PCB Tech - Common disadvantages of lead-free wave soldering of circuit boards

Common disadvantages of lead-free wave soldering of circuit boards

1. Selective wave soldering
When there are only a few areas on the bottom of the circuit board that need tin wave soldering, but want to save the cost of expensive carriers (Pallets), although you can switch to the hand soldering of SOlder Wire and Solder Iron, the Welding quality is not easy to grasp; if the stress increases, the dead angle is difficult to enter, the heat is insufficient, or the solder joints are too many, the labor cost is too high, etc.; even if both sides require socket wave soldering, the local solder wave soldering method is still selected Its necessity. Therefore, foreign suppliers have developed very special welding machines. With the cooperation of robots and conveyor belts, they can perform fixed-point or fixed-zone dip welding (Dip) or fixed-point dragging on the bottom surface of the board. Welding (Dray), and even manual surge welding of partial plates, etc., are described below.

(1), local plate surface flooding (Flood)
This kind of equipment is relatively simple. It only needs a large tin bath (for example, 50 cm*60 cm). A large stainless steel lid (Plate) with an opening is installed on the surface of the bath, and a large lid with a central opening to be welded is also set. A small cover with a wall can be replaced, and the inside of the wall is a full pool surface or a selective pool surface of molten tin. Put the flux-coated and preheated board, manually lay it flat and fix it on the wall, and then step on the foot key to make the central tin surface rise up, so the down part of the solder joint is held in 6-10 seconds Be welded inside. This kind of simple surge welding method can be changed in many ways, and it is very common in many single and double board circles, but its common disadvantage is that it is easy to short-circuit in close range.

(Two), fixed-point dip soldering (DiP)
A small amount of molten tin overflows from the fixed-point "gush nozzle" to form an arc-shaped surface, and then use the program to move the board to the position, and make it touch the fixed-point tin surface to be dipped and soldered firmly. With the cooperation of the robot, the conveyor belt and the software program, continuous processes such as flux coating, preheating, and touch welding can be carried out. This method can implement single-point dip soldering, and it can also move the board in a straight line for multi-point drag soldering. However, the automated process is not only expensive and very slow. In order to ensure sufficient heat, the tin temperature must also be set above 300°C to avoid cold soldering. This will make the iron content of stainless steel more likely to be corroded by high tin solder and long-term strong heat.

Since the surge welding time is slightly longer than wave welding, the average quality will also be better.

This selective dip soldering method can also be equipped with a special outlet plate for inrush tin on the large tin pool, and can carry out long and diversified solder inrush soldering. It is even possible to switch to a stainless steel jig with multi-grid openings for tin flooding, and conduct multi-point dipping in close pitch to reduce the short circuit between each other.

(Three), mobile drag welding (Drag)
Use the manipulator to accurately place the board on top of the 6mm diameter single-nozzle fountain, and then move the board along the solder joint track at a certain speed, so that the front and back solder joints of the local area can be welded one by one, which is called drag welding. . This approach can save the cost of a dedicated Nozz1e Plate, but the automation of the program is not only expensive but also very time-consuming. It can be used for a small number of high-unit-price plates. In response to low-unit-price mass-producers, they have to cut their love.

Second, the shortcomings that often occur in lead-free wave soldering
Some defects in lead-free wave solder joints are basically due to their physical nature, and the industry has no choice but to regard them as normal if it is unavoidable. Therefore, the international general specification IPC-A-610D has included certain deficiencies in the acceptance list, which is different from the previous lead solders, and readers should not know it. In addition, the quality problems of lead-free wave soldering are not the same as those of lead-free reflow soldering. It is necessary to thoroughly analyze the mechanism to avoid confusion. If certain wave soldering abnormal phenomena are caused by improper operation and management, they will still be regarded as defects in quality. Examples are as follows:

(1) There are cracks in the solder joints in the pin filling
Because the US and Japanese industries have long recognized SAC as the mainstream solder for wave soldering, and with the support of numerous reliability data through long-term research, it has almost become the best choice. In fact, SAC is not only more expensive, poor solderability, and easy to bite copper from the PCB, but also roughness and shrinkage cracks appear. However, as long as the crack does not penetrate to the bottom (referring to the surface of the pin or the backing surface of the board), IPC-A-610D determines that its quality is acceptable and acceptable. If the solder is changed to a cheaper one, the surface will be smoother and the occurrence of cracking will be greatly reduced.

(2) Fillet Lifting
Due to the large increase in heat of lead-free wave soldering (SAC or SCN), there is a large gap or mismatch between the Z expansion of the plate (55-60ppm/ degree Celsius) and the thermal expansion coefficient (CTE) of the solder itself (MiSmat Chment) ), so that in the rapid cooling and shrinkage after strong expansion, when the solder joint (20-22ppm/ degree Celsius) cannot keep up with the shrinkage of the plate, once the growth of the IM C during the period is poor, it will cause the copper ring and the solder The separation between. If the working MC is strong enough, the copper ring may be pulled off the board surface, or the solder itself may crack. As for those with lead solder due to its very soft nature, such defects rarely occur, unless it is a thick plate with deep holes that occasionally appear. The way to avoid it should focus on:

(3) Bridging
When a large amount of copper contamination occurs in alloy solders such as SAC or SCN, it will bring about the negative effect of rising melting point (mp). Due to the fact that the operating solder temperature cannot be adjusted simultaneously, the viscosity of the liquid material (Viscosity) increases. High speed (such as 110cm/min) will inevitably cause bridge short circuits between adjacent pins. Although reducing the production speed can cope with it for a while, the copper melting phenomenon will be worse. And lead-free soldering in this regard is not good. More than lead. The way to draw a salary from the bottom of the pot is to reduce the copper content in the tin bath, or use copper-free solder (such as SAC30 or SN, etc.) as a supplement when adding it to reduce its viscosity and improve its fluidity ( F1uiditY), so that the bridge can be relieved. In addition, a good flux can also reduce the occurrence of bridges. Some poor designs are too close to each other, the spacing should be relaxed. The use of nitrogen can also increase the activity of liquid tin and reduce Bridging; or adding a sacrificial hit at the end of the QFP wave that is prone to short-circuiting to gather excess solder and become a "tin thief".

(4) Solder Balling
Whether it is wave soldering or reflow soldering, whether it is lead or lead-free, solder balls have always been a problem that is difficult to eradicate. Most of the causes are caused by direct killer spattering. Usually the solvent in the flux is not preheated. If it can all be driven away, the top surface is easy to splash out the broken ball from the inside of the hole. If the green paint is not hardened enough to make it soft or too smooth at high temperature, the bottom surface is prone to adhesion, and the reasons for the broken balls on the two surfaces are different. Sometimes the OSP film is processed unevenly, or the bottom copper is oxidized after being stored for too long (for example, the stock is more than half a year), or the solder paste is cleaned and then reprinted, the OSP film has been removed by alcohol, causing the bare copper to rust. Poor wettability, tin splashing may also occur in the process of refusing tin. Some inferior through holes L will splash tin into balls when blow holes occur. At this time, reducing the pump speed or wave pressure of the booster will reduce the splashing of tin balls.

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