DIP Process Tips and Improvement Process

1. DIP process - curve analysis

1.Wetting time: The time it takes for the solder joint to start wetting after it comes into contact with the solder.

2.Dwell time: The length of 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.

3.Preheating temperature: refers to the temperature reached before the PCB comes into contact with the crest surface (see the table on the right).

4.Soldering temperature Soldering temperature is an extremely important soldering parameter, usually 50°C - 60°C higher than the solder melting point (183°C), in most cases the temperature of the soldering furnace. In actual operation, the temperature of the solder joints of the soldered PCB is lower than the furnace temperature, which is due to the heat absorption of the PCB. SMA Type Components Warm-up temperature Single-sided panel assemblies Through-hole devices and mixes 90 - 100 Double-sided panel assemblies Through-hole devices 100 - 110 Double-sided panel assemblies Mix-and-mix 100 - 110 Multilayer Boards Through-hole devices 115 - 125 Multilayer boards Assorted 115 - 125 2. Wave soldering problems Crest Surface: The surface of the wave is covered with a layer of oxide scale, which remains almost stationary over 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 pushes forward without wrinkles, 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 crest (A), the substrate and 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 the minimum state with the lead as the center, and the wetting force between the solder and the pad is greater than the cohesive force of the solder between the two pads, thus forming a full and round solder joint, and the excess solder leaving the tail of the crest falls back into the tin pot due to gravity to prevent bridging. Prevent bridging from happening

1. Use components/PCBs with good solderability.

2. Improve flux activity.

3. Increase the PCB preheating temperature and enhance the pad wetting performance.

4. Increase the solder temperature.

5.Remove harmful impurities, reduce solder cohesion, and make it easy to separate the solder between the two solder joints. A common preheating method in wave soldering machines

6.Air convection heating.

7.Infrared heater heating.

8.Heating is a combination of hot air and radiation.

3. Adjustment of process parameters

1. Wave height: refers to the height of the PCB in wave soldering, the 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 "bridge".

2. Conveying inclination: In addition to ensuring the level of the machine, the inclination angle of the conveyor should also be adjusted when the wave soldering machine is installed. By adjusting the inclination angle, the soldering time between the PCB and the crest surface can be adjusted, and the appropriate inclination angle helps the solder fluid to peel off the PCB faster and return it to the tin pot.

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

4. Influence of solder purity: In wave soldering, solder impurities mainly come from the copper leaching of the pads on the PCB, and excessive copper will lead to an increase in soldering defects.

5. Flux

6. Coordination of process parameters: The process parameters of the wave soldering machine, such as belt speed, preheating time, welding time and inclination angle, need to be coordinated with each other and adjusted repeatedly.

Fourth, welding defect analysis

1. POOR WETTING: THIS SITUATION IS AN UNACCEPTABLE DEFECT, AND THE SOLDER JOINT IS ONLY PARTIALLY WETTED. The causes and improvement methods are as follows:

(1) External pollutants such as oil, grease, wax, etc., can usually be cleaned with solvents, and such oil stains are sometimes stained when printing flux masks.

(2) Silicone oil is often used for demoulding and lubrication, often found on the substrate and parts feet, silicone oil is not easy to clean, you need to be very careful when using, especially when used as antioxidant oil, it is easy to cause problems, because it will evaporate on the substrate and cause poor tin staining.

(3) Oxidation often occurs due to poor storage conditions or substrate process problems, and the flux cannot be removed, which will cause poor tin staining, and the problem may be solved by secondary tin.

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

(5) Insufficient tin eating time or tin temperature will cause poor tin dipping, because tin melting needs enough temperature and time for wetting, usually the solder temperature should be higher than the melting point temperature 50 °C - 80 °C, and the total tin dipping time is about 3 seconds. Adjust the viscosity of the solder paste.

2. Poor local tinning: Similar to poor tinning, but the local tinning defect will not expose the copper foil surface, only a thin layer of tin, which cannot form a full solder joint.

3. Cold soldering or solder joints are not bright: solder joints seem to be broken and uneven, most of which are caused by vibration when the parts are cooled to form solder joints.

4. Solder joint cracking: It is usually caused by the mismatch of the expansion coefficient between the solder, the substrate, the through hole and the part foot, and should be improved from the aspects of substrate material, part material and design.

5. The amount of tin in the solder joint is too large: it is usually hoped that the solder joint is large, round and fat, but too large the solder joint may not be helpful for conductivity and tensile strength.

(1) The incorrect conveying angle of the tin pot will make the solder joint too large, and the tilt angle is adjusted at 1-7 degrees according to the design of the substrate, generally about 3.5 degrees, the larger the angle, the thinner the tin, and the smaller the angle, the thicker the tin.

(2) Increase the temperature of the tin bath, prolong the soldering time, and let the excess tin flow back to the tin bath.

(3) Increasing the preheating temperature can reduce the heat required for the substrate to dip into the tin and increase the soldering effect.

(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 easier it is to form tin bridges and tin tips.

6. Tin tip (icicle): This problem usually occurs in the DIP or WIVE soldering process, and ice-tip tin is found on the top of the part foot or solder joint.

(1) Poor solderability of the substrate, this problem is usually accompanied by poor tin staining, and the solderability of the substrate should be discussed, and the specific gravity of the flux can be improved.

(2) If the area of the gold road (PAD) on the substrate is too large, it can be improved by separating the gold path with green (solder-mask) paint wire, and in principle, the green (solder-mask) paint line can be separated into 5mm×10mm blocks on the Dajin pavement.

(3) If the temperature of the tin bath is insufficient and the dipping time is too short, the temperature of the tin bath can be increased, the soldering time can be prolonged, and the excess tin can be returned to the tin bath for improvement.

(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 the tin point will be cooled rapidly, and the excess solder cannot be pulled back to the tin bath by gravity and cohesion.

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

7. There is residual tin on the solder mask green paint:

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

(2) Incorrect substrate curing will cause this phenomenon, which can be baked at 120°C for two hours before insertion, and should be fed back to the substrate supplier in time.

(3) The solder slag 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 surface height of the tin bath (under normal conditions, the tin bath does not spray when the tin bath is stationary, the tin surface is 10mm height from the edge of the tin bath) can be improved.

8. White residue: After welding or solvent cleaning, white residue is found on the substrate, usually rosin residue, which does not affect the surface electrical resistance, but the customer does not accept it.

(1) Flux is usually the main reason, sometimes changing a flux can be improved, rosin flux cleaning often produces white spots, it is best to seek the assistance of flux suppliers, because the product is supplied by them, they are more professional.

(2) Residual impurities in the process of substrate making, long-term storage will also produce white spots, which can be cleaned with flux or solvent.

(3) Incorrect curing can also cause white spots, which are usually produced separately in a certain batch, and should be fed back to the substrate supplier in time and cleaned with flux or solvent.

(4) The flux used in the factory is not compatible with the substrate oxidation protective layer, which mostly occurs when the new substrate supplier or the flux brand is changed, and the supplier should be asked for assistance.

(5) 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 to shorten the storage time.

(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 is updated weekly, the immersion flux is updated every two weeks, and the spray flux is updated monthly).

(7) Use rosin flux, after the soldering furnace is parked for too long before cleaning, resulting in white spots, try to shorten the time of soldering and cleaning can be improved.

(8) The moisture content of the solvent of the cleaning substrate is too high, which reduces the cleaning ability and produces white spots, so the solvent should be renewed.

9. Dark residue and erosion marks: Usually black residue appears at the bottom or top of the solder joint, which is usually caused by improper use or cleaning of flux.

(1) The rosin flux is not cleaned immediately after soldering, leaving a black-brown residue, and it can be cleaned as far as possible in advance.

(2) The acidic flux is left on the solder joint, causing black corrosion color and cannot be cleaned, this phenomenon is common in hand soldering, use a weaker flux and clean it as soon as possible.

(3) If the organic flux is scorched at a higher temperature and produces black spots, confirm the temperature of the tin bath and use a higher temperature resistant flux instead.

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 the discovery of green substances should be regarded as a warning, and the cause must be identified immediately, especially when the green substances will increase, they should be paid attention to, and they can usually be improved by cleaning.

(1) Corrosion problem: It usually occurs on the bare copper surface or copper-containing alloy, when using non-rosin flux, this corrosive substance contains copper ions, which is green, and green corrosion is found, which can prove that the non-rosin flux is not cleaned properly after use.

 (2) COPPER ABIETATES is a compound of copper oxide and ABIETIC ACID (the main component of rosin), this substance is green but not corrosive and has high insulation, does not affect the quality but the customer will not agree, should be cleaned.

(3) The residue of PRESULFATE or similar residue on the substrate will produce green residue after soldering, and the substrate manufacturer should be required to do a cleanliness test after the substrate is made and cleaned to ensure the cleanliness quality of the substrate.

11. White corrosives: The white residues mentioned in item 8 refer to the white residues on the substrate, and this item refers to the white corrosives on the feet and metals of the parts, especially the metals with more lead components are more prone to produce such residues, mainly because chloride ions are easy to form lead chloride with lead, and then form lead carbonate (white corrosives) with carbon dioxide. When using rosin flux, rosin is insoluble in water and will encapsulate the chlorine-containing active agent and will not corrode, but if the improper solvent is used, only the rosin can be cleaned and cannot remove chloride ions, but will accelerate corrosion.

12. Pinholes and pores: pinholes are small holes in solder joints, pores are larger holes in solder joints and can see the 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. (1) Organic pollutants: the substrate and parts may produce gas to cause pinholes or pores, the source of pollution may come from the automatic transplanting machine or poor storage conditions, this problem is simple, can be cleaned with solvents, but if the contaminants are silicone oil, because it is not easy to be cleaned by solvents, other substitutes should be considered in the process. （

2) Moisture on the substrate: using a cheaper substrate material or a rougher drilling method, the hole is easy to absorb moisture, and it is caused by heating and evaporation during the soldering process, and the solution is to bake it in the oven at 120°C for two hours.

3) Brightener in electroplating solution: When a large number of brighteners are used for electroplating, the brightener is often deposited at the same time as gold, which is caused by volatilization at high temperatures, especially when gilding, the electroplating solution containing less brightener is used instead, which should be fed back to the supplier.

13. TRAPPED OIL: Oxidation prevents oil from being driven into the tin bath and gushing out of the jet to pollute the substrate, this problem is that the solder liquid level of the tin bath is too low, and it can be improved by adding solder to the tin bath.

14. Gray solder joints: There are two kinds of this phenomenon:

(1) After soldering, the color of the solder joint turns dark for a period of time (about half load to one year);

(2) The manufactured finished solder joints are gray.

(3) Impurities in the solder: The metal composition in the solder must be regularly inspected every three months.

(4) The flux will also produce a certain degree of gray and dark 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 can 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. (3) In solder alloys, those with low tin content (such as 40/60 solder) have gray solder joints.

15. The surface of the solder joint is rough: the surface of the solder joint is sand-like and protruding, and the overall shape of the solder joint remains unchanged.

(1) Crystallization of metal impurities: The metal composition in the solder must be regularly inspected every three months.

(2) Solder slag: The solder slag is pumped into the tin bath and gushes out through the jet stream, because the tin contains solder slag and the surface of the solder joint is sand-like, the solder liquid level of the solder 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.

(3) Foreign substances: such as burrs, insulating materials, etc., hidden in the feet of the parts, will also produce rough surfaces.

16. Yellow solder joints: caused by high solder temperature, immediately check whether the tin temperature and thermostat are faulty.

17. Short circuit: too large solder joints make the two solder joints connect.

(1) If the substrate does not eat tin for enough time and the preheating is insufficient, adjust the tin furnace.

(2) Poor flux: improper specific gravity and deterioration of flux.

(3) The direction of the substrate is not well matched with the tin wave, and the tin eating direction is changed.

(4) Poor circuit design: the distance between the lines or contacts is too close (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 separate, at this time, the thickness of white paint should be more than 2 times the thickness of the solder pad (Jindao). (5) If the contaminated tin or excessive oxide accumulation is caused by the PUMP belt and causes a short circuit, the solder in the tin bath should be cleaned or further updated completely.

Soldering is a process after SMT placement processing (except for special cases: PCB boards with plug-ins only), and the processing process is as follows:

<!--[if !supportLists]-->1.<!--[endif]-->Pre-processing of components The staff of the pre-processing workshop receive the materials at the material according to the BOM bill of materials, carefully check the material model and specifications and sign, carry out pre-production pre-processing according to the sample, and use automatic bulk capacitor foot shearing machine, transistor automatic molding machine, automatic belt molding machine and other molding equipment for processing. Requirements: (1) The horizontal width of the shaped component pin should be the same as the width of the positioning hole, and the tolerance should be less than 5%; (2) The distance between the component pins and the PCB pad should not be too large; (3) If the customer requires, the parts need to be formed to provide mechanical support and prevent the pads from warping.

<!--[if !supportLists]-->2.<!--[endif]-->Paste high-temperature adhesive tape, paste high-temperature adhesive tape → the board, and plug the tinned through holes and components that must be soldered later.

<!--[if !supportLists]-->3.<!--[endif]-->DIP plug-in processing staff need to bring an electrostatic ring to prevent static electricity, according to the component BOM list and component number diagram for insertion, plug-in should be careful and serious, no errors, leakage.

<!--[if !supportLists]-->4.<!--[endif]-->Check the inserted components, mainly to check whether they are inserted incorrectly or missing.

<!--[if !supportLists]-->5.<!--[endif]-->For PCB boards with no plug-in problems, the next step is wave soldering, which is automatically soldered by a wave soldering machine to make the components firm.

<!--[if !supportLists]-->6.<!--[endif]-->Remove the high-temperature adhesive tape, and then carry out inspection, this link is mainly visual inspection, through the naked eye to observe whether the soldered PCB board is in good condition.

<!--[if !supportLists]-->7.<!--[endif]-->Repair and repair the PCB board that has not been soldered to prevent problems.

<!--[if !supportLists]-->8.<!--[endif]-->Post-welding: The process set up for components with special requirements, because some components cannot be directly welded by the wave soldering machine due to the limitations of the process and the material itself, and need to be completed manually.

<!--[if !supportLists]-->9.<!--[endif]-->Perform functional tests on the PCB board after all components are soldered, test whether each function is normal, and repair and test if functional defects are detected.

The DIP plug-in post-soldering process and the SMT placement process are equally important, the DIP plug-in has a special workshop, the lead-free and leaded production lines are separated, and the quality control department strictly controls to prevent defective products from being listed.

In the whole process of wave soldering, we can see that all the links are closely connected and influence each other. For example, the problem of poor solder staining mentioned in the analysis of soldering defects may not only be caused by external contaminants, but also may be related to various factors such as the way the flux is used, the time and temperature of the tin. From these factors, we can further explore how to optimize the wave soldering process.

In the actual production process, with the continuous miniaturization and integration of electronic products, the requirements for the wave soldering process are getting higher and higher. In order to reduce welding defects, in addition to the precise control of individual process parameters such as crest height, transfer inclination, etc., it is also necessary to start with the quality control of raw materials. For example, the use of high-quality components and PCBs to ensure good solderability can effectively reduce the incidence of problems such as poor solder staining and solder joint cracking.

At the same time, the selection and use of flux is also crucial. Different types of fluxes behave differently in different soldering scenarios. For cases that are prone to white residues, if it is a rosin flux, it is necessary to consider its cleaning time to avoid white spots caused by too long parking. In terms of preventing bridging, it is necessary to consider not only improving the activity of the flux, but also increasing the PCB preheating temperature and removing harmful impurities.

In addition, with the increasingly stringent environmental requirements, it is also necessary to pay attention to the purity of the solder and the environmental friendliness of the flux in the wave soldering process. For example, some traditional solvents may be banned due to environmental conventions, which necessitates the search for suitable alternatives. In addition, the coordination between the individual parameters needs to be more precise in the adjustment of process parameters to adapt to changing production needs. In short, the wave soldering process needs to be comprehensively optimized from many aspects in order to meet the development requirements of the modern electronics manufacturing industry.