Mixed assembly process is characterized by: on one side of the PCB (side A) there are a variable number of IC components, and inserted through-hole components, on the other side of the PCB (side B) and many SMD resistive devices (sometimes also have IC), often called "mixed". It retains the advantages of low-cost through-hole components, commonly used in audio-visual products, such as CDs, DVDs.
Mixed assembly process operation process is: in the A side using solder paste - reflow soldering process soldering IC components; in the B side coated with SMD adhesive sent into the infrared oven curing; and then transferred to the A side, insert through-hole components; wave soldering B side; PCB finishing, cleaning, testing, general assembly.
Bonding SMD/SMC on PCB, its ultimate purpose is to solder, but only the components bonded to the PCB (excluding all bonding defects), and does not guarantee that SMA through wave soldering will definitely be soldered, this is because the chip components almost no leads, SMC / SMD in wave soldering has its own special characteristics.
Through-hole components wave soldering, component leads in contact with high-temperature solder wave and under the action of flux, the wetting force will prompt the solder to lead upward, so as to wet the entire pad and get a good welding effect, such as the hole on the PCB is a metal hole, the solder can also be extended through the metal hole to the other side of the PCB, and the formation of a full solder joint.
But the chip device because there is no pin, directly bonded to the PCB, components and PCB surface to form an acute angle, so that the flow of solder wave along the tangent direction impact resistance, capacitance of the surface, and not easy to reach the rectangular components and PCB plane formed by the corner, and with the increase in the thickness of the original more obvious. In this corner is easy to gather in the formation of solder bubbles and solder residues and leakage or poor welding. People often refer to this corner as the "solder dead zone".
SMD - wave soldering is another problem is usually a chip component end solder head for SnPb coating, there is good solderability, and in order to ensure the flatness of the PCB, the surface is usually coated with gold-plated or preheated flux, the flux effect is not as good as the use of SnPb alloy hot air leveling process. After the wave of solder, the two wetting time is not the same, usually SnPb end electrode only 0.1s, while the copper layer requires 0.5s, the component ends first contact with the solder, so it is also easy to cause a "solder dead zone. In order to solve the "solder dead zone" defects, usually using double wave soldering technology that increases the pulse wave so that the vertical direction of the impact of the solder wave "solder dead zone" in order to achieve good welding results.
In addition, low solid content solder should be used to reduce residues in the dead zone; increase the PCB preheating temperature to improve solderability; improve component alignment and reduce the dead zone corners to reduce the rate of bad solder joints.
Therefore, the SMC / SMD components in the wave soldering should be strictly in the PCB design should consider the direction of component alignment, as far as possible, the direction of the component pins perpendicular to the direction of movement of the wave soldering, IC components as far as possible on the A side of the PCB, less on the B side, the IC components must be placed on the B side, not only to pay attention to the direction of alignment, should also increase the auxiliary pad, flux Activity and density is also not negligible requirements, in addition, the curing strength of the components should meet the requirements, especially should not make the residual adhesive adhesion on the pads.
