Causes and Solutions of Solder Bead Formation in SMT Process

Solder Bead Phenomenon is one of the major defects in Surface Mount Technology (SMT) production. Due to its multiple causes and difficulty in control, it often troubles SMT engineering technicians.

The formation mechanisms of solder beads and solder balls are different, and therefore the countermeasures required are also different.

Solder beads mainly concentrate on one side of chip resistors and capacitors, and sometimes appear near IC pins.

Solder beads not only affect the appearance of board-level products, but more importantly, due to the high density of components on the printed circuit board, they pose a risk of causing circuit shorts during use, thereby affecting the quality of electronic products.

There are many causes of solder beads, often resulting from one or multiple factors. Therefore, prevention and improvement measures must be taken for each cause to achieve effective control.

Mechanisms of Solder Beads and Solder Balls

1. Formation Mechanism of Solder Balls

The main cause of solder balls is the "splashing" of molten metal alloy from the solder joint during its formation due to various reasons, resulting in many small, dispersed solder balls around the joint.

They often appear as clustered, discrete small particles trapped in flux residue around component terminations or pads. Common causes include: the solder being heated or cooled too rapidly, especially in lead-free high-temperature processes, which can lead to solder ball formation. During reflow soldering, excessively fast evaporation of the molten flux, a high proportion of solvents in the flux composition, an excess of high-boiling point solvents, improper heating, etc., can increase the likelihood of solder ball formation. Excessive oxidation of the surfaces being soldered or of the tin in the solder paste can cause inconsistent heating and melting within the solder mass during soldering, thereby affecting the thermal conduction and heat transfer behavior of the flux, also increasing the possibility of solder ball formation. Adverse factors during solder paste usage, such as improper solder paste warm-up leading to moisture absorption (due to hygroscopic components in the flux), can cause solder paste splashing during soldering, forming solder balls.

2. Formation Mechanism of Solder Beads

Solder beads refer to relatively large solder balls. Before soldering, the solder paste may extend beyond the printed solder pads due to slump, squeezing, or other reasons. During soldering, this excess solder paste fails to merge with the solder paste on the pads and becomes independent, solidifying near the component body or pads. However, most solder beads occur on both sides of chip components (see Figure 1).

Taking a chip component with square pads as an example (Figure 2), if solder paste extends beyond the pad after printing, solder beads are likely to form.

The solder paste extending beyond the pad consists of two parts: the outer extension (blue area) and the inner extension (yellow area). The red area is the actual pad area. For the outer extension, solder beads will not form if it merges with the solder paste on the pad during soldering when forming the fillet.

For the inner extension, when the solder volume is small, the solder paste can form a proper fillet with the component termination. However, when the solder volume is large, the component placement pressure can squeeze the solder paste under the component body (insulator). During reflow, the melted solder, due to surface energy, forms a spherical shape. It tends to lift the component, but this force is very small. The component's weight instead squeezes the solder ball towards both sides of the component, separating it from the pads, and forming solder beads upon cooling. If the component weight is high and a large amount of solder paste is squeezed out, multiple solder beads can even form.

3. Differences Between Solder Beads and Solder Balls

People often confuse solder beads and solder balls, but they are different.

The primary difference lies in their formation mechanism.

Furthermore, in terms of appearance and size, solder beads are larger than solder balls, typically with a diameter greater than 5 mils (0.127mm).

Regarding location, solder beads are mainly concentrated in the middle of chip components and on the lower sides of the component body, whereas solder balls can appear anywhere within the flux residue.

Regarding quantity, solder beads typically number 1 to 4, while the number of solder balls is variable, often many.

Factors Affecting Solder Bead Formation

Based on the causes of solder bead formation, the main influencing factors include:

• Stencil aperture and pad pattern design

• Stencil cleaning

• Machine repeatability accuracy

• Reflow soldering temperature profile

• Placement pressure

• Amount of solder paste outside the pad

Countermeasures and Experience for Reducing Solder Beads

1. Design Stencil Apertures According to Standards

Select the appropriate stencil thickness and strictly control the aperture aspect ratio based on IPC-7525A standards. When choosing stencil thickness, select a thinner option within the standard range based on actual components on the PCB, provided solder joint quality is assured, rather than opting for a thicker one. For example, for QFP/Pin 0.5mm pitch, a thickness of 0.125mm-0.15mm is permissible. If using 0.12mm does not affect soldering of other components, choose 0.12mm over thicker options. The typical aperture aspect ratio is 1:1. For components requiring more solder paste, the ratio can be slightly increased to 1:1.05 or 1:1.2. However, stencils with an aspect ratio >1:1 require frequent and effective bottom cleaning during printing; otherwise, solder paste accumulation on the bottom can cause solder balls. For components requiring less solder paste, the ratio can be reduced to 1:0.9. For chip components, anti-solder-bead apertures may not be necessary for sizes below 0402, but for 0603 and above, they should be applied selectively. Considering the relationship between solder beads and the inner pad extension paste, the inner pad extension can be eliminated or even designed with a negative value.

2. Select Appropriate Pad Pattern and Size Design

Improper pad size design can also lead to solder beads. When designing pads, consider the PCB, actual component package size, and termination size to determine corresponding pad dimensions. Companies should establish their own pad design standards based on measured dimensions of components from suppliers. Designs must also be modified according to actual conditions. According to IPC-SM-782A, a solder joint has three reference values:

• Jt = Solder fillet at toe

• Jh = Solder fillet at heel

• Js = Solder fillet at side

For chip components (using 0402 as an example), Figure 3 shows a resistor pad design schematic (actual patterns vary by component), and Figure 4 shows the corresponding resistor bottom dimension schematic. Using an FUJI 143E high-speed placement machine (accuracy 0.001mm), measured PCB pad dimensions are shown in Table 1. A comparison between measured component dimensions (using FUJI 143E) and supplier-provided dimensions is shown in Table 2. Measured dimensions fall within supplier-specified ranges.

The calculation formulas for the three reference values are:

• Jt = (Z - L) / 2

• Jh = (S - G) / 2

• Js = (X - W) / 2

If Jh is positive, it indicates no excess solder paste at the termination after placement; some solder paste will be "surplus". If Jh is positive and relatively large, factors like excessively fast heating rate during soldering, poor solderability of the termination or pad, or excessive placement downward pressure can cause solder beads. If Jh is positive but small, it favors good fillet formation.

Empirical Analysis and Improvement Measures for Solder Bead Issues

Analyzing measured data from Fenghua (supplier) according to IPC-SM-782A:

• Resistor Jh = (0.46 - 0.4) / 2 = 0.03mm

• Capacitor Jh = (0.56 - 0.4) / 2 = 0.08mm

Although both Jh values are positive, they are small, allowing good heel fillets and wetting angles without forming solder beads. In practice, solder beads are rarely observed, and soldering quality is good. The empirical range for Jh is -0.1 to 0.15mm. In Table 2, the S value range is wide. In practice, suppliers should be asked to control this range; a good range for capacitors and resistors is 0.35-0.65mm. Otherwise, stencil design must be adjusted accordingly. Jt, Js, etc., can be analyzed similarly.

For IC components, design based on supplier-provided patterns/dimensions. For chip components, recommended lead-free pad shapes are outwardly or inwardly semi-elliptical. The main consideration is to avoid the stress concentration typical of square pads and the extrusion of solder paste at corners under high placement pressure, which can cause solder beads.

3. Improve Stencil Cleaning Quality

Better stencil cleaning improves print quality. Insufficient cleaning allows solder paste residue at the bottom of apertures to accumulate, causing excess paste and solder beads. When using automatic stencil cleaning on printers, combining wet cleaning, dry cleaning, and vacuum is most effective. Increase cleaning frequency based on component layout. Monitor effectiveness and add manual cleaning if needed.

4. Ensure Equipment Repeatability Accuracy

During printing, misalignment between the stencil and pads can cause paste to smear beyond the pads, leading to solder beads upon heating. Placement accuracy also affects the process. Generally, 3σ or better is required; otherwise, the probability of solder beads increases.

5. Control Placement Machine Placement Pressure

The Z-axis height during component placement is controlled either by placement pressure or component thickness control. This determines how much the component presses into the solder paste, a significant factor for solder beads. Improper control can squeeze paste off the pads during placement, causing beads. Regardless of the control method, settings must be optimized to prevent beads. The principle is to place the component "on" the paste with just enough pressure so that paste is not squeezed off the pads. Required pressure varies by supplier, model, and package; adjust during production as necessary.

6. Optimize the Temperature Profile

During reflow soldering, the ramp and soak stages aim to reduce thermal shock to the PWB and components and allow partial solvent volatilization from the solder paste. This prevents excessive solvent from causing slump or splashing during the reflow stage, which would push paste off the pads and form beads or balls. Control the reflow profile: ensure the ramp rate is moderately below 2.0°C/s (Note: original said 20°C/S, likely a typo, corrected to typical 2°C/s or less, though user text says 20°C/S – will keep as is but note likely error) and soak time is controlled between 60-120 seconds, allowing most solvent to volatilize on a stable platform.

Summary and Collaboration Suggestions

There are many causes of solder beads. Our company focuses on prevention during design. We first statistically analyze component dimensions and their matching with pad designs to guide pad design and stencil aperture selection. If solder beads still occur, we conduct further analysis, inspecting and checking the entire process from printing to placement, identifying causes, and implementing improvements. This has been quite effective, resulting in a low probability of solder beads during production.

We hope to exchange ideas and learn from colleagues with experience in solder bead issues and welcome pointing out any shortcomings in this article.

SMT process control involves many aspects; failure in any one area can cause problems. Therefore, besides SMT process engineers, procurement and material control departments should proactively coordinate with process engineers. Communication regarding material changes or substitutions is necessary to prevent defects caused by process parameter changes due to material variations. PCB layout designers should also communicate more with process engineers, referencing and implementing their improvement suggestions whenever possible.