Tips & Tricks: Avoiding Solder Escape/Wick during Reflow

What is Solder Escape?

Solder escape, also known as solder wicking, is a common issue that occurs during the reflow soldering process in PCB Assembly. It happens when molten solder flows or “escapes” from its intended location on the PCB pad or component lead, often traveling along the component lead or up the side of the component body. This can result in poor solder joints, shorts between adjacent pins, and even damage to components.

Solder escape is particularly problematic with surface mount components that have closely spaced leads, such as QFP and QFN packages. The small size and tight pitch of these components make them more susceptible to solder bridging and short circuits caused by solder escape.

Causes of Solder Escape

There are several factors that can contribute to solder escape during reflow:

Excess Solder Paste

Applying too much solder paste to the PCB pads is a common cause of solder escape. When the solder paste melts during reflow, the excess solder can flow along the component leads or form bridges between adjacent pads.

Incorrect Solder Paste Viscosity

Using solder paste with the wrong viscosity can also lead to solder escape. If the paste is too thin, it may flow too easily and escape from the intended location. On the other hand, if the paste is too thick, it may not flow properly and result in poor solder joints.

Improper Stencil Design

The solder paste stencil plays a critical role in controlling the amount and placement of solder paste on the PCB. If the stencil apertures are too large or not properly aligned with the pads, it can result in excess solder paste and increased risk of solder escape.

Inadequate Preheat

Proper preheating of the PCB and components is essential for preventing solder escape. If the PCB is not heated evenly or to the correct temperature, the solder paste may not reflow properly, leading to solder escape and other defects.

Component Placement Issues

Misaligned or poorly seated components can also contribute to solder escape. If a component is not properly centered on the pads or is tilted, it can cause the solder to flow unevenly and escape from the intended location.

Tips for Preventing Solder Escape

Now that we understand the causes of solder escape, let’s look at some tips and best practices for preventing this issue during PCB assembly.

Tip 1: Optimize Solder Paste Stencil Design

A well-designed solder paste stencil is critical for preventing solder escape. Here are some guidelines to follow:

• Ensure stencil apertures are properly sized and aligned with the PCB pads
• Use appropriate stencil thickness for the component pitch and pad size
• Consider using stepped stencils or reduced apertures for fine-pitch components
• Implement proper stencil cleaning and maintenance procedures

Tip 2: Control Solder Paste Volume

Applying the correct amount of solder paste is key to avoiding solder escape. Too much paste can lead to bridging and shorts, while too little paste can result in poor solder joints. Some strategies for controlling paste volume include:

• Use a solder paste dispenser with precise volume control
• Implement a closed-loop print process with paste inspection and feedback
• Monitor and adjust print parameters such as squeegee pressure and speed
• Regularly calibrate and maintain paste dispensing equipment

Tip 3: Select Appropriate Solder Paste

Using the right solder paste for your application can help minimize solder escape. Consider the following factors when selecting a paste:

• Choose a paste with the appropriate viscosity and flow characteristics for your components and PCB design
• Use a paste with good slump resistance to prevent bridging and shorts
• Consider using a paste with a higher melting point for improved process control
• Ensure proper storage and handling of solder paste to maintain consistency

Tip 4: Optimize Reflow Profile

A well-tuned reflow profile is essential for preventing solder escape and achieving good solder joint quality. Key considerations include:

• Ensure proper preheating to activate the flux and dry the paste
• Use a temperature profile that matches the solder paste specifications
• Avoid excessive peak temperatures that can cause solder balling and escape
• Implement a controlled cool-down to prevent thermal shock and warpage

Tip 5: Improve Component Placement Accuracy

Accurate component placement is critical for preventing solder escape, especially with fine-pitch components. Some strategies for improving placement accuracy include:

• Use a high-precision pick-and-place machine with vision alignment
• Implement a robust fiducial recognition system for accurate board registration
• Regularly calibrate and maintain placement equipment
• Use appropriate nozzles and placement pressure for each component type

FAQ

Q1: What is solder bridging?

A1: Solder bridging is a defect that occurs when solder inadvertently connects adjacent pins or pads on a PCB, creating an unintended electrical connection or short circuit. Solder bridging is often caused by solder escape or excess solder paste.

Q2: Can solder escape be detected by visual inspection?

A2: Yes, solder escape can often be detected by visual inspection after the reflow process. Signs of solder escape include solder balls or spikes on component leads, solder bridging between adjacent pins, and excessive solder buildup on component bodies or PCB pads.

Q3: How can solder escape be reworked?

A3: If solder escape is detected after reflow, it can often be reworked using a soldering iron or hot air tool to remove the excess solder. However, rework can be time-consuming and may damage the PCB or components if not done carefully. Prevention is always the best approach.

Q4: Can solder escape be caused by the PCB design?

A4: Yes, PCB Design Factors such as pad size, shape, and spacing can contribute to solder escape. Pads that are too large or too close together may allow solder to flow between them, while pads with sharp corners or insufficient solder mask can promote solder wicking up the component lead.

Q5: What role does flux play in preventing solder escape?

A5: Flux helps to remove oxides and improve the wetting and flow characteristics of the solder. Using a solder paste with an appropriate flux chemistry can help to control the flow of solder and prevent escape. However, too much flux can also contribute to solder balling and bridging, so it’s important to strike a balance.

By understanding the causes of solder escape and implementing these tips and best practices, you can significantly reduce the occurrence of this defect in your PCB assembly process. Regular process monitoring, equipment maintenance, and operator training are also essential for achieving consistent, high-quality results.