1. Common defects in reflow soldering

 

 

 

In the assembly and production of modern electronic products, factors such as inappropriate reflow oven temperature curve settings, unreasonable material selection, and poor soldering environment may produce many welding defects, which may eventually lead to long-term reliability problems. Common major welding defects include poor wettability (lack of solder, cold solder, empty solder), solder balls, solder beads, tombstones (suspension bridges), bridging, component cracking, excessive intermetallic compound growth, Solder joint voids, etc. Therefore, understanding the basic reasons for the occurrence of various defects and carrying out reasonable material and process combination and optimization can improve the quality of assembly welding and ensure its long-term reliability.

 

 

 

2. Factors affecting the success rate of reflow soldering

 

 

 

Overall, the common factors that affect the quality of reflow soldering mainly include the following aspects.

 

 

 

(1) PCB pad design

 

 

 

The assembly quality of SMT has a direct and very important relationship with PCB pad design. If the PCB pad design is correct, a small amount of skew during mounting can be corrected by the surface tension of the molten solder during reflow soldering (self-positioning or self-correction effect); on the contrary, if the PCB pad design is incorrect, even if The mounting position is very accurate, but welding defects such as component position deviation and tombstones will appear after reflow soldering.

 

 

 

According to the analysis of the solder joint structures of various components, in order to meet the reliability requirements of solder joints, PCB pad design should master the following key elements.

 

 

 

① Symmetry - The pads at both ends must be symmetrical to ensure the balance of the surface tension of the molten solder.

 

 

 

② Pad spacing - ensure proper overlap between component electrodes or pins and pads. Spacing that is too large or too small can cause welding defects.

 

 

 

③ Remaining size of the solder pad - the remaining size after the component electrode or pin is overlapped with the solder pad must ensure that the solder joint can form a meniscus.

 

 

 

④ Pad width - should be basically consistent with the width of the component electrode or pin. Here we take the solder joints of rectangular chip components as an example. The structure is shown in the figure below.

 

 

 

 

 

 

 

If the above design rules are violated, welding defects will occur during reflow soldering, and PCB pad design problems are difficult to solve using process methods during production.

 

 

 

● When the pad spacing G is too large or too small, tombstones and shifts will occur because the component soldering ends cannot overlap the pads during reflow soldering.

 

 

 

● When the pads are large and small asymmetrically, or when the electrodes of two components are designed on the same pad, tombstones (suspension bridges) and shifting phenomena may also occur due to asymmetric surface tension.

 

 

 

● If the via hole is designed on the pad, the solder will flow from the via hole, resulting in insufficient solder paste, as shown in the figure below.

 

 

 

 

 

(2) Solder paste quality and correct use of solder paste

 

 

 

There are certain requirements for the metal powder content in the solder paste, the oxygen content of the metal powder, the viscosity and thixotropy of the solder paste.

 

 

 

● If the solder paste contains high metal powder, the metal powder will spatter as the solvent evaporates when the reflow soldering heats up. At this time, if the oxygen content of the metal powder is high, spattering will be intensified and solder beads will be formed.

 

 

 

● If the viscosity of the solder paste is too low or the shape retention (thixotropy) of the solder paste is not good, the solder paste pattern will collapse after printing and even cause adhesion. Welding defects such as solder beads and bridges will also be formed during reflow soldering.

 

 

 

● Improper use of solder paste, for example, taking the solder paste out of the low-temperature cabinet and using it directly. Since the temperature of the solder paste is lower than room temperature, water vapor condensation occurs, that is, the solder paste absorbs moisture in the air, and after stirring, the water vapor is mixed in the solder paste, causing the reflow soldering to heat up. When the metal powder is evaporated, the water vapor evaporates and brings out the metal powder to produce solder balls. At high temperatures, the water vapor will also oxidize the metal powder, spatter and form solder beads, and even cause poor wetting.

 

 

 

(3) Component soldering terminals and pins, and PCB substrate pad quality

 

 

 

If the soldering terminals and pins of components and the pads of the PCB substrate are oxidized or contaminated, or the PCB becomes damp, welding defects such as poor wetting, virtual soldering, solder beads, and voids will occur during reflow soldering.

 

 

 

(4) Solder paste printing quality

 

 

 

According to statistics, under the premise that the PCB design is correct and the quality of components and printed boards is guaranteed, nearly 70% of surface assembly quality problems are caused by the solder paste printing process. Whether the printing position is correct (printing accuracy), the amount of solder paste, whether the solder volume is uniform, whether the solder paste pattern is clear, whether there is adhesion, whether the PCB surface is contaminated by solder paste, etc., all directly affect the quality of PCB assembly and welding.

 

 

 

There are many factors that affect printing quality, mainly the following factors.

 

 

 

① Screen quality. Screen printing is contact printing, so the thickness of the screen and the size of the opening determine the printing quality of the solder paste. Too much solder paste will cause bridging, and too little solder paste will cause insufficient solder or virtual soldering. The shape of the stencil opening and whether the opening is smooth will also affect the demoulding quality. The opening of the stencil must have the bell mouth facing downwards, otherwise the solder paste will be brought out from the chamfer of the bell mouth when demoulding.

 

 

 

② The printing process of solder paste. The viscosity, printability (rollability, transferability), thixotropy, and service life at room temperature of the solder paste will all affect the printing quality. If the printability of the solder paste is not good, in severe cases the solder paste will just slide on the stencil. In this case, the solder paste cannot be printed at all.

 

 

 

③ Printing process parameters. Solder paste is a thixotropic fluid and is viscous. When the scraper moves forward at a certain speed and angle, it exerts a certain pressure on the solder paste, pushing the solder paste to roll in front of the scraper, creating the pressure required to inject the solder paste into the mesh. The viscous friction of the solder paste causes the solder paste to shear at the intersection between the scraper and the screen. The shear force reduces the viscosity of the solder paste, which is beneficial to the smooth injection of the solder paste into the mesh.

 

 

 

There are certain constraints between the squeegee speed, squeegee pressure, the angle between the squeegee and the stencil, and the viscosity of the solder paste. Therefore, only by correctly controlling these parameters can the printing quality of the solder paste be guaranteed. For example, if the squeegee pressure is too high, the solder paste pattern will adhere during printing; if the printing speed is too fast, the amount of solder paste may be insufficient. If the residual solder paste at the bottom of the stencil is not wiped off in time, the solder paste will contaminate areas other than the pads during printing. These factors will cause welding defects such as bridging, false soldering, and solder beads.

 

 

 

④ Equipment accuracy. When printing high-density and small-pitch products, the printing accuracy and repeatability of the printing machine will also play a certain role. If the printing machine is not equipped with a visual alignment system, even if the manual pattern alignment is very fine, the pad pattern of the PCB will not match the mesh. The board mesh patterns completely overlap, but the processing errors of the PCB still cannot be solved.

 

 

 

⑤ Impact on the use, management and environmental factors of recycled solder paste. Ambient temperature, humidity and environmental hygiene all affect the quality of solder joints. Recycled solder paste and new solder paste should be stored separately. If the ambient temperature is too high, the viscosity of the solder paste will be reduced. When the humidity is too high, the solder paste will absorb moisture in the air. If the humidity is too low, it will accelerate the volatilization of the solvent in the solder paste. Dust in the environment will Mixing into solder paste can cause pinholes in solder joints.

 

 

 

(5) The three factors that affect the placement quality of mounted components are:

 

 

 

① The components are correct. It is required that the type, model, nominal value, polarity and other characteristic marks of each assembly number component must comply with the requirements of the product's assembly drawing and detailed list, and cannot be pasted in the wrong position.

 

 

 

② The position is accurate. The electrodes or pins of the component should be as aligned and centered as possible with the pad pattern. The mounting position of components must meet process requirements. Because the self-positioning effect of chip components with two electrodes is relatively large, when mounting, the two electrodes in the length direction of the component only need to overlap the corresponding pads, and 1/2 of the width direction overlaps the pads. It can self-calibrate during reflow soldering. But if one of the electrodes does not overlap the pad, it will shift or tombstone during reflow soldering. For SOP, SOJ, QFP, PLCC and other components, the self-positioning effect is relatively small, and the mounting offset cannot be corrected through the self-correction effect of reflow soldering. Therefore, it is necessary to ensure that 3/4 of the pin width is on the pad when mounting, and the toe and heel of the pin should also be on the pad. If the mounting position exceeds the allowable deviation range, it must be manually adjusted before entering the reflow soldering furnace for soldering. Otherwise, it must be repaired after reflow soldering, which will cause a waste of man-hours and materials, and even affect product reliability. During the production process, when it is found that the mounting position exceeds the allowable deviation range, the mounting coordinates should be corrected in time.

 

 

 

When manual mounting, the mounting position is required to be accurate, and the pins and pads are aligned and centered. Do not place them incorrectly. Drag the solder paste for alignment to avoid adhesion of the solder paste pattern and cause bridging.

 

 

 

③ The patch pressure (patch height) must be appropriate. The welding end or pin of the component should be immersed in the solder paste to a depth of 1/2 of the printed thickness of the solder paste. For general components, the solder paste extrusion amount (length) should be less than 0.2mm, and for small-pitch components, the solder paste extrusion amount (length) should be less than 0.1mm. If the patch pressure is too small, the welding ends or pins of the components will float on the surface of the solder paste. The solder paste cannot stick to the components and will easily cause positional movement during transfer and reflow soldering. In addition, because the z-axis height is too high, components falling from a high place during placement will cause the placement of the placement to shift. If the patch pressure is too high and the amount of solder paste is extruded, it is easy to cause the solder paste to stick, and it is easy to cause bridging during reflow soldering, and in severe cases, the components may be damaged.

 

 

 

(6) Reflow soldering temperature curve

 

 

 

① Peak temperature and reflow time. The temperature curve is the key to ensuring welding quality. The temperature rise slope and peak temperature of the real-time temperature curve and the solder paste temperature curve should be basically the same. The heating rate before 160℃ is controlled at 1~2℃/s. If the temperature rise slope is too large, on the one hand, the components and PCB will heat up too quickly, easily damaging the components and deforming the PCB; on the other hand, the solvent in the solder paste will evaporate too quickly, easily splattering metal components, and producing solder beads. . For leaded solder Sn37Pb, the peak temperature is generally set to be 20 to 40°C higher than the melting point of the solder alloy in the solder paste, and the reflow time is 30 to 60 seconds; while for lead-free alloys (such as SAC305), it is generally set to be higher than the melting point of the solder paste. The liquefaction line height of the medium solder alloy is 15~35℃, and the reflow time is 40~70s. Low peak temperature or short reflow time will result in insufficient soldering, and in severe cases, the solder paste will not melt. If the peak temperature is too high or the reflow time is too long, it will easily cause the metal powder to oxidize, affect the welding quality, and even damage the components and PCB.

 

 

 

②Set the reflow soldering temperature curve. Set according to the temperature profile of the solder paste used. Solder pastes with different alloy contents and flux compositions have different temperature curves, and should be carried out in accordance with the temperature curve provided by the solder paste manufacturer.

 

 

 

(7) Quality of reflow soldering equipment

 

 

 

The quality of reflow soldering equipment is closely related to the equipment. The main parameters affecting the quality of reflow soldering are:

 

 

 

① The temperature control accuracy should reach ±0.1~±0.2℃ (the sensitivity of the temperature sensor must meet the requirements).

 

 

 

② The transverse temperature difference of the transmission belt is required to be less than ±2℃, otherwise it will be difficult to ensure the welding quality.

 

 

 

③ The width of the conveyor belt must meet the maximum PCB size requirements.

 

 

④ The longer the heating zone length and the greater the number of heating zones, the easier it is to adjust and control the temperature curve. Generally, temperature zones 4 and 5 are selected for medium and small batch production. A heating zone length of about 1.8m can also meet the requirements. In addition, the upper and lower heaters should have independent temperature control to facilitate adjustment and control of the temperature curve;

 

 

 

⑤ The maximum heating temperature is generally 300~350℃. If lead-free solder or metal substrate is considered, 350℃ or above should be selected.

 

 

 

⑥ The conveyor belt must run smoothly. Vibration of the conveyor belt will cause displacement.

 

 

 

From the above analysis, it can be seen that the quality of reflow soldering is closely related to PCB pad design, component solderability, solder paste quality, PCB processing quality, production line equipment, process parameters of each SMT process, and even the operation of the operator. close relationship. At the same time, it can also be seen that PCB design, PCB processing quality, component and solder paste quality, etc. are the basis for ensuring the quality of reflow soldering, because these problems are difficult or even impossible to solve in the production process.

 

 

 

Therefore, as long as the PCB design is correct, the PCB manufacturing, components and solder paste are all qualified, and the reflow soldering quality can be controlled through the process parameters of each process such as printing, mounting, and reflow soldering.