Wave soldering related parameters and principles and analysis of failure after furnace
Date: 2015/1/17 15:48
Relevant parameters and principles of wave soldering and poor analysis after furnace preheating 1. When the solvent component of the flux passes through the preheater, it will be heated and volatilized. Therefore, the high temperature gasification of the solvent components when passing through the liquid surface can be avoided to cause the phenomenon of cracking, and finally the quality of tin particles can be prevented. • 2. The temperature of the parts mounted on the product to be immersed in tin is slowly increased when passing through the preheater, which can avoid the damage of the parts caused by the physical effect of sudden heat during the over-peak. • 3. After the preheated parts or terminals pass the wave peak, the welding temperature of the solder joint will not be greatly reduced due to the lower temperature, so as to ensure that the welding meets the temperature requirements within the specified time. The role of crest one and crest two • Crest one is mainly: there is a shadow effect on the SMD patch. Due to the "shielding effect" of the solder, more serious quality problems are prone to occur, such as missing welding and insufficient weld seams. • Wave two is mainly the quality of the solder joints, which can repair and prevent the occurrence of defects such as continuous welding, tipping, false welding, and burrs. In fact, the main purpose of installing a cooling device is to accelerate the solidification of the solder joints. When the solder joints are solidified, the surface cooling and the internal cooling rate of the solder joints will increase, forming tin cracks. The exhaust gas in the PCB board formed tin holes, pinholes and other defects. After the cooling device was installed, the cooling speed of the solder joints was accelerated, and the solder joints quickly solidified after leaving the wave peak, which greatly reduced the occurrence of similar situations. Spray system function • The flux system is the first link to ensure the quality of the solder. Its main function is to uniformly apply the flux, remove the oxide layer on the soldering surface of the PCB and components, and prevent re-oxidation during the soldering process. The coating of the flux must be uniform and try not to build up, otherwise it will lead to short circuit or open circuit. • There are a variety of flux systems, including spray, jet, and foam. At present, spray-type flux systems are generally used, and no-clean fluxes are used. This is because the solid content of no-clean fluxes is very small, and the non-volatile flux content is only 1/5 to 1/20. Therefore, a spray flux system must be used to apply the flux, and an anti-oxidation system must be added to the solder system to ensure that a uniform, fine and thin flux coating is obtained on the PCB, so that it will not be caused by the first wave. The scrubbing effect and the volatilization of the flux cause insufficient flux amount, which leads to solder bridging and tipping. • There are two ways of spraying: one is to use ultrasonic to hit the flux to make the particles smaller, and then spray it onto the PCB. The second is to use a fine nozzle to spray the flux under a certain air pressure. This spraying is uniform, small in size, easy to control, and spray height / width can be adjusted automatically, which is the mainstream for future development. Transport The main purpose of the conveyor belt is to send the circuit board into the wave soldering machine, along the flux addition zone, preheating zone, wave zone, cooling zone, etc. Flux function • The word FLUX comes from the Latin meaning of "Flowinsoldering", but its role here is not only to help flow, but also other functions. • The main functions of the flux are: 1. Remove the oxide film on the surface of the welding metal; 2. Form a liquid protective film on the surface of the solder to isolate the surrounding air at high temperature to prevent reoxidation of the metal surface; 3. Reduce the surface of the solder Tension to increase its diffusion ability; 4. At the moment of welding, the molten solder can be replaced to complete the welding smoothly. • Mainly "assisted heat conduction", "removing oxides", "reducing the surface tension of the material being welded", "removing the oil on the surface of the material being welded, increasing the welding area", "preventing reoxidation", etc. Some influencing factors of solder and some factors influencing tin: flux flow, specific gravity, rosin content, as well as its activity and temperature resistance. Preheating temperature, over-feed speed, guide rail angle, welding time, temperature difference between two waves, distance between two waves, waveform, wave velocity, height of two waves, uneven peaks, direction of passing furnace, pad design is too large, The pad design is too close, there is no tin holding point, the copper content of tin, PCB quality, PCB moisture, environmental factors, tin furnace temperature. Some solutions to Lianxi • 1. Improper preheating temperature. Too low temperature will cause poor activation of the flux or insufficient surface temperature of the PCB, resulting in insufficient tin temperature, poor liquid solder's wettability and fluidity, and bridge connections between adjacent lines; 2. PCB boards The surface is not clean. When the board surface is not clean, the fluidity of the liquid solder on the PCB surface will be affected to a certain extent, especially at the moment of detachment, the solder is blocked between the solder joints and forms a bridge; • 3, the solder is impure, the solder If the combined impurities exceed the allowed standards, the characteristics of the solder will change, and the wetting or fluidity will gradually deteriorate. If the antimony content exceeds 1.0%, the arsenic exceeds 0.2%, and the cadmium exceeds 0.15%, the solder flowability will decrease by 25%. , And arsenic containing less than 0.005% will dewet; 4, poor flux, poor flux can not clean the PCB, reducing the solder's wettability on the copper foil surface, resulting in poor wetting; 5. PCB dip The tin is too deep. This situation is likely to occur in IC-type components or through-hole components with high pin density. The essential reason for its formation is that the soldering time is too long, the flux is completely decomposed or the tin is not smooth, and the solder joints are not in good condition. Desoldering in the state of the device; 6. The component pins are too long, which causes the component bridges because the excessively long pins cause the adjacent solder joints to not be “single” removed from the solder wave, or too long. Lead is soaked in tin temperature for too long, leading The surface of the flux is scorched, and the fluidity of the solder between the pins is deteriorated, resulting in the possibility of bridge formation; • 7. The PCB board is holding and walking at a speed that should be as high as possible during the soldering process. Adjust under the conditions that meet the soldering time, and set the preheating temperature to meet the activation conditions of the flux. Any inconsistency in the above links (low temperature, high temperature, incorrect tin temperature, insufficient tin immersion time, etc.) will cause bridging. On the other hand, there is a certain relationship between the matching of the speed and the relative flow velocity of the solder wave crest. When the “force” in the forward direction of the PCB and the “force” in the forward flow of the solder wave can cancel each other out, this state is the best soldering state. At this time, the desoldering point formed by the PCB on the solder is “0”. . This situation is relatively strong for ICs and plug-in components. 8. PCB board welding angle, the larger the angle, the smaller the probability that the joints will be coplanar when the front and rear joints are off the peak. The smaller the chance of bridging. However, the angle of soldering is determined by the wetting characteristics of the solder itself. Generally speaking, the lead soldering angle is adjustable between 4 ° and 9 ° according to the PCB design, and the lead-free soldering is adjustable between 4 ° and 6 ° according to the customer's PCB design. It should be noted that in the large-angle soldering process, the tin immersion front of the PCB board may not eat enough tin to become tin. At this time, the PCB board is caused by the heat to the concave center. If this happens, the soldering should be appropriately reduced. angle. 9. Poor PCB design. This kind of situation is common when the component density is high or the pad shape design is poor or the IC components are soldered in the wrong direction. 10 The PCB board is deformed. This situation will result in inconsistent pressure wave depths at the three locations on the left, middle, and right of the PCB, and cause the tin flow to be poor where the tin is eaten, which may easily cause bridging. The factors of PCB deformation are as follows: (1) Preheating or solder temperature is too high; (2) PCB board is too tightly clamped; • (3) The transfer speed is too slow, and the PCB board is too long at high temperature. False welding refers to the failure to form a copper-tin alloy with a suitable thickness on the welding surface, which is mainly caused by insufficient wetting. The reasons are as follows: 1. The preheating temperature is too low. This situation will lead to poor activation of the flux or the solder joint temperature is too low to reach the temperature required for wetting at the moment of welding, which is common in fiberboard. The treatment plan is based on the temperature curve. 2. The transportation speed is too fast. The reason for this situation is that the fast chain speed causes the PCB to have insufficient temperature in the preheating zone or insufficient time to soak in the crest. The treatment plan is based on the temperature curve. 3. Poor PCB design. This situation is often caused by poor soldering direction of high-density SMT components or small packages. The treatment plan should be modified as much as possible on the premise that the design can be improved. Secondly, in the furnace, the impact velocity of the first wave should be accelerated as much as possible, and the infiltration time of about 2 seconds should be guaranteed. The formation of through-hole plug-in components is common when the component pins are thin, but the through-hole design is too large. 4. Flux is bad. This situation is usually caused by poor welding of large parts of the board. However, the flux and spray amount of the flux are within the control range that meets normal production. The reason is that the solder joints cannot be cleaned properly. Contaminants prevent solder from infiltrating the pads. • 5. Parts or pads are oxidized. This situation is seen in several through-hole components in the entire PCB. When such conditions occur, it can be clearly seen that the parts or pads are covered by contaminants (rust or oil stains). At this time, incoming materials management and storage management of components or PCB should be strengthened. Of course, after this happens, the virtual welding can be solved by manual repair, which is caused by the different mechanism of wave welding and manual welding. 6, tin temperature is not suitable, this situation is common in fiberboard. When the tin temperature is low, the fiber board absorbs a large amount of heat, and the tin temperature supply at the contact point with the PCB board is insufficient, resulting in excessive solder cooling, which makes the solder's fluidity worse, the wetting force decreased, and the solder joints cannot be wetted. When the temperature of the solder is too high, the surface tension of the solder itself increases, the adhesion is reduced, the capillary function is reduced, and the flow is deteriorated. At the instant of de-tinning, the solder on the surface of the solder joint is pulled back to the solder bath by the solder in the solder bath. This leads to dry solder joints and less tin. The treatment plan is subject to the temperature curve. Chain jitter. This situation occurs in production by accident on a single-chip PCB, and there are many component bridges on the PCB. When this happens, the maintenance of the equipment should be strengthened. Attention must also be paid to whether the titanium claws are damaged, resulting in poor splints, which will cause the chain to shake. After the peak-point PCB board is subjected to wave soldering, the solder on the solder joint is in the shape of milkstone or water column is called the tip. Its essence can be understood as the solder is subject to gravitational stress greater than the solder internal stress. The causes are as follows: 1. The PCB transfer speed is inappropriate. The setting of the transmission speed should meet the requirements of the welding process. Generally, if the speed is suitable for the welding process, the formation of the pull tip can be irrelevant. 2. Dipping too deep. It will cause the solder joints to be completely coked before they are detached. Because the surface temperature of the PCB is too high, a lot of solder will be accumulated on the solder joints due to the poor flowability of the PCB de-tinned solder, forming a pull tip. Should appropriately reduce the depth of tin or increase the welding angle. 3. Poor or too little flux. This reason will cause the solder to be unable to wet on the surface of the spot to be soldered, and the solder's diffusivity on the surface of the copper foil is extremely poor. At this time, a large area of sharpening will be generated on the PCB. • 4, the preheating temperature or tin temperature deviation is too large. When the temperature is too low, the temperature of the solder surface will drop too much after the PCB enters the solder, resulting in poor fluidity. A large amount of solder will accumulate on the surface of the solder joint to cause sharpening. The temperature too high will cause the flux to scorch and make the solder The wettability and diffuseness of the polymer deteriorate, and a sharpened tip may be formed. 5. The transmission angle is too low. The PCB transmission angle is too low, and the solder is easy to accumulate on the surface of the solder joint when the fluidity is relatively poor. In the process of solder condensation, the gravitational force is greater than the internal stress of the solder, forming a pull tip. 6. Solder wave velocity. The solder wave's scouring force on the solder joint is too low, and the fluidity of the solder is in a poor state, especially lead-free tin. The solder joint will attract a large number of solder joints, which may cause excessive solder and lead to sharpening. After soldering on the tin-pillar PCB, the solder joints are cylindrical. The reasons for its formation are as follows: Poor flux: 1. Generally, poor flux is easy to form bridges and false soldering, and it is sharp, but compared to some components, it is also possible to form tin pillars, especially in Process holes between multilayer printed circuits on fiberboard. 2. The surface of the pad is oxidized. The surface of the through-holes of the fiberboard with more than two layers is oxidized, and the interior is normal. The solder is wetted and extended in the through-holes during soldering, but not wetted and extended on the surface of the pad. Formation of tin pillars. 3. The immersion time is too long. After the immersion time is too long, the inside of the through-hole may be wetted, and the flux on the surface of the solder joint will be coked, resulting in poor solder flowability and excessive adsorption on the surface of the pad to form tin pillars . Poor solder joint gloss • The solder joint gloss is dull and matte. In the case of no-lead flux, the solder joints are dim and dull without counting defects, because the characteristics of the lead-free solder determine the state of the solder joint. The bad causes are as follows: • 1. Poor solder quality. The solder is seriously contaminated by impurity metals. Its own characteristics have changed. There are obvious black impurities on the surface of the solder joint. Matte, the reason is that the temperature during soldering is too low. • 3, PCB pads are slightly oxidized, and the solder joint surface after soldering is rough in this case.