SMT on the tin bad solution

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 wave soldering, the PCB touches the surface of the tin wave front, the oxide scale breaks, and the tin wave in front of the PCB smoothly advances, which indicates that the entire oxide scale moves the wave soldering machine at the same speed as the PCB.

Ⅰ.Solder joint forming: When the PCB enters the front end of the wave crest (A), the substrate and the pin are heated, and before leaving the crest face (B), the whole PCB is immersed in the solder, that is, bridged by the solder, but at the moment of leaving the tail of the wave crest, a small amount of solder adheres to the pad due to the action of wetting force, and due to the surface tension, there will be a minimum shrinkage 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. As a result, full, rounded solder joints are formed, and excess solder that leaves the tail of the crest falls back into the pot due to gravity.

1. Use components/PCBs with good solderability to prevent bridging

2. Increase the preheating temperature of the PCB and increase the wetting performance of the pad

3. Use components/PCBs with good solderability. 5. Remove harmful impurities and reduce the cohesion of the solder, so as to facilitate the separation of the solder between the two solder joints.

4. Improve the activity of flux. 1. Air convection heating

5. Increase the preheating temperature of the PCB and enhance the wetting performance of the pad. 3. Heating by combining hot air and radiation

6. Increase the temperature of the solder. 1. Wetting time: refers to the time when wetting begins after the solder joint is in contact with the solder

7. Remove harmful impurities and reduce the cohesion of the solder so that the solder between the two solder joints can be separated. 3. Preheating temperature: The preheating temperature refers to the temperature reached before the PCB comes into contact with the crest surface (see the table on the right).

II.. Common preheating methods in wave soldering machine The soldering temperature is a very important soldering parameter, usually higher than the solder melting point (183 °C) 50 °C ~ 60 °C Most of the cases refer to the temperature of the soldering furnace When the actual operation, the temperature of the soldered PCB solder joint is lower than the furnace temperature, which is the result of PCB heat absorption

1. Air convection heating. Single-sided assemblies Through-hole devices and assembly 90~100

2. Infrared heater heating. Double-sided panel assembly 100~110

3. Heating by a combination of hot air and radiation. Multi-layer board 115~125

III.. Analysis of wave soldering process curve

1. Crest height: Crest height refers to the height of PCB tin in wave soldering. The value is usually controlled at 1/2~2/3 of the thickness of the PCB, and too large will cause the molten solder to flow to the surface of the PCB, forming a "bridge"

2. Wetting time: refers to the time when the wetting starts after the solder joint comes into contact with the solder.

3. Hot air knife: The so-called hot air knife is a narrow and long "cavity" with an opening placed below the SMA just after the SMA leaves the welding peak, and the narrow and long cavity can blow out hot air flow, especially like a knife, so it is called "hot air knife"

4. 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. 5. Flux

5. Preheating temperature: refers to the temperature reached before the PCB comes into contact with the crest surface (see the table on the right). Wave soldering defect analysis:

6. The soldering temperature is an unacceptable disadvantage that only part of the solder joints are soldered. The reasons for this and how to improve it are as follows:

Soldering temperature is a very important soldering parameter, usually 50°C - 60°C higher than the melting point of the solder (183°C), in most cases the temperature of the soldering furnace. In actual operation, the temperature of the soldered PCB joints is lower than the furnace temperature, because the PCB is endothermic. 1-2. SILICON OIL is usually used for mold release and lubrication, usually found on the substrate and parts feet, and SILICON OIL is not easy to clean, so it should be used very carefully, especially when it is used as antioxidant oil, problems often occur, because it will evaporate on the substrate and cause poor tin staining

SMA Type Component Preheating temperature 1-4The way of dipping the flux is incorrect, and the cause is that the foaming air pressure is unstable or insufficient, resulting in the unstable or uneven foam height and the substrate part is not stained with the flux

Single-sided assemblies Through-hole devices and mixes 90 - 1002Poor local tin staining: This situation is similar to poor tinning, except that the local tin staining defect will not expose the copper foil surface, and only a thin layer of tin cannot form a full solder joint

Double-sided Panel Assemblies Through-hole Devices 100 - 1104.Solder joint cracking: This situation is usually caused by the expansion coefficient between the solder, the substrate, the via, and the part foot, which is not matched, and should be improved in the substrate material, part material and design

Double-sided Panel Assemblies Mixed 100 - 1105-1.Incorrect conveying angle of tin pot will cause the solder joint to be too large, and the inclination angle is from 1 to 7 degrees according to the design method of the substrate?#123; The general angle is about 3.5 degrees, the larger the angle, the thinner the tin, the smaller the angle, the thicker the tin

Multilayer board through-hole device 115 - 1255-3.Increasing the preheating temperature can reduce the heat required for the substrate to get tin, and the soldering effect has been added

Multilayer board mixed 115 - 1256Tin tip (icicle) :

IV.3 Wave soldering process parameter adjustment 6-1.Poor solderability of the substrate, this problem is usually accompanied by poor tin staining, this problem should be discussed by the solderability of the substrate, and it can be improved by increasing the specific gravity of the flux

1. Wave height: The height of the PCB in wave soldering, its value is usually controlled at 1/2 - 2/3 of the thickness of the PCB board, too high will cause the molten solder to flow to the surface of the PCB, forming a "bridging". 6-3. If the temperature of the tin bath is too short, the soldering time can be improved by increasing the temperature of the tin bath and extending the soldering time, so that the excess tin can flow back to the tin bath

2. Conveying inclination: In addition to making the machine horizontal, the inclination angle of the conveyor should also be adjusted when the wave soldering machine is installed. Through the tilt adjustment, the soldering time between the PCB and the wave surface can be adjusted, and the appropriate inclination angle helps the solder liquid to peel off from the PCB faster and return it to the tin pot. 6-5. When hand soldering, the tin tip is generated, usually because the temperature of the soldering iron is too low, and the solder temperature is insufficient to immediately retract due to cohesion to form a solder joint

3. 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, like a knife, so it is called "hot air knife". 7-1. There are some substances that are not compatible with the flux during the production of the substrate, and after overheating, the adhesive adhesive solder is formed to form a tin wire, which can be cleaned with acetone (*chemical solvent banned by the Montreal Convention), chlorinated enes and other solvents, if it cannot be improved after cleaning, there is a possibility that the substrate layer material is incorrect, and this accident should be reported to the substrate supplier in time

4. Influence of solder purity: In the wave soldering process, solder impurities mainly come from the copper leaching of the pads on the PCB, and excess copper will lead to more soldering defects. 7-3. The solder slag is pumped into the tin bath and then sprayed out and causes the substrate surface to be stained with solder slag, this problem is relatively simple, good tin pot maintenance, and the correct tin surface height of the tin bath (under normal conditions, the tin surface is 10mm high from the edge of the tin bath when the tin bath does not spray still).

5. Flux is usually the main cause of this problem, sometimes it can be improved by using another flux, rosin flux often produces a day shift during cleaning, the best way at this time is to seek the assistance of the flux supplier, the product is that they supply them more professionally

6. Coordination of process parameters: The process parameters of the wave soldering machine, belt speed, preheating time, welding time and inclination angle need to be coordinated and adjusted repeatedly. INCORRECT CURING CAN ALSO CAUSE DAY SHIFTS, USUALLY A SINGLE BATCH IS GENERATED, AND SHOULD BE REPORTED BACK TO THE SUBSTRATE SUPPLIER IN TIME AND CLEANED WITH FLUX OR SOLVENT

V.. Wave soldering defect analysis: Because the solvent used in the substrate manufacturing process changes the substrate material, especially the solution in the nickel plating process often causes this problem, it is recommended that the storage time is as short as possible

1. POOR WETTING: USE ROSIN FLUX, AFTER THE SOLDERING FURNACE WAITING TIME IS TOO NINE BEFORE CLEANING, RESULTING IN DAY SHIFT, TRY TO SHORTEN THE TIME OF SOLDERING AND CLEANING CAN BE IMPROVED

This is an unacceptable defect, and only part of the solder joint is stained with tin. The causes and improvement methods are as follows: 9. Dark residues and erosion marks: Usually black residues occur at the bottom or top of the solder joint, and this problem is usually caused by incorrect use of flux or cleaning

1 - 1. External contaminants such as oils, greases, waxes, etc., which can usually be cleaned with solvents, are sometimes picked up when printing flux masks. 9-2. The acidic flux left on the solder joint causes black corrosion color, and can not be cleaned, this phenomenon is often found in hand soldering, use the weaker flux and clean it as soon as possible

1 - 2. Silicone oil is usually used for mold release and lubrication, usually found on the substrate and part feet, silicone oil is not easy to clean, so be very careful when using, especially when it is used as an antioxidant oil, it often has problems, because it will evaporate and stick to the substrate and cause poor tin staining. 10. Green residue: Green is usually caused by corrosion, especially electronic products but not completely, because it is difficult to distinguish whether it is green rust or other chemical products, but generally speaking, the discovery of green substances should be a warning signal, and the cause must be found immediately, especially this green substance will become larger and larger, and should be very careful, usually with cleaning to improve

1 - 3. Oxidation often occurs due to poor storage or substrate process problems, and the flux cannot be removed, which will cause poor tin staining, and the secondary tin may solve this problem. COPPER ABIETATES IS A COMPOUND OF COPPER OXIDE AND ABITIC ACID (THE MAIN COMPONENT OF ROSIN), WHICH IS GREEN BUT BY NO MEANS CORROSIVE AND HAS HIGH INSULATION, WHICH DOES NOT AFFECT THE QUALITY BUT THE CUSTOMER WILL NOT AGREE TO CLEAN IT

1 - 4. The way of dipping 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. 11. White corrosives: The eighth item talks about white residues refers to the white residues on the substrate, and this project talks about the white corrosives on the feet of parts and metals, especially on the metals with more lead-containing components, which are more likely to form such residues, mainly because chloride ions are easy to form lead chloride with lead, and then form lead carbonate (white corrosives) with carbon dioxide. Only cleaning rosin can not remove chloride ions, which will accelerate corrosion. 

1 - 5. Insufficient tin eating time or tin temperature will cause solder dipping defects, because the solder 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. 12-1. Organic pollutants: the substrate and the feet of the parts may produce gas and cause pinholes or pores, the source of pollution may come from the automatic grafting machine or poor storage conditions, this problem is relatively simple, as long as it is cleaned with solvent, but if the contaminant is found to be SILICONOIL because it is not easy to be cleaned by solvents, other substitutes should be considered in the process

VI.. Partial tin staining: This situation is similar to tinning defects, except that local tinning defects do not expose the copper foil surface, only a thin layer of tin, and cannot form a full solder joint. Brightener in electroplating solution: When using a large number of brighteners for electroplating, the brightener is often deposited with gold at the same time, and volatilized when encountering high temperatures, especially when gilding, switch to the electroplating solution containing less brightener, of course, this should be fed back to the supplier

1. Cold soldering or solder joints are not bright: The solder joints seem to be broken and uneven, most of which are caused by the vibration of the parts when the solder is about to cool down to form a solder joint. Solder joint graying: This phenomenon is divided into two types: (1) After a period of soldering, (about half a load to a year) the color of the solder joint becomes dark. 2) The manufactured finished solder joint is gray

2. Solder joint cracking: This situation is usually caused by the mismatch of the expansion coefficient between the solder, the substrate, the via, and the part foot, and should be improved in terms of substrate material, part material, and design.  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

3. The amount of tin in the solder joint is too large: Usually when evaluating a solder joint, it is hoped to get a large, round and fat solder joint, but in fact, too large a solder joint may not be helpful for conductivity and tensile strength. In solder alloys, those with low tin content (such as 40/60 solder) also have darker solder joints

3 - 1. The solder joint will be too large due to the incorrect conveying angle of the tin pot, the tilt angle is 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, the smaller the angle, the thicker the tin. Crystallization of metal impurities: The metal composition of the solder must be checked regularly every three months

3 - 2. Increase the temperature of the tin bath, extend the soldering time, and let the excess tin flow back to the tin bath. Foreign substances: such as burrs, insulating materials, etc., hidden in the foot of the part, will also produce a rough surface

3- 3. Increasing the preheating temperature can reduce the heat required for tinning on the substrate and increase the soldering effect. Short circuit: An excessively large solder joint causes two solder joints to meet

3 - 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, the easier it is to short circuit, the lower the specific gravity, the thinner the tin, but the easier it is to form tin bridges and tin tips. Defective flux: improper specific gravity of flux, deterioration, etc

VII: Tin tip (icicle): Poor line design: too close to each other between lines or contacts (there should be a spacing of more than 0.6mm); If it is an arranged solder joint or IC, you should consider stealing solder pads, or use text white paint to distinguish them, and the thickness of the white paint should be more than 2 times the thickness of the solder pad (gold path).

This problem usually occurs during the DIP or WIVE soldering process, where ice-tipped tin is found on the top of the part or on the solder joint. Wave soldering technology plays a crucial role in electronics manufacturing, and as mentioned in the article, it involves numerous process parameters and possible soldering defects. From the crest height, the transfer inclination angle to the control of process parameters such as hot air knives, every link directly affects the quality of welding. For example, if the crest height exceeds 1/2 - 2/3 of the thickness of the PCB, there will be a "bridging" phenomenon of molten solder flowing to the PCB surface, which fully demonstrates the importance of precise control of process parameters.

When it comes to soldering defects, poor soldering is one of the common problems. This paper analyzes in detail the various causes of poor tin dipping, such as external contaminants, the influence of silicone oil, incorrect flux dipping method, and insufficient tin eating time and temperature. These reasons indicate that the welding process is a complex system, and deviations in any one link can lead to defects.

For local tin staining defects, although similar to tinning defects, it has its own uniqueness, it does not expose the copper foil surface, only forms a thin layer of tin, and cannot form a full solder joint. This may be related to the brightener in the electroplating solution, especially in gold plating, a large number of electroplating solutions containing more brighteners are used, which will be caused by volatilization at high temperatures, so you can consider switching to electroplating solutions containing less brighteners.

In the control of the size of the solder joint, the factors such as the conveying angle of the solder pot, the temperature of the tin bath, the preheating temperature and the specific gravity of the flux are related to each other. If the conveying angle of the tin furnace is too large, the tin is thin, and if it is too small, the tin is thick; Increasing the temperature of the tin bath and extending the soldering time can reduce the amount of tin in the solder joint; Increasing the preheating temperature helps to reduce the heat requirement of the substrate and increase the soldering effect. Adjusting the specific gravity of the flux can avoid the problem of short circuit due to too high specific gravity, which leads to too thick tin, or too low specific gravity, which is easy to form tin bridges and tin tips.

In addition, the appearance and performance of solder joints are also affected by a variety of factors. Cold soldering or solder joints that don't shine can be caused by the vibration of the part as the solder cools; Solder joint cracking is due to a mismatch in the coefficient of expansion between the solder, substrate, via, and part foot; Dark solder joints can be caused by flux residue or low tin content in the solder alloy.

Short circuit problems are often caused by factors such as oversized solder joints, poor flux, or poor circuit design. For example, if the distance between the line or contact is less than 0.6mm, it is prone to short circuit. For tin tip (icicle) problems, it is common on the top of the part foot or solder joint in the DIP or WIVE soldering process, which is also related to various factors in the soldering process, such as the use of flux, circuit design, etc. In conclusion, all aspects of wave soldering require fine tuning and strict control to ensure the quality of the weld.