PCBA Tech - PCBA processing solder paste welding process

The interconnect pins of various PCBA surface mount components on the circuit board, whether it is a protruding pin, a hook pin (J-Lead), a ball pin, or a pinless but only solder pad, must be on the board first Solder paste is printed on the bearing pad, and each "foot" is temporarily positioned and pasted before it can be permanently soldered by melting the solder paste. Reflow in the original text refers to the process in which small spherical particles of solder that have been melted in the solder paste are melted and welded again by various heat sources to become solder joints. The general PCBA industry irresponsibly directly quotes the Japanese term "reflow soldering", which is actually not appropriate and fails to fully express the correct meaning of Reflow Soldering. If it is literally translated as "remelt" or "reflow", it is even more inexplicable.

1. Selection and storage of solder paste:

At present, the latest international standard for solder paste is J-STD-005. The choice of solder paste should focus on the following three points, in order to maintain the best consistency of the printed paste layer:

(1) The size of the tin particles (powder or balls), alloy composition specifications, etc., should depend on the size of the solder pads and pins, as well as the volume of the solder joints and the soldering temperature.

(2) What is the activity and cleanability of the flux in the solder paste?

(3) What is the content of the solder paste's Viscosity and the metal weight ratio?

After the solder paste is printed, it also needs to be used for the placement of the parts and the positioning of the pins, so its positive tackiness (tackiness) and negative collapse (slump), as well as the actual opening after the original packaging. The working life (Working Life) is also taken into consideration. Of course, it has the same view as other chemicals, that is, the long-term stability of solder paste quality should definitely be considered first.

Secondly, the long-term storage of solder paste must be placed in the refrigerator. It is more ideal to adjust to room temperature when taking it out. This will prevent the condensation of dew in the air and cause water accumulation in the printed dots, which may cause tin splashing during high-temperature soldering., And the solder paste after opening each vial should be used up as much as possible. The remaining solder paste on the screen or steel plate should not be scraped back, and stored in the remaining material of the original container for reuse.

2. Soldering and pre-baking of solder paste:

For the distribution and application of solder paste on the solder pads on the board, the most common mass production methods are the "Screen Print" or the Stencil Plate printing method. In the former screen, the screen itself is only a carrier, and a precise patterned stencil (Stencil) needs to be attached separately to transfer the solder paste to various solder pads. This screen printing method is more convenient and inexpensive to make the screen, and it is very economical for a small number of diverse products or the process of making samples. However, because it is not durable and the precision and processing speed are not as good as steel plate printing, the former is rarely used in mass production PCBA assemblers.

As for the steel plate printing method, local chemical etching or laser ablation processing method must be used to carry out double-sided precision hollowing for 0.2mm thick stainless steel plates to obtain the required openings so that the solder paste can be pressed and leaked The printing is performed on the solder pads on the board surface. The sidewalls must be smooth to facilitate the passage of solder paste and reduce its accumulation. Therefore, in addition to etching the hollow, electropolishing (Electropolishing) is required to remove the hair. Even electroplating nickel is used to increase the lubricity of the surface to facilitate the passage of the solder paste.

In addition to the two main methods mentioned above, there are two common methods for distributing solder paste: Syringe Dispensing and Dip Transfer for small batch production. The injection method can be used when the surface of the board is uneven and the screen printing method cannot be used, or when the solder paste has not many spots and the distribution is too wide. However, the processing cost is very expensive because there are few dots. The amount of solder paste coating is related to the inner diameter of the needle tube, air pressure, time, particle size, and adhesion. As for the "multi-point transfer method", it can be used for fixed arrays of packaged substrates (substrates) such as a small board. The amount of transfer is related to the degree of adhesion and the size of the tip.

Some solder pastes that have been spread need to be pre-baked (70~80 degree Celsius, 5~15 minutes) before placing the parts on the pins to drive away the solvent in the paste, so as to reduce the subsequent high temperature welding Solder ball caused by medium splashing, and reduced voiding in the solder joints; but this kind of printing and then heating and baking will make the solder paste that reduce the adhesion easily occur when stepping on the foot Collapse. In addition, once the pre-bake is excessive, it may even accidentally cause poor soldering properties and solder balls afterwards due to the oxidation of the particle surface.

Three, high temperature welding (Reflow)

1. General

High-temperature welding is the use of infrared light, hot air or hot nitrogen, etc., to make the solder paste printed and attached to each pin undergo high-temperature melting and become solder joints, which is called "fusion welding". At the beginning of the rise of SMT in the 1980s, most of its heat sources were derived from Radiation infrared (IR) units with the best heating efficiency. Later, in order to improve the quality of mass production, hot air was added, or even the infrared ray was completely abandoned and only hot air units were used. Recently, in order to "no-clean", it has to be further changed to "hot nitrogen" for heating. Under the condition that it can reduce the oxidation of the surface of the metal to be welded, "hot nitrogen" can maintain quality and take into account environmental protection. Naturally, it is the best way, but the increase in cost is extremely lethal.

2. Infrared and hot air

Common infrared rays can be roughly divided into:

(1) "Near IR" with a wavelength of 0.72~1.5µm, which is close to visible light.

(2) "Middle IR" (Middle IR) with a wavelength of 1.5 ~ 5.6 µm.

(3) And "Far IR" (Far IR) with a lower thermal energy wavelength of 5.6~100µm.

The advantages of infrared welding are: high heating efficiency, low equipment maintenance cost, the disadvantage of "tombstone" is reduced compared with steam welding, and it can be operated together with high-temperature hot gas. The disadvantage is that there is almost no upper limit temperature, which will often cause burns, and even cause discoloration and deterioration of the parts to be welded due to overheating, and can only weld SMD but not PTH plug-in component feet.

3. PCBA automatic conveying process:

The overall temperature profile of connection welding (Profile); there are three levels of preheating (heat absorption), welding and cooling. In each level, there are several zones (Zones). Those with fewer zones (3-4 zones) have a slower conveying speed (26cm/min), and those with more zones (above 7 zones) speed up (close to 50cm/ min) The temperature control is also more accurate. Generally, 6 stages are more suitable for batches. The appropriate time for the whole line is between 4-7 minutes.

Preheating can make the board surface temperature reach 150 degree Celsius, and the flux can be activated within 90-150 seconds at 120 degree Celsius to remove rust stains and prevent them from rusting again. The higher the Tg temperature of the sheet, the better, because the plastic material above Tg will not only show soft plasticity, which will greatly damage the dimensional stability, but also the PTH will easily break when the expansion in all directions (X.Y.Z) increases. Each board with different material numbers has its best conveying speed, but the residence time of the general welding zone can be specified between 30-60 seconds, and the appropriate welding temperature is 220 degree Celsius. Practical standard operating procedures (SOP) should be formulated separately before mass production.