The SMT assembly process and welding process of PCB board are introduced
SMT assembly process is closely related to every process step before soldering, including capital investment, PCB design, component solderability, assembly operation,
flux selection, temperature/time control, solder and crystal structure, etc.
At present, the most commonly used solder for wave soldering is eutectic tin-lead alloy: tin 63%; Lead 37%, should always grasp the solder temperature in the solder pot,
its temperature should be higher than the alloy liquid temperature 183℃, and make the temperature uniform. In the past, a solder pot temperature of 250 ° C was regarded
as the "standard".
With the innovation of flux technology, the uniformity of solder temperature in the whole solder pot has been controlled, and a preheater has been added, and the
development trend is to use a lower temperature solder pot. It is common to set the temperature of the solder pot in the range of 230 -- 240 ° C. Usually, components do not
have uniform thermal mass, and it is necessary to ensure that all solder joints reach sufficient temperature in order to form qualified solder joints. The important issue is to
provide enough heat to raise the temperature of all leads and pads, thus ensuring the fluidity of the solder and wetting both sides of the solder joint. The lower temperature
of the solder reduces the thermal impact on the component and substrate, which helps to reduce the formation of scum. At lower strengths, the combination of the solder
coating operation and the solder compound allows for sufficient flux at the crest outlet, which reduces the generation of burls and solder balls.
The composition of the solder in the solder pot is closely related to time, that is, it changes with time, which leads to the formation of scum, which is the reason for the
removal of residue and other metal impurities from the welded components and the loss of tin in the welding process. These factors can reduce the fluidity of solder. In the
procurement, the maximum limit of metal trace scum and tin content of solder to be specified in the individual standards (such as IPC/ J-STD-006 are clearly specified).
The requirements for solder purity during soldering are also specified in the ANSI/J-STD-001B standard. In addition to the scum limit, for 63% tin; The minimum tin content
in 37% lead alloy shall not be less than 61.5%. The concentration of gold and copper in the organic swimming layer on the wave solder assembly accumulates faster than in
the past. This aggregation, together with significant tin loss, can cause a loss of solder fluidity and create soldering problems. Rough, granular solder joints are often caused
by scum in the solder. Due to the accumulation of scum in the solder pot or the residual of the component itself, the dim, rough, granular solder joint may also be a sign of low
tin content, not the local special solder joint, or the result of tin loss in the tin pot. This appearance may also be caused by vibration or impact during solidification.
The appearance of the solder joint can directly reflect the process problem or material problem. The analysis of the solder pot is important to maintain the "full pot" state of
the solder and to follow the process control plan. "Dumping" THE SOLDER OUT OF THE SOLDER POT due TO SCUM IS usually NOT NECESSARY, AS regular applications REQUIRE
THE SOLDER to BE added to the SOLder pot so that the solder pot is always full. In the case of tin loss, adding pure tin helps to maintain the desired concentration. To monitor
compounds in tin POTS, routine analysis should be performed. If tin is added, it should be sampled and analyzed to ensure the correct proportion of solder. Too much scum is
another tricky problem. There is no doubt that there is always scum in the solder pot, especially when welding in the atmosphere. Using "chip peaks" This is useful for soldering
high-density components, because the surface of the solder exposed to the atmosphere is too large to oxidize the solder, so more scum is produced. When the surface of the
solder in the solder pot is covered by a scum layer, the oxidation rate slows down.
In welding, more scum is produced due to the turbulence and flow of wave peaks in the tin pot. The usual recommended method is to skimming the scum. If skimming is done
frequently, more scum will be produced and more solder will be used. Scum may also be included in the crest, resulting in crest instability or turbulence, which requires more
maintenance of the liquid composition of the solder pot. If the amount of solder in the tin pot is allowed to be reduced, scum from the solder surface will enter the pump,
which is likely to happen. Sometimes, granular solder joint will be mixed with floating slag. The scum, which was initially found, may have been caused by rough peaks and
could have blocked the pump. The tin pot shall be equipped with adjustable low capacity solder sensor and alarm device.
2 wave peak
In the wave crest welding process, the wave crest is the core. The most critical step is the transfer of preheated, solder-coated, stain-free metal to a welding station on a
conveyor belt, where it is exposed to solder at a certain temperature, and then heated so that the solder chemically reacts and the solder alloys are interconnected by wave
crest forces. At present, the commonly used symmetrical wave peak is called the main wave peak, set pump speed, peak height, infiltration depth, transmission Angle and
transmission speed, to provide a full range of conditions for good welding characteristics. The DATA SHOULD BE PROPERLY ADJUSTED SO THAT THE solder operation
should be slowed down AND slowly stopped after leaving the crest (the exit end). The PCB runs with the crest and eventually pushes the solder to the outlet. In the most
hanging case, the surface tension of the solder and the crest of the optimized plate run, and zero relative motion is achieved between the component and the crest at the
outlet. This shucking area is where the solder is removed from the plate. Sufficient inclination should be provided without defects such as bridging, burrs, drawing and
soldering balls. Sometimes, hot air flow is required at the crest outlet to ensure that possible Bridges are excluded. After the surface mount element is mounted on the
bottom of the plate, sometimes, the flux is compensated or bubbles are formed in the "harsh crest" region behind, while the crest leveling is carried out before using
the turbulent chip crest. The high vertical velocity of the turbulent wave peak helps to ensure the contact of the solder with the lead or pad. The vibration section behind
the leveling laminar wave peak can also be used to eliminate air bubbles and ensure that the solder achieves satisfactory contact with the assembly. The welding station
should basically: high purity solder (according to the standard), peak temperature (230 ~ 250 ° C), total time to contact the peak (3 ~ 5 seconds), the depth of the printed
board immersed in the peak (50 ~ 80%), the realization of parallel transmission track and the solder content in the tin pot in the state of parallel peak and track.
3 Cooling after wave peak welding
Usually, a cooling station is added to the tail of the wave welding machine. In order to limit the tendency of copper tin intermetallic compounds to form solder joints,
another reason is to accelerate the cooling of the assembly and avoid plate displacement when the solder is not fully cured. Fast cooling of assemblies to limit exposure of
sensitive components to high temperatures. However, consideration should be given to the hazardous nature of the thermal shock of the components and solder joints from
the aggressive cooling system. A well-controlled "soft and stable" forced gas cooling system should not damage most components. This system is used for two reasons: it
allows the plate to be handled quickly without hand gripping, and it ensures that the component temperature is lower than the temperature of the cleaning solution.
Of concern is the latter cause, which may be responsible for the foaming of some of the flux residues. Another phenomenon is that there is sometimes a reaction with some
flux scum, so that the residue "can not be cleaned out". There is no formula to ensure that the data set at the welding station meets the requirements of all machines, all designs,
all materials used, and process material conditions and requirements. Every step of the process must be understood. In a word, in order to obtain the best welding quality and
meet the needs of users, it is necessary to control each process step before welding and during welding, because each step of the whole assembly process of SMT is related to
each other and interact with each other. Any problem in one step will affect the overall reliability and quality. The SAME IS TRUE FOR WELDING OPERATIONS, SO ALL PARAMETERS,
TIME/TEMPERATURE, AMOUNT OF SOLDER, flux COMPOSITION, TRANSFER SPEED, ETC., SHOULD BE STRICTLY CONTROLLED. For defects in welding, the cause should be identified
as soon as possible, the analysis should be carried out, and corresponding measures should be taken to eliminate various defects that affect the quality in the bud. In this way, to
ensure that the production of products are in line with the technical specifications.