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The factors affecting the quality of reflow soldering are divided into the quality factors and external influence factors of the reflow soldering equipment itself. The following is a detailed explanation of the reflow soldering.
affect reflow soldering quality’s elements of equipment.
1.Temperature control precision reach ±1℃, Influencing the temperature control accuracy and the reflow soldering method is related to the heating and heat transfer method:
a: Heating wire type heating element (usually imported nickel-chromium wire wound heating element),this type heating element generally has a high exchange rate and a long service life.
b: Infrared tube heating element (using far-infrared heating tube with imported material), this kind of heating element has radiation type and good uniformity, but it will make color temperature difference, mainly used in heat compensation area, not suitable for welding.
c: Heating tube type heating body, such heating tube has low heat efficiency, if it does not pass hot air, the heating surface uniformity is not good (usually less) heating heat transfer mode: full hot air circulation type (good) and hot air circulation + infrared composite (good) and full infrared (poor)
2. The transverse temperature difference of the conveyor belt requires less than 5 °C, otherwise it is difficult to guarantee the welding quality.
3. The conveyor belt width needs match the maximum PCB size requirements.
According to your PCB selection mesh belt width: PCB 200MM mesh belt application choice 300MM, general: 300 350 400 450 500MM 600MM choice width is larger, reflow soldering power will be larger, so the choice is appropriate is the most important.
4. The longer the length of the heating zone and the greater the number of heating zones, the easier it is to adjust and control the temperature profile. Generally, small and medium-sized batch production of reflow furnaces with a heating zone length of about 1.8 m can meet the requirements. In addition, the upper and lower heaters should be independently temperature controlled to adjust and control the temperature profile.
5.The maximum heating temperature is generally 300~350 °C. If you consider lead-free solder or metal substrate, you should choose 350℃ or above.
Different reflow soldering properties are different. The low-grade products generally have insufficient thermal insulation layer. The maximum temperature is usually less than 300 degrees. Generally, the design temperature can reach 300 degrees. The reflow design of the mid-range or higher is more than 320-350. Reflow soldering reaches 400 degrees.
6. The conveyor belt should run smoothly, and the belt vibration will cause welding defects such as displacement, suspension bridge and cold welding.
In the middle and low reflow soldering, the transmission mechanism is relatively simple and there will be a little vibration.
7. Good reflow soldering is generally designed with anti-interference circuit in the control circuit. The frequency changer and external surge voltage will affect the stability of reflow soldering. If it is not done, the manufacturer's equipment is not recommended.
8. The temperature curve test function should be available. If the device does not have this configuration, the temperature curve collector should be purchased.
9. In response to voltage fluctuations and momentary power loss, it is recommended that a computer reflow soldering standard be equipped with a UPS (ongoing power supply) to protect the system and PCB output.
External factors affecting the quality of reflow soldering
There are impurities in reflow soldering in the reflow soldering furnace, which affects the quality of reflow soldering.
After the reflow soldering is completed, if the reflow soldering technician does not timely repair and clean the reflow soldering, there may be problems in the next reflow soldering, which will affect the reflow soldering effect. Because the welding scraps generated during use are not cleaned up after reflow soldering, the degree of friction between the various components of the reflow soldering equipment is increased, which affects the subsequent work and causes flaws in the soldered products.
2. The water vapor in the reflow soldering circuit board affects the quality of reflow soldering.
When the circuit board is reflowed, the moisture in the vicinity of the through hole on the circuit board is heated to become steam. If the metal plating on the hole wall is thin or void, the water vapor will be removed through the hole wall. If there is solder in the hole, the water vapor will create a void (needle eye) in the solder when the solder solidifies, or the solder will be extruded on the front side of the printed board. Produce a solder ball. The solder balls produced on the reverse side of the printed board (ie, the surface touching the peak) are caused by improper setting of some process parameters in the wave soldering. If the flux coating amount is increased or the preheating temperature is set too low, the composition of the flux may be affected. When the printed board enters the peak, the excess flux is evaporated by the high temperature, and the solder is splashed from the tin bath. Irregular solder balls are produced on the printed board surface.
The temperature of the preheating zone of the reflow soldering machine should be set so that the temperature of the board surface is as low as 100 °C. A proper preheating temperature not only eliminates the solder balls, but also prevents the board from being deformed by thermal shock. There are many reasons for the occurrence of solder balls in wave soldering, but in general, it is mainly caused by moisture in the material.
3、 Before the reflow soldering of the circuit board brush solder paste is not brushed to lead to tin, less tin, or due to solder paste quality problems will lead to reflow soldering products caused by batch reflow soldering.
4、SMT placement machine before the reflow soldering placement stickers, stickers, mislabeling, etc., will also lead to poor batch after reflow soldering products.
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