Flux coating quantity
Requirements in the bottom of the printed board have a thin layer of solder, to be uniform, not too thick, for the cleaning process, special attention should not be excessive. The flux coating shall be set according to the flux coating system of the wave soldering machine and the type of solder used. Flux coating method mainly has two ways: brushing and foaming and quantitative injection.
When brushing and foaming, the specific gravity of the flux must be controlled. The specific gravity of flux is controlled between 0.82-0.84 (the proportion of liquid rosin flux). In the welding process with the extension of time, the flux in the volatile solvent will gradually increase the proportion, the flux; the viscosity increases, liquidity becomes poor, effects of solder wetting of the metal surface, prevent wetting of molten solder on the metal surface, caused by welding defects. Therefore, the traditional way of brushing and foaming when timing measurement of flux proportion, if found increases immediately with diluent to adjust to the normal range; however, cannot add too much thinner, the low proportion of flux decline in the role will cause adverse effects on welding quality. In addition, we should also pay attention to continuously increase the flux in the solder groove, and not lower than the minimum limit.
When using a quantitative injection method, the flux is sealed in the container and does not volatilize, does not absorb moisture in the air and is not contaminated, so the flux composition can remain unchanged. The key requirement is that the nozzle can control the amount of spraying, and the nozzle should be cleaned frequently. The spray hole can’t be blocked.
Preheat temperature and time
1. evaporate the solvent in the flux, which reduces the gas when welding.
2. in the flux of rosin and surfactant began decomposition and activation, can remove the PCB pads, components and pin ends of the oxide film on the surface and other pollutants, and to prevent re oxidation of the metal surface under high temperature.
3. make the printed boards and components fully preheated to avoid rapid heating during welding, producing thermal stress, damage to printed boards and components.
The preheating temperature and time of the printed board shall be determined according to the size, thickness, size and number of the printed circuit board and the number of components to be loaded. Preheat temperature is 90 – 130 DEG C (PCB surface temperature), multilayer board and more attaching components, preheating temperature is upper limit. The warm-up time is controlled by the speed of the conveyor belt. Such as the preheating temperature and preheating time is low or too short, the flux in the volatile solvent is not fully formed during welding caused by gas pores, solder welding defects such as; as preheating temperature is high or the preheating time is too long, the flux is the flux decomposition in advance, loss of activity, can also cause hair thorn, welding defects such as bridging. Therefore, to properly control the top heating temperature and time, the optimum preheat temperature is applied to the bottom of the PCB before wave soldering. The tack is sticky (see table).
Welding temperature and time
Welding process is a complex process of interaction between welding metal surface, molten solder and air. It is necessary to control welding temperature and time. If the welding temperature is low, the viscosity of liquid solder is large, and it can not be wetted and diffused on the metal surface. It is easy to produce defects such as sharp point and bridge joint and rough surface of solder joint. Such as welding temperature is too high, easy to damage components, but also because the flux is carbonized, lose the activity, the solder joint oxidation speed up, produce solder joint hair black, solder joint is not full and so on.
The wave crest temperature is generally 250 + 5 degrees (must measure the actual wave crest temperature). As the heat is a function of temperature and time, the heat at which the solder joints and components are heated increases with time. The welding time of wave soldering is controlled by adjusting the speed of the conveyor belt, and the speed of the belt shall be adjusted according to the length, preheating temperature and welding temperature of different models of wave crest welding machine. For each solder joint, contact the crest time to indicate the welding time, and the weld time is 3-4 seconds.
Climbing angle and crest height of the printed board
The climbing angle of the printed board is 3 – 7 DEG C. This is achieved by adjusting the inclination angle of the wave soldering machine transmission device.
The climbing angle and help to remove the residual gas generated by the flux around the solder joints and components, when mixed with SMD THC, because the hole is relatively small, it should be appropriate to add in the climbing angle. The contact time by adjusting the tilt angle can be adjusted with the PCB peak, the greater the inclined angle, each weld contact peak time is shorter, the
welding time is short; the inclination is small, each solder contact peak time is longer, the welding time is long. Add a seal plate and the solder joint and the ramp angle for stripping solder wave. When the solder joint leaves the wave crest if the solder joint and the solder wave’s stripping speed is too slow, easy to cause the bridging. The proper wave crest height makes the solder wave increase the pressure and velocity of the solder joint, which is beneficial to solder wetting the metal surface and flowing into the small hole, and the crest height is generally controlled at the 2/3 of the printed board thickness.
Comprehensive adjustment of process parameters
The comprehensive adjustment of process parameters is very important to improve the quality of wave soldering.
Welding temperature and time are the primary conditions for forming good solder joints. The welding temperature and time are related to the preheating temperature, the temperature of the solder wave, the angle of inclination and the transmission speed. In order to adjust the process parameters, the welding temperature and time must be guaranteed. The first wave crest of double wave soldering is at about 235~240 DEG C / 1s, and the second peaks are around 240-260 DEG C / 3s.
Welding time = contact length/transmission speed of solder joint and wave crest
The contact length between solder joints and wave crests can be measured with a scale
Requirements for surface mount components
The surface of the metal electrode assembly components should choose three layer end structure, thermal shock and welding end components can withstand two times above 260 DEG C wave soldering, welding components do not damage or deformation, chip component metal ends without flaking phenomenon (hat).
Two. Requirements for plug-in components
For example, if the short insert soldering process is adopted, the plug-in element must be performed and the component pin shall be exposed to the printed board surface 0.8-3mm.
Three. Requirements for printed circuit boards
The substrate should be able to withstand the thermal shock of 260 RF4 / 50s, and the copper foil has good anti-stripping strength. The solder resists film has enough adhesion at high temperature and no wrinkling after welding. The printed circuit board is usually made of epoxy fiber cloth.
Four 、 the warpage of the printed circuit board is less than 0.8-1.0%.
Five 、 requirements for PCB design
For printed circuit board mounting components using the wave soldering process must be designed in accordance with the characteristics of mount components, components layout and arrangement should follow the direction of smaller components before and try to avoid blocking each other in principle.