How to solve the problem of solder balls caused by mismatched pads during production

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How to solve the problem of solder balls caused by mismatched pads during production [Copy link]

Author | Wang Huigang Wang Huidong

Preface:
There are many problems in the production process of PCBA, among which short circuit failure caused by solder beads is always hard to prevent. There are many ways to solve this problem, whether it is to improve production, improve process, or optimize the design source. For many years, people have expressed their opinions and discussed endlessly. This issue of DFM case analysis may give a clearer answer...

Text:
With the continuous improvement of the integration of electronic products, the requirements for PCBA process are getting higher and higher. For example, the widespread application of 01005 size components in smart wearable products and mobile communication products, and the application of dense pitch QFN and CSP packages have increased the complexity of SMT process. In order to meet the reliability requirements of the product, the formation of good solder joints depends on reasonable pad design, appropriate solder paste amount, appropriate furnace temperature zone line, etc. Among them, the design process of the steel mesh is the core part of improving the yield of the SMT process, which is also a huge challenge for engineers who have been in front-line production for many years. Only when everyone has accumulated rich production experience and solid technical skills can they quickly deduce the cause and mechanism of defects when handling abnormalities on the production line, and solve the problem quickly and effectively.

In this issue, we will share with you the common problems of solder bead defects in several packages in the SMT process, and how to solve them through the optimization of steel mesh openings. At the same time, when optimizing the steel mesh openings, we must also avoid the problem of less tin in the solder joints due to the reduction of the opening area.

The phenomenon of solder beading is usually described as follows:

solder beading refers to irregular solder balls formed on non-solder points on the PCB surface due to solder paste splashing or residue, overflowing from the pad, etc. during the soldering process. The probability of solder beading around the following components is relatively common in the SMT process, and the usual distribution locations are as follows:

• Around the pads of resistors and capacitors
• Module components, shielding case and surrounding
• Inductors, magnetic beads, crystal oscillators, LED lights, MOS tubes, fuses
• Solder beads appear on the sides of bottom flat package components such as filters and LGA packages

The following figure shows a defective example of solder beads on the bottom of the resistor and capacitor and around the module pins.

Figure 1: RC package Figure 2: Module package Figure 3: Crystal oscillator package

There are many reasons why solder balls are formed:

• From a design perspective, the PCB pad design is unreasonable, and the large grounding pad of the special packaged device extends too far beyond the device pin.
• The material package does not match the pad size, and the component body is pressed on the pad, causing solder paste to overflow.
• After solder paste printing, the patch pressure is too high, and part of the solder paste is squeezed out of the pad to the bottom of the component body or the outside of the pad. During reflow soldering, part of the squeezed solder paste cannot be normally returned to the pad, forming solder beads.
• During the reflow soldering process, the heating slope of the temperature curve is too fast, and the flux solvent in the solder paste vaporizes violently to produce explosions, resulting in tin powder splashing and forming tin beads around the pad.
• During the solder paste printing process, since the bottom of the steel mesh is not cleaned, there is residual tin powder around the PCB pad, which can also lead to the formation of tin beads during the reflow soldering process.
• If the steel mesh opening design is directly opened 1:1 according to the pad size in the gerber file without any review and optimization, the solder paste will be printed on the full opening, and the solder paste will be squeezed out of the pad after the component is mounted, forming solder beads after reflow soldering.

Treatment strategies and preventive measures:

• When the inner distance of the chip resistor and capacitor pad design is less than the "Gap" value of the IPC-SM-782 standard, the process engineer must perform special processing according to the anti-tin bead opening method when optimizing the steel mesh. If the hole is opened according to the Gerber file provided by the customer, the hole will not be cut inward, resulting in a smaller inner distance of the hole (as shown in Figure 1 below).

The inner distance of the opening is less than 0.3mm. The inner distance of the opening of the 0402 package is optimized to be between 0.35 and 0.5mm.

• There are several ways to prevent solder beads from opening holes:

The inner distance of 0603 components is kept at 0.8mm. Anti-tin bead treatment. Inner cutting and outer expansion method is used to prevent tin bead and reduce tin

Open trapezoidal anti-solder bead method Open "U" shaped anti-solder bead method

• The anti-tin bead opening method for module packaging is mainly optimized based on the size and shape of the material pins:

Pad size 1.8*1.27mm Optimized steel mesh opening size 2.1mm*1.26

• For the type of crystal oscillator with a bottom pad package size, the anti-solder ball opening is also different.

The inner distance size is: 1.3mm optimized inner distance 2mm rounded corners, 2mm width opening

• How to open the pads for the long strip pins of QFP and PLCC packages

Width incision pin width 1:0.9 opening, inner distance greater than 0.2mm

• Generally speaking, the shrinkage method and the hole opening and bridging method are used to prevent the occurrence of tin bridging after solder paste printing.
• When encountering products with high complexity of mixed components, a local step thickness opening method is used

For example, the components on the board include both the smallest package 0201 and 01005 components, and components with large pads or positioning holes (such as headphone sockets, card sockets, connectors, etc.). In order to ensure that the pads of all components meet the good tinning effect, the stepped steel mesh opening method is used to take both requirements into account. The thickness is kept larger at the pad position of large-sized components, and
the thickness is kept smaller at the pad position of small package. Generally, the steel mesh thickness of 0201 and 01005 components is 0.08mm, and
the step thickness of the local position with large tinning amount of large pads is 0.15-0.18mm to meet the tin amount of solder joints.

• Refer to IPC-SM-782 when designing PCB pads. The inner distances of pads for components of different package sizes are as follows:

• When engineers optimize and design steel mesh openings before production, they also refer to the steel mesh opening design guide IPC-7525 and measure the area ratio and width-to-thickness ratio according to the shape and size of the component package pins to ensure the solder paste printing effect.

1) The ratio of the steel mesh opening area to the hole wall side area is generally recommended to be greater than 0.66.

2) The ratio of the steel mesh opening width to the steel mesh thickness is generally recommended to be greater than 1.5.

Summarize:

• To prevent the occurrence of tin beads, we check the package size and pad design during the DFM review, mainly considering reducing the amount of tin on the bottom of the component, thereby reducing the chance of solder paste squeezing out of the pad. The optimized opening method and size are different for different package components, and should be optimized based on the actual component specifications and specific process parameters.
• In summary, optimizing the size of steel mesh openings is a fast and efficient solution to the problem of tin beads. Of course, the causes of tin beads are varied and the solutions are also different. For example, optimizing the reflow oven temperature curve, machine mounting pressure, workshop environment, and solder paste reheating and stirring before printing are also important means to solve the problem of tin beads. Experienced engineers from E Company share a few points with you:

1) Optimize the temperature curve setting of the reflow oven. The temperature rise slope should not be too fast during the preheating stage. The temperature rise slope setting
should be less than 2℃/second. Especially when the components on the complex server motherboard are too dense, ensure that the components are preheated evenly and smoothly.

2) Control the mounting pressure of LED packaged components during machine mounting.

3) The solder paste should be strictly reheated for more than 4 hours before printing, and stirred for 3-5 minutes during use.
During the production process, process engineers can summarize the shape and
size of the steel mesh openings when dealing with and solving various problems. It is very important to analyze and optimize the point-to-point analysis according to the poor phenomenon of the solder joints, and to continuously summarize experience and optimize the amount of tinning according to actual problems. It is very important to standardize the management of the steel mesh opening design, otherwise it will directly affect the production pass rate.

This is exactly:
the adverse effects of tin beads are large, and
short circuit failures are mostly caused by it.
Various solutions are carefully considered, and
the steel mesh optimization method is the best.

led2015

When designing individually, many welding problems can be solved manually. However, when mass producing, they are complex problems that we don’t have time to control.