pre-heat too rapidly or two slowly, two failures can occur. During the pre-heat phase, the solvents evaporate from the solder paste. The soak profile can be divided into five zones: pre-heat, pre-flow or soak, ramp-up, reflow, and cool down.
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