Impact on Soldering Defects and Reliability
The placement of components on a PCB directly affects soldering quality and product reliability. Improper placement or component layout, such as positioning components in areas with significant flexural stress or high mechanical stress, can lead to solder joint cracking or failure under stress.
Components with high thermal capacity, if placed unevenly or on large PCBs prone to warping, may experience uneven heating during soldering, compromising soldering quality and long-term product stability.
High Thermal Capacity Components and Their Stability
For components with high thermal capacity, LCSC provides inductors and capacitors with efficient heat dissipation, ensuring the thermal stability of the PCB during operation.
Let’s take a look at a Samsung surface mount ceramic capacitor CL10C200JB8NNNC C1648 available at LCSC. It has a capacitance of 20pF, with a tolerance of ±5%, a rated voltage of 50V, and a C0G (also known as NP0) material. This material provides excellent temperature stability, maintaining almost constant capacitance values within a temperature range of -55°C to +125°C. C0G capacitors are ideal for applications that require high precision and stability. C0G is one of the most stable types of ceramic capacitors. Under identical operating conditions, the real tolerance of C0G capacitors is around 7%, while X7R capacitors exhibit a real tolerance of 33%.
Capacitor Material and Performance Comparison
The performance of ceramic capacitors is closely related to their dielectric material. The type of dielectric material used in ceramic capacitors determines the stability of the capacitance and their suitability for different environments.
- Class I ceramic capacitors (such as NPO, C0G) exhibit the most stable capacitance values.
- Class II ceramic capacitors (such as X7R, X5R, Y5V, Z5U, etc.) experience larger fluctuations with temperature changes.
For precision applications, Class I capacitors deliver more reliable performance.
Challenges in Reworkability
Connector Spacing Issues
When connector spacing is too tight, the connectors, which are typically tall components, end up positioned closely together after assembly. During rework, this tight arrangement makes it difficult to use disassembly tools. For instance, when using tweezers or a desoldering tool to remove a connector, the limited space can make it hard to operate precisely, often causing the adjacent connectors to be bumped, which may result in pin deformation or damage to the solder joints. Additionally, when reinstalling a new connector, ensuring accurate placement becomes challenging due to the obstruction of adjacent connectors, making proper installation difficult.
Improper component layout of large and small components
For example, placing large components (such as power inductors) above smaller components (like small-sized oscillators) can create difficulties during rework. To access the oscillator below, the large power inductor must first be removed. However, power inductors are typically heavy with thick leads, and during removal, external forces may be transferred to the oscillator, potentially causing it to be squeezed or bumped, resulting in damage. Moreover, the process of removing the large power inductor itself is complex, and the heat from soldering tools can affect nearby components, potentially damaging other unrelated parts.
Component Layout issues in high-density component areas
In PCB design, placing a large number of small surface-mount components (such as resistors or capacitors in 0402 or 0201 packages) in a confined area can cause challenges during rework. When one component fails and requires repair, the extremely small spacing in component layout between these components makes it difficult for soldering tools (like hot air guns or soldering irons) to target only the faulty component accurately. During rework, the adjacent components’ solder joints may melt due to heat, causing component displacement or pin short-circuits. Additionally, after performing rework in such a high-density area, it becomes difficult to ensure the quality that the solder joints of other un-reworked components remain unaffected.
The Risk of Short Circuit
Bridging Caused by Small Component Spacing
During the SMT process, when the spacing between different components is less than 0.5mm, bridging is highly likely to occur. Due to the tight spacing, even slight design flaws in the stencil or minor issues during the printing process can cause solder bridges. This could lead to short circuits between different pins or components. This can severely impact the functionality of the product.
Improper Component Layout of Large Components
If we place two large components, with significant thickness, closely together, it can affect the pick-and-place operation. During the placement of the second component, the pick-and-place machine may collide with the first component, triggering a safety shutdown. This not only interrupts the production process, reducing efficiency, but may also damage both the machine and the components, increasing production costs. For double-sided PCBs with large components on both sides, the placement process could cause components on the opposite side to fall off. Therefore, it is recommended to place large components on the same side whenever possible, and the lighter components should be placed first during the assembly process to prevent detaching issues.
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