SMT assembly process

- Oct 17, 2018-

1 solder

At present, the most commonly used solder for wave soldering is eutectic tin-lead alloy: 63% tin;Lead 37%, should always keep solder in solder pot temperature, its temperature should be higher than that of alloy liquid temperature 183 ℃, and the temperature uniformity.In the past, the temperature of 250 ℃ soldering pot is regarded as the "standard". 

With the innovation of solder technology, the uniformity of solder temperature in the whole solder pot is controlled, and the preheater is added.In the range of 230-240 ℃ set temperature of the solder pot is very common.Generally, components do not have uniform thermal mass and it is necessary to ensure that all solder joints are at sufficient temperature to form qualified solder joints.The important issue is to provide enough heat to raise the temperature of all leads and pads to ensure fluidity of solder and wet both sides of solder joints.Low solder temperature will reduce the thermal impact on the elements and substrate, help to reduce the formation of dross, in the lower intensity, the flux coating operation and the flux compound together, so that the wave crest outlet has enough flux, thereby reducing burrs and solder ball production. 

The solder composition in the solder pot is closely related to time, that is, it changes with time, thus resulting in the formation of scum, which is the reason to remove residue and other metal impurities from the welded components and the cause of tin loss in the welding process.These factors can reduce the fluidity of solder.The maximum limit of tin content of metallic micro scum and solders to be specified in the procurement shall be specified in each standard (as defined in IPC/ j-std-006).In welding process, requirement for purity solder in the ANSI/J - STD - 001 - b also has stipulated in the standard.In addition to the limitation on the scum, for 63% tin;37% lead alloy of tin content prescribed minimum shall not be less than 61.5%.The gold on the wave soldering components and organic swimming layer copper concentration converge faster than in the past.This kind of gathering, plus a significant loss of tin, can make the solder loss of liquidity, and welding problems.Looks rough, the granular solder joints is often due to solder the scum.Because the scum of the solder pot of agglomeration or component itself inherent remains bleak, coarse granular solder joints may also be a sign of low content of tin, not local special solder joints, are the result of the loss of tin in tin pan.This appearance could also be in the process of solidification, due to vibration or shock. 

The appearance of the solder joint can directly reflects the process or material problems.To maintain the solder pot "" filled with state and in accordance with the process control plan to check the solder pot analysis is very important.Due to solder pot with scum "dump" solder in solder pot, usually is unnecessary, because in the application of conventional requirements to add solder, tin pan to make the solder tin pan is always full.In the case of loss of tin, adding pure tin helps to maintain the desired concentration.In order to monitor the compounds of tin pot, should carry on the routine analysis.If you add a tin, should sampling analysis, in order to ensure that the solder composition proportion is correct.Too much scum is a tricky question.There is no doubt that scum always exist in the solder pot, especially when such by welding in the atmosphere.Using chips "wave" for the welding of high density component is helpful, because the exposed to the atmosphere of the surface of the solder is too big, and make the solder oxide, so will produce more scum.Solder in solder pot with scum layer covering surface, oxidation rate is slow. 

In welding, the tin pan in the wave of the scum of the flow and turbulence and will produce more.Is recommended to use the conventional approach is to skim scum, if regular prime cut, will produce more of the dross, and consumed more solder.Scum could also inclusions in wave crest, lead to wave of instability or turbulence, so demanded more of the liquid component of solder pot of maintenance.If allowed to decrease in the tin pan solder, solder surface scum will enter the pump, the phenomenon is likely to happen.Sometimes, granular solder joints can mixed scum.Originally found scum, may be caused by the rough crest, and is likely to block pump.Tin pot, the solder should be equipped with adjustable low capacity sensor and alarm device. 

Two peaks

In wave soldering process, the wave is the core.Can be preheated, coated with flux, no contamination of metal by the conveyor belt to welding workstation, contact with certain temperature solders, and then heat, the flux can produce chemical reaction, solder alloy by wave form interconnection power, this is the most crucial step.At present, the commonly used symmetric wave known as the main wave, set the pump velocity, wave height, infiltration depth, the transmission Angle and transmission speed, good welding characteristics to provide a full range of conditions.Data should be properly adjusted, left behind the wave crest (outlet) should make solder run slow down, and stop running slowly.PCB with the final wave run to push the solder to exports.In the case of the hanging, the surface tension of the solder wave running and optimization of the plate, the component and the outlet which can realize zero relative motion between wave crest.This is an area of shell is achieved to remove solder on the board.Should provide sufficient Angle, does not produce bridge, burr, wire drawing, and defects such as solder ball.Sometimes, wave exports have hot wind flow, to ensure that ruled out form the bridge.Installed in the bottom of the plate after SMT components, sometimes, flux compensation or form of behind "demanding wave" area of the bubbles, and wave before leveling, using turbulent wave chip.High vertical turbulent wave speed help ensure solder with lead or contact welding plate.In the leveling of the laminar flow vibration wave at the back of the part can be used to eliminate air bubbles, ensure the solder to achieve satisfactory contact components.Welding workstation should be basically: high purity solder (according to the standard), peak temperature (230 ~ 250 ℃), contact crest the total time (3 ~ 5 seconds), depth of PCB in wave (50 ~ 80%), parallel transfer orbit and wave parallel to orbit in the state of flux content in tin pan. 

Cooling after welding of 3 wave crests

A cooling station is usually installed at the end of the wave crest welding machine.In order to limit the copper tin compound formed between the trend of the solder joint, another reason is that speed up the process of cooling components, the solder not completely cure, avoid the board shift.Rapid cooling components, to limit the sensitive components are exposed to high temperatures.However, the hazards of the thermal shock of the erosive cooling system to the elements and solder joints should be considered.A "stable" soft control good, forced-air cooling system should not damage most of the components.Using this system for two reasons: to quickly processing board, instead of a hand holding, and ensures the component temperature is lower than the temperature of the cleaning solution.After what people care about is a reason, it may be the cause of certain flux residue bubbles.Another phenomenon is sometimes react with some solder dross phenomenon, in this way, the residue "wash not to drop".In welding workstation data set that meet all of the machine, all the design, using all of the material and technological conditions and demands no pattern can meet these requirements.Must understand the whole process every step of the process of operation.4 conclusion in a word, to get the best welding quality, meet the needs of users, must be controlled before welding, welding of each processing step, because the whole SMT assembly process of each step is associated with each other, interact with each other, as there is something wrong with the step within the shadow to the reliability and quality of the whole.Welding operations, too, so must be strictly control all the parameters, time/temperature, amount of solder and flux components and transmission speed and so on.For the defect of the welding, should as soon as possible to find out the cause, were analyzed, and the corresponding measures, will affect the quality of various defects in the bud.In this way, can guarantee to produce a product meet the specification.