Common quality defects in reflow soldering and their solutions

The quality of reflow soldering is influenced by various factors, among which the most important factors are the temperature curve of the reflow soldering furnace and the composition and quantity of solder paste. Nowadays, high-performance reflow soldering furnaces can easily and accurately control and adjust temperature curves. In the trend of high density and miniaturization, the printing of solder paste has become a key factor in the quality of reflow soldering. The three factors of solder paste, template, and printing can all affect the quality of solder paste printing.

Monument phenomenon

In reflow soldering, chip components often exhibit the phenomenon of standing up, known as standing up, also known as suspension bridge or Manhattan phenomenon. This is a common defect in reflow soldering technology.

Reason for occurrence: The fundamental reason for the phenomenon of standing a monument is the imbalance of wetting forces on both sides of the component, resulting in an unbalanced torque at both ends of the component, leading to the occurrence of the phenomenon of standing a monument.

The following situations can cause an imbalance in wetting force on both sides of the component during reflow soldering.

1. Unreasonable pad design and layout.

If the pad design and layout have the following defects, it will cause an imbalance in wetting force on both sides of the component.

One of the solder pads on both sides of the component is connected to the ground wire, or one side of the solder pad has an area that is too large, resulting in uneven heat capacity at both ends of the solder pad

The temperature difference on the surface of the PCB is too large, resulting in uneven heat absorption on both sides of the component solder pads;

The temperature unevenness occurs at both ends of the solder pads of small chip components around large devices such as QFP, BGA, and heat sinks.

Solution: Improve pad design and layout

2. Solder paste and solder paste printing.

The low activity of solder paste or poor solderability of components can lead to uneven wetting force of solder pads due to different surface tension after melting of solder paste. The printing amount of solder paste on the two pads is uneven, and the excess side will absorb more heat due to the solder paste, resulting in delayed melting time and imbalanced wetting force.

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

3. Patch.

Uneven force distribution in the Z-axis direction can result in uneven depth of immersion of components into solder paste, and uneven wetting force on both sides during melting due to time difference. Deviating components from the solder pads can directly lead to a monument being erected.

Solution: Adjust the process parameters of the SMT machine.

4. Furnace temperature curve.

The working curve of PCB heating is incorrect, resulting in a large temperature difference on the board surface. Usually, these defects occur when the reflow soldering furnace body is too short and the temperature range is too few.

Solution: Adjust the appropriate temperature curve for each product.

5. Oxygen concentration in N2 reflow soldering.

The use of N2 protected reflow soldering can increase the wetting force of the solder, but more and more reports indicate that the phenomenon of standing a monument increases when the oxygen content is too low; It is generally believed that controlling the oxygen content around (100-500) × 10-6mg/m3 is the most suitable.

Tin bead

Tin beads are one of the common defects in reflow soldering, and their causes are multifaceted, not only affecting the appearance but also causing bridging.

Tin beads can be divided into two types. One type appears on one side of the chip component, often in the form of an independent large spherical shape; Another type appears around the IC pins in the form of scattered small beads.

1. The temperature curve is incorrect.

The reflow soldering curve can be divided into four sections, namely preheating, insulation, reflow, and cooling. The purpose of preheating and insulation is to raise the surface temperature of the PCB to 150 ℃ within 60-90 seconds, and to keep it warm for about 90 seconds. This not only reduces the thermal shock of the PCB and components, but also ensures that the solvent of the solder paste can partially evaporate, avoiding splashing caused by too much solvent during reflow soldering, resulting in the solder paste rushing out of the solder pad and forming solder beads.

Solution: Pay attention to the heating rate and adopt a moderate preheating to provide a good platform for most of the solvent to evaporate.

2. The quality of solder paste.

The metal content in solder paste is usually (90 ± 0.5)%. If the metal content is too low, it can lead to excessive flux composition. Therefore, excessive flux can cause flying beads due to its low volatility during the preheating stage.

An increase in water vapor and oxygen content in solder paste can also cause flying beads. Due to the fact that solder paste is usually refrigerated, there is no guarantee of recovery time when taken out of the refrigerator, which can lead to the entry of water vapor; In addition, the lid of the solder paste bottle should be tightly closed after each use. Failure to do so in a timely manner can also lead to the entry of water vapor.

The solder paste printed on the template should be treated separately after completion. If it is placed back in the original bottle, it will cause the solder paste in the bottle to deteriorate and produce solder beads.

Solution: Choose high-quality solder paste and pay attention to the storage and usage requirements of solder paste.

3. Printing and SMT

In the printing process of solder paste, there may be a deviation between the template and the solder pad. If the deviation is too large, it can cause the solder to flow out of the solder pad, making it easy to produce solder beads after heating. In addition, poor printing working environment can also lead to the generation of tin beads. The ideal printing environment temperature is (25 ± 3) ℃ and relative humidity is 50%~65%.

Solution: Carefully adjust the clamping of the template to prevent loosening; Improve the printing work environment.

The pressure on the Z-axis during the SMT process is also an important cause of solder beads, which is often not easily noticed. Some SMT machine Z-axis heads are positioned based on the thickness of the components. If the Z-axis height is not adjusted properly, it can cause the solder paste to be squeezed out of the solder pad at the moment the components are attached to the PCB. This part of the solder paste will form solder beads during welding. The size of the tin beads produced in this situation is slightly larger.

Solution: Readjust the Z-axis height of the SMT machine.

The thickness and opening size of the template. The thickness and opening size of the template are too large, which can lead to an increase in the amount of solder paste used and also cause solder paste to flow out of the solder pad, especially for templates manufactured by chemical corrosion methods.

Solution: Choose a template with appropriate thickness and design the opening size. Generally, the opening area of the template is 90% of the solder pad size, which will improve the situation of the solder beads.

4. Core suction phenomenon

Core suction phenomenon, also known as core pulling phenomenon, is one of the common welding defects, commonly seen in gas phase reflow soldering. Core suction phenomenon refers to the phenomenon of solder detaching from the solder pad and rising along the pins between the pins and the chip body, usually resulting in serious virtual soldering.

Cause of occurrence:

The main reason is that the thermal conductivity of component pins is high, so the temperature rises rapidly, resulting in solder preferentially wetting the pins. The wetting force between solder and pins is much greater than that between solder and pads. In addition, the upward warping of pins can exacerbate the occurrence of core suction phenomenon.

Solution:

For gas-phase reflow soldering, the SMA should be preheated thoroughly before being placed in the gas-phase furnace;

The solderability of PCB pads should be carefully checked, and PCBs with poor solderability should not be used in production;

Fully value the coplanarity of components, and devices with poor coplanarity should not be used in production.

In infrared reflow soldering, the organic flux in the PCB substrate and solder is a good absorbing medium for infrared radiation, while the pins can partially reflect infrared radiation. Therefore, compared to other materials, solder is preferentially melted, and the wetting force between solder and pads is greater than that between solder and pins. Therefore, solder does not rise along the pins, and the probability of core suction phenomenon is much smaller.

5. Bridging

Bridging is one of the common defects in SMT production, which can cause short circuits between components. When encountering bridging, it must be repaired.

There are many reasons that can cause bridging, and there are four common ones:

1. Quality issues with solder paste.

The metal content in solder paste is relatively high, especially after a long printing time, which can easily lead to an increase in metal content and lead to IC pin bridging;

The solder paste has low viscosity and flows out of the solder pad after preheating;

The solder paste has poor slump and flows out of the solder pad after preheating.

Solution: Adjust the solder paste ratio or use high-quality solder paste instead.

2. Printing system.

The printing machine has poor repeatability accuracy and uneven alignment (poor alignment of steel plates and PCB), resulting in solder paste printing outside the solder pads, which is more common in the production of fine pitch QFPs;

The incorrect design of the size and thickness of the steel plate window, as well as the uneven Sn-Pb alloy coating in the PCB pad design, result in an excessive amount of solder paste.

Solution: Adjust the printing machine and improve the PCB pad coating layer.

3. Paste and place.

Excessive pressure during pasting and solder paste overflowing under pressure are common reasons in production. In addition, insufficient SMT accuracy, component displacement, and IC pin deformation can also easily lead to bridging.

4. Preheat.

The heating rate of the reflow soldering furnace is too fast, and the solvent in the solder paste cannot evaporate in time.

Solution: Adjust the Z-axis height of the SMT machine and the heating speed of the reflow soldering furnace.

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