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I. Form of “solder balls” and relevant standards
Standard 1: "No solder balls allowed" stipulated in the standard MIL-STD-2000;
Standard 2: "Less than 5 per square inch" stipulated in the standard IPC-A-610C.
In the IPC-A-610C standard, the minimum insulation gap shall be 0.13 mm, and the solder balls with a diameter less than this value are considered to be qualified; those with a diameter greater than or equal to 0.13 mm are not qualified, so manufacturers must take corrective measures to avoid this phenomenon.
Common forms of solder balls and their sizes are shown in the pictures below:
II. Causes of "solder balls" and preventive measures
During the soldering process of "SMT surface mounting", various factors, such as the "temperature, time, solder paste quality, printing thickness, steel mesh (mold plate) manufacturing, and mounting pressure" of the reflow soldering may lead to the generation of "solder balls".
(I). Cause one: poor solder paste quality
1. Metal content in solder paste.
The mass ratio of metal content in the solder paste is about 89% to 91%, and the volume ratio is about 50%. Usually, the higher the metal content, the closer the arrangement of the metal powder in the solder paste, and the more opportunities for the combination among tin powder particles and the less possibility to be blown apart during gasification. In this case, it is not easy to form "solder balls"; if the metal content is reduced, the probability to generate "solder balls" would be increased.
2. Content of oxides in solder paste.
The oxide content in the solder paste also affects the soldering effect. The higher the oxide content, the greater the surface tension of the melted metal powder during the process of fusion with the solder pad. What’s more, in the "reflow soldering stage", the content of the oxides on the surface of the metal powder would be higher, which is not conducive to the complete "wetting" of the melted solder, resulting in the generation of small solder balls.
3. Particle size of metal powder in solder paste.
The metal powder in the solder paste is a very tiny and nearly circular ball, and the spherical diameter of the commonly used solder powder is about 25-45μm. The oxide content in the finer powder is relatively lower, which can relieve the phenomenon of “solder balls” to some extent.
4. Thermal collapse resistance of solder paste.
In the preheating stage of reflow soldering, if the thermal collapse resistance of the solder paste is not good, the printed solder paste begins to collapse before reaching the welding temperature (before the solder begins to melt), and some of the solder paste flows outside the solder pad. When entering the soldering zone, the solder begins to melt. Due to the action of internal stress, the solder paste shrinks into solder joints and begins to infiltrate and climb to the soldering end. Sometimes, due to the lack of soldering flux or other reasons, the solder paste stress is insufficient, and a small part of the solder paste outside the solder pad does not shrink back, and it would form a “solder ball” when completely melted.
(II). Cause two: improper use
1. Generation of “solder balls” during the passing through reflow furnaces.
The flow soldering process can be divided into four stages as follows: preheating, heat preservation, soldering and cooling. The "preheating stage" is to slowly heat up the printed board and surface mount components to a temperature between 120 °C and 150 °C, which can remove volatile solvents in the solder paste and reduce the thermal shock to the components. During this process, the gasification phenomenon would happen in the solder paste. At this time, if the bonding force among the metal powder in the welding paste is less than the force generated by the soldering flux gasification, there will be a small amount of "soldering powder" flowing or flying out of the solder pad, and in the "soldering" stage, these "soldering powder" would also melt, thus forming "solder balls". Thus, it can be concluded that "the higher the preheating temperature, the faster the preheating speed, the more likely to aggravate the gasification phenomenon of the soldering flux, cause collapse or splash, and further form solder balls". Therefore, we can utilize a more moderate preheating temperature and preheating speed to control the formation of "solder balls".
2. Printing thickness and amount of the solder paste on the printed board
The printing thickness of the solder paste is a key production parameter, and its value is usually between 0.15-0.20 mm. Too thick or too much solder paste would easily lead to "collapse" and form "solder balls". During the production of steel meshes (mold plates), the size of the solder pad would determine the size of the mold plate opening. Usually, in order to avoid excessive printing amount of the solder paste, the size of the printing hole is controlled to be less than about 10% of the corresponding contact area of the solder pad. Relevant results have shown that this can reduce the generation of “solder balls” to a certain extent.
3. If the mounting pressure is too large during the mounting process, when the component is pressed on the solder paste, there is a possibility that some solder paste is squeezed under the component or a small amount of tin powder flies out, and this part of solder powders would be melted in the soldering stage to form "solder balls". Therefore, an appropriate mounting pressure should be applied during the mounting process.
4. Generally, the solder paste needs to be refrigerated for storage. Before use, it is a must to make it return to room temperature before opening the package. If the solder paste temperature is too low and its package is opened, some moisture would form on its surface, which will cause the solder powder to fly out after preheating, and make the melted soldering flux splash in the soldering stage to further form "solder balls". In most regions of our country, the air humidity is relatively high in summer. So, when the solder paste is taken out of the refrigerated environment, it must be kept at room temperature for 4-5 hours before opening the bottle cap.
5. The production or working environment can also affect the formation of "solder balls". If the printed board is stored in a wet warehouse for a long time, small water beads would be found in the packaging bag of the printed board, and these moisture, similar with those absorbed by the solder paste, would affect the soldering effect and cause the formation of "solder balls". Therefore, if conditions permit, the printed board or components shall be dried to a certain extent before mounting, printing and soldering, which can effectively inhibit the formation of "solder balls".
6. The shorter the exposure time of the solder paste to the air, the better. This is also a principle of using the solder paste. After some solder paste is taken out, the lid shall be closed immediately. Especially, the inside lid must be pressed down tightly so that the air between the lid and the solder paste can be squeezed out, otherwise the service life of the solder paste would be affected to some extent, and it would simultaneously cause the solder paste to dry more quickly or absorb moisture in the next use, thus forming "solder balls".
(III). Prevention and control of "solder balls"
1. Selection of solder paste
When a kind of solder paste is selected, it shall be put into trial use under the existing process conditions, so that we can verify the applicability of the supplier's solder paste to its own products and processes, and also preliminarily understand the specific performance of the solder paste in actual use. During the process of solder paste evaluation, attention should be paid to various common parameters, such as "the ratio of soldering oil to soldering powder, particle size of tin ball" and so on.
2. Control and improvement of "SMT surface mounting" process
(1). The solder paste shall be stored strictly in accordance with the storage conditions and temperature requirements provided by the supplier. Generally, the solder paste should be stored under refrigerated conditions of 0-10℃;
(2). After the solder paste is taken out and before use, it should be warmed at room temperature. Its container must not be opened before it completed returns to room temperature.
(3). During the stirring process, the stirring method and stirring time provided by the supplier shall be applied for stirring;
(4). During the printing process, attention should be paid to the strength of printing, and the cleanliness of the surface of the steel mesh. The redundant solder paste residue on the surface of the steel mesh should be wiped away in a timely manner so as to prevent the PCB surface from being polluted in this process and the formation of solder balls during the soldering process;
(5). During the reflow soldering process, the work should be carried out in strict accordance with the reflow soldering curve, and must not be adjusted at will. At the same time, the difference between the reflow soldering curve and the standard curve should be checked frequently and relevant corrective measures shall be taken accordingly;
(6). During the "SMT surface mounting" process, the "opening mode" and "opening rate" of the steel mesh (mold plate) are very likely to cause some defects in the solder paste’s "printing characteristics" and "soldering characteristics", which would further cause the formation of “solder balls”.
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