The solution of bad SMT soldering

Crest Surface: The surface of the wave is covered by a layer of oxide scale, which remains almost stationary along the entire length of the solder wave. During the wave soldering process, when the PCB touches the surface of the tin wave front, the oxide scale breaks, and the tin wave in front of the PCB will smoothly move forward, which indicates that the entire oxide scale moves the wave soldering machine at the same speed as the PCB.

Solder joint forming: When the PCB enters the front of the wave crest (A), the substrate and the pins are heated, and before leaving the crest face (B), the entire PCB is immersed in solder, that is, bridged by solder. However, at the moment of leaving the tail of the wave crest, a small amount of solder adheres to the pad due to the wetting force, and due to the surface tension, it will shrink to a minimum with the lead as the center, and the wetting force between the solder and the pad is greater than the cohesion of the solder between the two pads, resulting in a full, rounded solder joint. Excess solder leaving the tail of the wave crest falls back into the pot due to gravity.

Prevent bridging from happening

<!--[if !supportLists]-->1.<!--[endif]-->Use components/PCBs that are solderable.

<!--[if !supportLists]-->2.<!--[endif]-->Improves the activity of the flux.

<!--[if !supportLists]-->3.<!--[endif]-->Increase the preheating temperature of the PCB and enhance the wetting performance of the pads.

<!--[if !supportLists]-->4.<!--[endif]-->Increase the temperature of the solder.

<!--[if !supportLists]-->5.<!--[endif]-->Remove harmful impurities and reduce solder cohesion to facilitate solder separation between two solder joints.

A common preheating method in wavesoldering machines

<!--[if !supportLists]-->1.<!--[endif]-->Air convection heating.

<!--[if !supportLists]-->2.<!--[endif]-->Infrared heater heating.

<!--[if !supportLists]-->3.<!--[endif]-->Heating is a combination of hot air and radiation.

Analysis of wave soldering process curves

<!--[if !supportLists]-->1.<!--[endif]-->Wetting time: Refers to the time when the wetting begins after the solder joint comes into contact with the solder.

<!--[if !supportLists]-->2.<!--[endif]-->Dwell time: The time from the time a solder joint on the PCB touches the crest surface to the time it leaves the crest face, and the dwell/solder time is calculated as: dwell/solder time = crest width/velocity.

<!--[if !supportLists]-->3.<!--[endif]-->Preheating temperature: refers to the temperature reached before the PCB comes into contact with the crest surface (see the table on the right).

<!--[if !supportLists]-->4.<!--[endif]-->Soldering temperature Soldering temperature is a very important soldering parameter, usually 50°C - 60°C higher than the melting point of solder (183°C), most of the time refers to the actual operating temperature of the soldering furnace, the temperature of the soldering joint of the soldered PCB is lower than the furnace temperature, this is the result of PCB heat absorption. SMA Type Component Preheating temperature Single-sided panel assembly Through-hole and mixed 90 - 100 Double-sided panel assembly Through-hole 100 - 110 Double-sided panel assembly Mixed 100 - 110 Multilayer board Through-hole device 15 - 125 Multilayer board Mixed 115 - 125

Wave soldering process parameter adjustment

<!--[if !supportLists]-->1.<!--[endif]-->Wave height: The height of the PCB in wave soldering is usually controlled at 1/2 - 2/3 of the thickness of the PCB, and too large will cause molten solder to flow to the surface of the PCB, forming a "bridging".

<!--[if !supportLists]-->2.<!--[endif]-->Conveying inclination: In addition to making the machine horizontal when the wave soldering machine is installed, the inclination angle of the conveyor should also be adjusted, through the inclination adjustment, the soldering time between the PCB and the crest surface can be adjusted, and the appropriate inclination angle will help the solder liquid and the PCB to peel off faster and return it to the tin pot.

<!--[if !supportLists]-->3.<!--[endif]-->Hot air knife: The so-called hot air knife is a narrow and long "cavity" with an opening placed under the SMA just after the SMA leaves the welding peak, and the narrow and long cavity can blow out hot air flow and is shaped like a knife, so it is called "hot air knife".

<!--[if !supportLists]-->4.<!--[endif]-->Effect of solder purity: In the wave soldering process, the impurities of the solder mainly come from the copper leaching of the pads on the PCB, and excessive copper will lead to an increase in soldering defects.

<!--[if !supportLists]-->5.<!--[endif]-->Flux

<!--[if !supportLists]-->6.<!--[endif]-->Coordination of process parameters: The process parameters of the wave soldering machine need to be coordinated and repeatedly adjusted between the belt speed, preheating time, welding time and inclination angle.

Wave soldering defect analysis:

<!--[if !supportLists]-->1.<!--[endif]-->POOR WETTING: THIS IS AN UNACCEPTABLE DEFECT WHERE ONLY PART OF THE SOLDER JOINT IS WETTED. Analyze the causes and ways to improve as follows: 1 - 1. External contaminants such as oils, greases, waxes, etc., which can often be cleaned with solvents, are sometimes picked up when printing flux masks.

 1 - 2. SILICON OIL is commonly used for mold release and lubrication, and is usually found on the substrate and part feet, while SILICON OIL is not easy to clean, so it is necessary to be very careful when using it, especially when it is used as an antioxidant oil, which often causes problems, as it can evaporate and stick to the substrate and cause poor tin staining.

1 - 3. Oxidation often occurs due to poor storage conditions or substrate process problems, and when the flux cannot be removed, it will cause poor tin staining, and secondary tin may solve this problem.

 1 - 4. The method of dipping the flux is incorrect, because the foaming air pressure is unstable or insufficient, resulting in the unstable or uneven foam height, so that the substrate part is not stained with flux.

1 - 5. Insufficient tin eating time or tin temperature will cause tin staining defects, because the molten tin needs enough temperature and time to wet, usually the solder temperature should be between 50°C - 80°C above the melting point temperature, and the total soldering time is about 3 seconds.

Adjust the viscosity of the solder paste.

1. Poor local tin staining: This situation is similar to poor tinning, except that local tinning defects will not expose the copper foil surface, only a thin layer of tin, and cannot form full solder joints.

<!--[if !supportLists]-->2.<!--[endif]-->Cold soldering or solder joints do not shine: solder joints look broken and uneven, most of the reason is that the parts are about to cool down to form solder joints.

<!--[if !supportLists]-->3.<!--[endif]-->Solder joint cracking: This is usually caused by a mismatch in the coefficient of expansion between the solder, the substrate, the via, and the part foot, and should be improved in terms of substrate material, part material, and design.

<!--[if !supportLists]-->4.<!--[endif]-->Too much tin in the solder joint: When evaluating a solder joint, it is often desirable to have a large, round and full solder joint, but in fact, too large a solder joint may not necessarily help with conductivity and tensile strength.

<!--[if !supportLists]-->5.<!--[endif]-->The solder joint is too large due to the incorrect conveying angle of the tin furnace, and the tilt angle is adjusted from 1 to 7 degrees according to the design of the substrate, the general angle is about 3.5 degrees, the larger the angle, the thinner the tin, and the smaller the angle, the thicker the tin.

5 - 2. Increase the temperature of the tin bath, extend the soldering time, and allow excess tin to flow back into the tin bath.

5 - 3. Increasing the preheating temperature can reduce the heat required for the substrate to get tinned and increase the soldering effect.

5 - 4. Change the specific gravity of the flux, slightly reduce the specific gravity of the flux, usually the higher the specific gravity, the thicker the tin and the easier it is to short circuit, the lower the specific gravity, the thinner the tin, but the more likely it is to cause tin bridges and tin tips.

<!--[if !supportLists]-->6.<!--[endif]-->Tin Tip (Icicle): This problem usually occurs during the DIP or WIVE soldering process, where ice-tin is found on the tip of the part foot or on the solder joint.

6 - 1. Poor solderability of the substrate, which is usually accompanied by poor soldering, should be discussed from the aspect of substrate solderability, and the specific gravity of the flux can be tried to improve.

6 - 2. If the area of the PAD on the substrate is too large, the green (solder-mask) paint line can be used to separate the gold track to improve, and in principle, the green (solder-mask) paint line can be used to separate the gold road into 5mm×10mm blocks on the Dajin pavement.

6 - 3. If the temperature of the tin bath is insufficient and the dipping time is too short, it can be improved by increasing the temperature of the tin bath and extending the soldering time, so that the excess tin can flow back into the tin bath.

6 - 4. The angle of the cooling air flow after the wave peak is wrong, and it should not be blown in the direction of the tin bath, otherwise it will cause the solder point to cool rapidly, and the excess solder cannot be pulled back to the tin bath by gravity and cohesion.

6 - 5. When the tin tip is generated during hand soldering, the temperature of the soldering iron is too low, resulting in insufficient solder temperature, and the solder joint cannot be immediately retracted due to cohesion.

<!--[if !supportLists]-->7.<!--[endif]-->Solder mask green paint with tin residues:

7 - 1. Some substances that are not compatible with flux remain during the production of the substrate, which are melted after overheating to produce sticky adhesive solder to form tin wire, which can be cleaned with acetone (* chemical solvent banned by the Montreal Convention), chlorinated alkenes and other solvents, if it cannot be improved after cleaning, it may be that the substrate layer material is incorrect, and it should be fed back to the substrate supplier in time.

7 - 2. Incorrect substrate curing can cause this phenomenon, which can be baked at 120°C for two hours before insertion, and this accident should be reported to the substrate supplier in time.

7 - 3. The solder dross is pumped into the tin bath and then sprayed out, resulting in the substrate surface stained with solder slag, this problem is relatively simple, good solder pot maintenance, the correct tin height of the tin bath (generally normal conditions, when the tin bath does not spray stationary, the tin surface is 10mm from the edge of the tin bath).

<!--[if !supportLists]-->8.<!--[endif]-->White residue: White residue on the substrate, usually rosin, found after soldering or solvent cleaning, which does not affect the surface electrical properties but is not acceptable to the customer.

8 - 1. Flux is usually the main cause of this problem, sometimes it can be improved by switching to another flux, rosin flux often produces white spots when cleaning, at this time it is best to seek the assistance of the flux supplier, the product is supplied by them, they are more professional.

8 - 2. Impurities remain in the substrate manufacturing process, and white spots will also occur under long-term storage, which can be cleaned with flux or solvent.

8 - 3. INCORRECT CURING CAN ALSO CAUSE WHITE SPOTS, USUALLY IN A SINGLE BATCH, WHICH SHOULD BE REPORTED TO THE SUBSTRATE SUPPLIER AND CLEANED WITH FLUX OR SOLVENT.

8 - 4. The flux used in the factory is not compatible with the substrate oxide protective layer, and the supplier should be asked to assist in the event of a new substrate supplier or a change of flux brand.

8 - 5. Since the solvents used in the substrate manufacturing process can change the substrate material, especially the solution in the nickel plating process, it is recommended that the storage time be as short as possible.

8 - 6. If the flux is used for too long and deteriorates when exposed to air to absorb water vapor, it is recommended to update the flux (usually the foaming flux should be updated weekly, the immersion flux should be updated every two weeks, and the spray flux should be updated monthly).

8 - 7. If you use rosin flux, you can stop it for too long after passing through the soldering furnace before cleaning, which will cause white spots, and it can be improved by shortening the soldering and cleaning time as much as possible.

8 - 8. If the moisture content of the solvent in the cleaning substrate is too high, it will reduce the cleaning ability and produce white spots, so the solvent should be renewed.

<!--[if !supportLists]-->9.<!--[endif]-->Dark residue and erosion marks: Typically black residue appears at the bottom or tip of the solder joint, which is usually caused by incorrect flux use or cleaning.

9 - 1. The rosin flux is not cleaned immediately after soldering, leaving a black-brown residue, so try to clean it in advance.

9 - 2. Acidic flux left on the solder joints causes a black corrosive color that cannot be cleaned, this phenomenon is common in hand soldering, use a weaker flux and clean it as soon as possible.

9 - 3. Organic fluxes scorch at higher temperatures to produce black spots, confirm the temperature of the tin bath, and switch to a higher temperature resistant flux.

<!--[if !supportLists]-->10.<!--[endif]-->Green residue: Green is usually caused by corrosion, especially in electronics, but not entirely, because it is difficult to tell if it is green rust or other chemical products, but the discovery of green substances should be considered as a warning sign and the cause must be identified immediately, especially when this green substance will become larger and larger, it should be taken with special care and can usually be improved with cleaning.

10 - 1. Corrosion problems: Usually occurs on bare copper surfaces or copper-containing alloys, using non-rosin fluxes, which contain copper ions so they are green, and when this green corrosion is found, it is proof that it has not been cleaned properly after using non-rosin fluxes. 10 - 2. COPPER ABIETATES is a compound of copper oxide and ABIETIC ACID (the main ingredient of rosin), this substance is green but by no means corrosive and has high insulation, does not affect the quality but the customer will not agree, should be cleaned.

10 - 3. Residues of PRESULFATE or similar residues on the substrate production will produce green residues after soldering, and the substrate manufacturer should be required to do a cleanliness test after the substrate is made and cleaned to ensure the quality of substrate cleanliness.

<!--[if !supportLists]-->11.<!--[endif]-->White corrosives: The eighth item deals with white residues on substrates, while this item discusses white corrosives on the feet and metal of the parts, especially on metals with a high content of lead, which are more likely to form such residues, mainly because chloride ions tend to form lead chloride with lead and lead carbonate (white corrosives) with carbon dioxide. When using rosin flux, because rosin is insoluble in water, it will encapsulate the chlorine-containing active agent without corrosion, but if the solvent is used improperly, only cleaning the rosin can not remove chloride ions, which will accelerate corrosion.

<!--[if !supportLists]-->12.<!--[endif]-->Pinholes and pores: Pinholes are small holes found in solder joints, pores are larger holes in solder joints and can be seen inside, pinholes are usually empty inside, pores are large holes formed by complete ejection of internal air, and the reason for the formation of solder is that the solder has solidified before the gas is completely discharged.

12 - 1. Organic contaminants: the substrate and the feet of the parts may produce gases to cause pinholes or pores, the source of contamination may come from the automatic grafting machine or poor storage conditions, this problem is relatively simple, as long as it is cleaned with solvents, but if the contaminants are found to be SILICON OIL, because it is not easy to be cleaned by solvents, other substitutes should be considered in the manufacturing process.

12 - 2. Moisture on the substrate: If you use a cheaper substrate material or use a rougher drilling method, it is easy to absorb moisture at the through-hole, and it is caused by high heat evaporation during the soldering process, and the solution is to bake it in the oven at 120°C for two hours.

12 - 3. Brightener in electroplating solution: When a large amount of brightener is used for electroplating, the brightener is often deposited at the same time as gold, and it is volatilized when encountering high temperatures, especially when gilding, the electroplating solution containing less brightener is used, of course, this should be fed back to the supplier.

<!--[if !supportLists]-->13.<!--[endif]-->TRAPPED OIL: Oxidation prevents oil from being pumped into the tin bath and gushing out of the jet stream to contaminate the substrate, which can be solved by adding solder to the bath.

<!--[if !supportLists]-->14.<!--[endif]-->Solder joint graying: This phenomenon is divided into two types: (1) the color of the solder joint turns dark for a period of time (about half load to one year) after soldering; (2) The manufactured finished solder joints are gray.

14 - 1. Impurities in the solder: The metal composition in the solder must be checked regularly every three months.

14 - 2. The flux will also produce a certain degree of gray color on the hot surface, such as RA and organic acid flux left on the solder joint for too long will also cause slight corrosion and gray color, which should be improved by cleaning immediately after soldering. SOME INORGANIC ACIDS CAN CAUSE ZINC OXYCHLORIDE, WHICH CAN BE CLEANED WITH 1% HYDROCHLORIC ACID AND THEN WASHED WITH WATER.

14 - 3. In solder alloys, the solder joints with low tin content (e.g. 40/60 solder) are also darker.

<!--[if !supportLists]-->15.<!--[endif]-->Rough surface of the solder joint: The surface of the solder joint is a sand-like protruding surface, while the overall shape of the solder joint remains unchanged.

15 - 1. Crystallization of metal impurities: The metal composition of the solder must be checked regularly every three months.

15 - 2. Solder slag: The solder slag is pumped into the tin bath by the pump and gushes out through the jet flow, because the tin contains solder slag and the surface of the solder joint has a sand-like protrusion, it should be that the solder liquid level of the tin bath is too low, and the solder tank can be improved by adding solder to the tin bath and cleaning the tin bath and PUMP.

15 - 3. Foreign substances, such as burrs and insulating materials, can also produce rough surfaces when they are hidden in the feet of the parts.

<!--[if !supportLists]-->16.<!--[endif]-->Yellow solder joints: It is caused by the high solder temperature, immediately check the tin temperature and whether the thermostat is faulty.

<!--[if !supportLists]-->17.<!--[endif]-->Short circuit: An excessively large solder joint causes two solder joints to meet.

17 - 1. The substrate does not eat tin for enough time and the preheating is insufficient, so you can adjust the tin furnace.

17 - 2. Defective flux: improper specific gravity of flux, deterioration, etc.

17 - 3. The direction of the substrate is not well matched with the tin wave, and the tin direction is changed.

17 - 4. Poor circuit design: the lines or contacts are too close to each other (there should be a spacing of more than 0.6mm), if it is a permutated solder joint or IC, it should be considered to steal solder pads, or use text white paint to distinguish them, and the thickness of white paint should be more than 2 times the thickness of the solder pad (gold path).

17 - 5. Contaminated tin or excessive oxide accumulation is short-circuited by the PUMP belt, and the solder in the tin bath should be cleaned or further updated completely.

18. Wave soldering technology is of great significance in the field of electronic manufacturing, as mentioned above, it involves multiple links and numerous process parameters. From the characteristics of the crest face to the process of forming solder joints, each step needs to be precisely controlled. When it comes to preventing bridging, many of the measures taken are based on a deep understanding of the welding materials and the environment. For example, the use of components/PCBs with good solderability is the key to radically reducing soldering problems. Just like choosing high-quality building materials to build a house, high-quality components and PCBs are the foundation of good soldering.

19. In the preheating method of wave soldering machine, air convection heating, infrared heater heating and heating combined with hot air and radiation have their own advantages and disadvantages. The preheating temperature corresponding to different SMA types and components is also clearly defined, which reflects the rigor of the process. For example, the preheating temperature of 115 - 125 for multilayer board mixing, a temperature range that has been extensively practiced and tested to ensure the best results can be achieved during the welding process.

The adjustment of wave soldering process parameters is even more complex and diverse. The crest height is controlled at 1/2 - 2/3 of the thickness of the PCB board, and parameters such as conveying inclination angle and hot air knife need to be carefully adjusted. These parameters are interrelated, just like the individual gears in a precision machine, and a change in one parameter may affect the entire welding result. For example, the change of the transfer inclination angle not only affects the soldering time between the PCB and the crest surface, but also affects the peeling effect of the solder fluid and the PCB.

20. In wave soldering defect analysis, the causes of various defects are intricate. POOR TIN STAINING CAN BE CAUSED BY A VARIETY OF FACTORS, SUCH AS EXTERNAL CONTAMINANTS, SILICON OIL, STORAGE CONDITIONS, AND THE WAY THE FLUX IS WETTED. This requires that every possible factor be investigated in the production process. For example, if the foaming air pressure is unstable or insufficient, the substrate will not be stained with flux, resulting in poor soldering.

21. Cold soldering or solder joints are mostly caused by vibration when the solder is about to cool down to form solder joints, which reminds us to pay attention to the stability of the equipment in the production process, especially whether there is abnormal vibration in the tin pot transportation. In the case of solder joint cracking, improvements need to be made in terms of substrate material, part material, and design, which indicates that problems that may arise during the welding process need to be considered at the product design stage.

The problem of too much tin in the solder joint involves multiple adjustment directions, such as the solder pot conveying angle, tin bath temperature, preheating temperature, flux specific gravity, etc. The adjustment of each factor needs to be considered comprehensively, and one cannot be neglected at the expense of the other. For example, increasing the preheating temperature can reduce the amount of heat required for the substrate to get solder and increase the soldering effect, but it may also have an impact on other aspects.

22. Tin tip, solder mask green paint left on the tin residue, white residue, dark residue and erosion traces, green residue, white corrosion, pinholes and pores, TRAPPED OIL, solder joint gray, solder joint surface rough, yellow solder joint, short circuit and other problems, also have their own causes and solutions. This further illustrates the complexity of the wave soldering process, which requires strict control at every step to ensure the quality of the final product.