SMT welding quality defects - - reflow soldering quality defects and Solutions

The phenomenon of standing tablets often occurs when chip elements are rewelded.

Reasons: The basic reason for the phenomenon is that the wetting force on both sides of the element is not balanced, so the torque on both ends of the element is not balanced, which leads to the phenomenon of stele erection.

The following conditions will cause uneven wetting on both sides of the components during reflow.

The pad design and layout are not reasonable. If the pad design and layout have the following defects, the wetting force on both sides of the component will be unbalanced.

One of the two side pads of the element is connected with the ground wire or the area of one side pad is too large, so the heat capacity of both ends of the pad is not uniform.

The temperature difference across the surface of the PCB is too large to cause uneven heat absorption on both sides of the pad.

Large parts of QFP, BGA, and small chip parts around the radiator will have uneven temperature on both ends of the pad.

Solution: change the pad design and layout.

Solder paste and solder paste printing problems. Solder paste activity is not high or poor solderability of components, solder paste melting, surface tension is not the same, will cause the pad wettability imbalance. Two pads of solder paste printing quantity is uneven, more side will be due to the heat absorption of solder paste, melting time lag, resulting in imbalance in wettability.

Solution: Select solder paste with higher activity to improve solder paste printing parameters, especially the window size of the template.

The displacement of the patch along the Z axis results in uneven depth of penetration into solder paste and imbalance of wetting force on both sides due to time difference during melting. If the patch is displaced, the stele will be erected directly.

Solution: adjust the placement process parameters.

If the furnace temperature curve is incorrect, if the furnace body is too short and the temperature zone is too small, the working curve of heating PCB will be incorrect, and the difference of humidity on the board will be too large, resulting in the imbalance of wetting force.

Solution: adjust the appropriate temperature curve according to each product.

Oxygen concentration in nitrogen reflow soldering. Nitrogen shielded reflow soldering can increase the wettability of solder, but more and more examples show that the phenomenon of erecting steles increases when oxygen content is too low; it is generally considered that the oxygen content should be controlled in the negative 6th power of (100-500)*10.

Tin Pearl

Tin beads are one of the most common defects in reflow soldering, which not only affect the appearance but also cause bridging. Tin beads can be divided into two types: one appears on the side of chip components, usually in a large independent sphere; the other appears around the IC pin and is scattered beads.

The purpose of preheating and heat preservation is to make the surface temperature of PCB rise to 150 C within 60-90 s and keep it for about 90 s. This can not only reduce the thermal shock of PCB and its components, but also ensure that the solder paste can be partially volatilized and avoided. Spatter caused by too many solvents during the reflow soldering causes solder paste to rush out of the pad to form tin beads.

Solution: Pay attention to the heating rate, and take a moderate preheating, so that it has a good platform for most of the solvent volatilization.

Quality of solder paste

The metal content in solder paste is usually (90 + 0.5), too low metal content will lead to excessive flux composition, so too much flux will be difficult to volatilize during the preheating stage and lead to take-off beads.

Increased vapor and oxygen content in solder paste can also cause flying beads. Since solder paste is usually refrigerated, if the recovery time is not ensured, water vapor will enter when the solder paste is removed from the refrigerator. In addition, the lid of solder paste bottle should be tightly sealed after each use, and if not tightly sealed in time, water vapor will also enter.

The solder paste printed on the template is finished. The remaining part should be treated separately. If it is put back into the original bottle, the solder paste in the bottle will deteriorate and the solder beads will also be produced.

Solution: choose high quality solder paste, pay attention to solder paste maintenance and use requirements.

Printing and placement

In solder paste printing process, due to the offset between the template and the pad, if the offset is too large, the solder paste will be immersed outside the pad, and the solder beads will easily appear after heating.

Solution: carefully adjust the clamping of templates to prevent loosening. Improve printing environment.

The pressure of Z-axis is also an important reason for solder beads in the process of mounting, but it is often not noticed. The Z-axis head of some mounting machines is positioned according to the thickness of the components. If the Z-axis height is not adjusted properly, the solder paste will be extruded to the pad immediately after the component is mounted on the PCB. This part of solder paste will be extruded to the pad during welding. Forming tin beads. In this case, the size of tin beads is slightly larger.

Solution: re adjust the height of the Z axis of the placement machine.

Thickness and opening size of formwork. Excessive thickness and opening size of formwork will lead to the increase of solder paste dosage and lead to solder paste overflowing outside the pad, especially for touch plates manufactured by chemical etching.

Solution: Choose the appropriate thickness of the mold and the design of the opening size, the general template opening area is 90.

Sucking phenomenon

The phenomenon of wicking, also known as core pulling, is one of the common welding defects. It is often found in gas reflow soldering. Wicking causes solder to break away from the pad and go up along the lead to the pin and the chip body, usually forming a serious virtual soldering phenomenon. The wetting force between the solder and the pad is much greater than that between the solder and the pad. In addition, the upward warping of the lead will aggravate the wicking phenomenon.