What is SMT reflow carrier? Why is reflow Carrier sometimes used in SMT production? When need to use a full process carrier?
What is reflow carrier
The so called SMT reflow carrier is used to hold the PCB and then put it into the reflow oven. The reflow carrier usually has a positioning column to fix the PCB to prevent it from moving away from the position or deformation.
Some more advanced reflow carriers will add a cover, usually for FPC, and install magnets on the carrier and download the device as suction cup fastening, which can more accurately avoid the deformation of the board. Some hard-board PCB will also use the cover plate to fix the sliding of some parts when passing through the furnace.
What are purposes of using SMT reflow carrier
Reduce PCB deformation
Prevent heavy parts from falling
The reflow carrier can be designed to hold certain parts that are likely to fall off during the second weld after the first side of the piece, usually some heavy parts, such as the mesh line connector.
The above two points are actually related to the high temperature area of SMT reflow oven. Most of the products now use the lead-free process. The melting temperature of the lead-free SAC305 amount of solder paste is 217℃, and217℃ ~ 225℃ for SAC0307 solder paste. The maximum temperature of the back welding is generally recommended to be between 240 and 250℃.
However, for cost consideration, we generally choose FR4 plates above Tg150. In addition, the thickness of the circuit board is getting thinner and thinner, from 1.6mm to 0.8mm, and even 0.4mm PCB. Such a thin PCB is more prone to deformation problems due to high temperature when it is baptized by SMT reflow.
SMT reflow carrier is to overcome the PCB deformation and parts fall out of the problem, it generally uses the positioning column to fix the positioning hole of PCB, effectively maintain the shape of PCB when the plate deformation at high temperature to reduce the deformation of the plate, of course, there must be other reinforcement to assist the middle position of the plate because of the influence of gravity may bend and sink the problem.
In addition, it is possible to support the heavy part to ensure that the parts do not fall off by taking advantage of the non-deformable nature of the vehicle. However,the design of the reflow carrier must be very careful to avoid excessive support points lifting the part and causing inaccurate printing of solder paste on the second side.
What are properties needed for an ideal SMT reflow carrier
1 The softening deformation temperature should be higher than 300℃, to be able to be reused, without deformation. This is the main requirement.
2 To have a small thermal expansion. Thermal expansion and cold contraction are the characteristics of general materials. If the thermal expansion of the reflow carrier is too large, it will destroy the PCB.
3 Material can be processed and better be light. Because of the operators need to pick up and put the reflow carrier, which means that heavy reflow carrier is not suitable for general electronics factory operations.
4. The best material suppose to be not easy to absorb heat or heat dissipation fast. Because the reflow carrier should be able to use repeatedly, if the temperature cannot be lowered quickly enough to be reached by human hands after backsoldering, more loads must be prepared, which will increase the cost.
5. The material should be as cheap as possible and can be mass-produced.
6.Generally, the common material of the furnace carrier is aluminum alloy. In addition, high carbon steel and magnesium alloy are used to make the reflow carrier. Although aluminum alloy is lighter than the general iron metal, it still has a certain weight.
And the material of aluminum alloy is easy to absorb heat, after the furnace needs to wear heat insulation gloves or wait for a period of cooling time to pick up, the operation is a little to it is not very convenient. In addition, there is a special reflow carrier,a relatively economical material, but the service life is short, but recently there are some reactions will be allergic phenomenon.
What is full process carrier
Common SMT reflow carrier only use when the board through the welding furnace, that is to say,process before the welding furnace such as solder paste, patch hit piece no need to use reflow carrier, which can reduce the number of reflow carrier.
However, with the increasingly thin and dense PCB, and the higher requirements for solder paste printing precision, if the circuit board has already been deformed before solder paste printing, the solder paste printing position will be offset, the thickness of the solder paste will also change, which are usually not good for the fine pitch parts. When the above problems occur, the best way is to solve them through design changes.
If all the designs fail to help, we have to start to use SMT full process carrier, which is actually the same as the reflow carrier. The only difference is that the solder paste printing process needs to be considered, so the PCB must be higher than the surface of the vehicle after being put into the carrier. Even the positioning column must be followed, otherwise the solder paste will be a problem.
It can be expected that the number of full course SMT reflow carrier will be increased by multiple times, depending on the length of the SMT production line. For mass-produced products, the entire cost may be just a small amount of money, but for small and diverse products, the total cost of these vehicles can almost buy a car
Please carefully evaluate the production cost and quality risk brought by the design, and using SMT reflow carrier needs to calculate the cost of loading and unloading. Considering the concept of total cost and contact with the reliable PCB manufacturer.