**Why is the PCB Board Deformed and How to Prevent It**
1. **The Hazards of PCB Board Deformation**
During reflow soldering, most PCB boards are prone to bending and warping. In more severe cases, this can lead to issues such as component misplacement, cold solder joints, and tombstone effects. So, how can this problem be addressed?
1. **The Risks of PCB Deformation**
In an automated surface-mount assembly line, if the PCB is not flat, it can result in misalignment, causing components to be improperly inserted or mounted on the board’s pads and holes. This can even damage the automatic insertion machines. After soldering, if the board bends, it becomes difficult to trim component leads neatly. Furthermore, the warped board may not fit properly into the chassis or socket of the machine, leading to additional challenges for the assembly plant. With the ongoing development of surface-mount technology toward higher precision, speed, and automation, the requirement for flatness in PCBs has become even more critical, especially considering they host various components.
According to IPC standards, the maximum allowable deformation for PCBs with surface-mount devices is 0.75%, and for PCBs without surface-mount devices, it is 1.5%. In practice, to meet the demands of high-precision, high-speed assembly, some manufacturers set even stricter deformation limits. For example, several of our customers specify a maximum deformation of 0.5%, and in some cases, the requirement is as low as 0.3%.
**1. PCB Board Composition and Deformation**
A PCB board consists of materials such as copper foil, resin, glass cloth, and others, each with different physical and chemical properties. When these materials are pressed together, thermal stress inevitably develops, leading to deformation. Moreover, during PCB processing, the board undergoes various stages, including high temperatures, mechanical cutting, and wet treatments, all of which significantly impact the board’s deformation. In conclusion, the causes of PCB deformation are complex and multifaceted, with distortion or warping during processing becoming one of the most challenging issues faced by PCB manufacturers.
**2. Causes of PCB Board Deformation**
The deformation of a PCB can be analyzed from several perspectives, including the materials, structure, pattern distribution, and processing methods. This article discusses various causes and potential improvement methods.
Uneven copper surface areas on the PCB can exacerbate bending and warping.
Typically, large copper areas are used for grounding purposes on the circuit board, and sometimes also for the Vcc layer. When these large copper areas are not evenly distributed across the board, it leads to uneven heat absorption and dissipation. This results in thermal expansion and contraction. If the expansion and contraction do not occur uniformly, it creates varying stresses, leading to deformation. If the temperature reaches the board’s upper Tg limit, the board begins to soften, causing permanent deformation.
The connection points (vias) between different layers of the circuit board also restrict the board’s expansion and contraction.
Modern PCBs are mostly multi-layer boards, and there are rivet-like connection points (vias) between the layers. These points, including through holes, blind holes, and buried holes, create constraints that limit the expansion and contraction of the board, indirectly contributing to bending and warping.
**3. Additional Causes of PCB Deformation:**
**(1) The weight of the circuit board itself can cause denting and deformation.**
In reflow soldering, a chain typically drives the PCB forward in the oven, with the two sides of the board serving as fulcrums to support it. If there are heavy components on the board or if the board is too large, the added weight may cause the center to sag, resulting in bending.
**(2) The depth of the V-Cut and connecting strips can influence deformation.**
The V-Cut process is a major contributor to structural weakness in the board. By cutting grooves into the original large sheet, V-Cuts can lead to significant deformation.
**4. Influence of Materials, Structures, and Patterns on Deformation**
The PCB board is formed by pressing together a core board, prepreg, and outer copper foil. When the core and copper foil are pressed, both materials heat up and deform. The extent of this deformation is determined by the coefficient of thermal expansion (CTE) of each material.
– The CTE of copper foil is approximately 17 × 10⁻⁶.
– The Z-direction CTE of a typical FR-4 substrate at the Tg point ranges from (50–70) × 10⁻⁶.
– Above the Tg point, the CTE increases to (250–350) × 10⁻⁶, while the X-direction CTE typically mirrors that of copper foil, due to the presence of glass cloth.
**Tg Point Considerations:**
At a specific temperature, known as the glass transition temperature (Tg), the substrate material of a PCB changes from a “glass state” to a “rubber state,” losing its rigidity. This temperature, which varies by material, represents the highest temperature at which the substrate remains rigid. At this point, typical PCB substrates not only soften and deform but also experience a sharp decline in both mechanical and electrical properties.
– Standard Tg for a PCB substrate is above 130°C, while high Tg is usually above 170°C, and medium Tg is around 150°C.
– PCBs with Tg ≥ 170°C are classified as high-Tg boards.
Higher Tg values enhance heat resistance, moisture resistance, chemical resistance, and overall stability. This is particularly important in lead-free processes, where high-Tg PCBs are commonly used.
**5. High-Tg Circuit Boards:**
High-Tg materials are essential for the evolving electronics industry, especially for high-functionality and multi-layered designs. Advanced mounting technologies like SMT and CMT, which involve smaller apertures, finer wiring, and thinner boards, require substrates with high heat resistance to support these advancements.
The primary difference between standard FR-4 and high-Tg FR-4 lies in mechanical strength, dimensional stability, adhesion, water absorption, and thermal decomposition at high temperatures, particularly after moisture absorption. High-Tg materials offer superior performance in these areas.
When the core board and inner-layer patterns differ in material characteristics, thickness, or pattern distribution, deformation can occur. If the core board’s pattern distribution is uneven or the material thickness is not uniform, it can lead to deformation. Similarly, asymmetry in the laminate structure or pattern distribution causes significant differences in the CTE of various layers, leading to deformation during lamination. This can be explained using the following principles.
Consider two core boards with a substantial CTE difference. For instance, if core board A has a CTE of 1.5 × 10⁻⁵/°C and the core’s length is 1000mm, the prepreg bonds the two cores during the lamination process through softening, flowing, filling, and curing.
As shown in the analysis, the uniformity of the laminate structure and material type directly impacts the CTE difference between the core boards and copper foil. This difference in thermal expansion and contraction during the lamination process is retained in the prepreg sheet, eventually resulting in PCB deformation.
1. **The Hazards of PCB Board Deformation**
During reflow soldering, most PCB boards are prone to bending and warping. In more severe cases, this can lead to issues such as component misplacement, cold solder joints, and tombstone effects. So, how can this problem be addressed?
1. **The Risks of PCB Deformation**
In an automated surface-mount assembly line, if the PCB is not flat, it can result in misalignment, causing components to be improperly inserted or mounted on the board’s pads and holes. This can even damage the automatic insertion machines. After soldering, if the board bends, it becomes difficult to trim component leads neatly. Furthermore, the warped board may not fit properly into the chassis or socket of the machine, leading to additional challenges for the assembly plant. With the ongoing development of surface-mount technology toward higher precision, speed, and automation, the requirement for flatness in PCBs has become even more critical, especially considering they host various components.
According to IPC standards, the maximum allowable deformation for PCBs with surface-mount devices is 0.75%, and for PCBs without surface-mount devices, it is 1.5%. In practice, to meet the demands of high-precision, high-speed assembly, some manufacturers set even stricter deformation limits. For example, several of our customers specify a maximum deformation of 0.5%, and in some cases, the requirement is as low as 0.3%.
**1. PCB Board Composition and Deformation**
A PCB board consists of materials such as copper foil, resin, glass cloth, and others, each with different physical and chemical properties. When these materials are pressed together, thermal stress inevitably develops, leading to deformation. Moreover, during PCB processing, the board undergoes various stages, including high temperatures, mechanical cutting, and wet treatments, all of which significantly impact the board’s deformation. In conclusion, the causes of PCB deformation are complex and multifaceted, with distortion or warping during processing becoming one of the most challenging issues faced by PCB manufacturers.
**2. Causes of PCB Board Deformation**
The deformation of a PCB can be analyzed from several perspectives, including the materials, structure, pattern distribution, and processing methods. This article discusses various causes and potential improvement methods.
Uneven copper surface areas on the PCB can exacerbate bending and warping.
Typically, large copper areas are used for grounding purposes on the circuit board, and sometimes also for the Vcc layer. When these large copper areas are not evenly distributed across the board, it leads to uneven heat absorption and dissipation. This results in thermal expansion and contraction. If the expansion and contraction do not occur uniformly, it creates varying stresses, leading to deformation. If the temperature reaches the board’s upper Tg limit, the board begins to soften, causing permanent deformation.
The connection points (vias) between different layers of the circuit board also restrict the board’s expansion and contraction.
Modern PCBs are mostly multi-layer boards, and there are rivet-like connection points (vias) between the layers. These points, including through holes, blind holes, and buried holes, create constraints that limit the expansion and contraction of the board, indirectly contributing to bending and warping.
**3. Additional Causes of PCB Deformation:**
**(1) The weight of the circuit board itself can cause denting and deformation.**
In reflow soldering, a chain typically drives the PCB forward in the oven, with the two sides of the board serving as fulcrums to support it. If there are heavy components on the board or if the board is too large, the added weight may cause the center to sag, resulting in bending.
**(2) The depth of the V-Cut and connecting strips can influence deformation.**
The V-Cut process is a major contributor to structural weakness in the board. By cutting grooves into the original large sheet, V-Cuts can lead to significant deformation.
**4. Influence of Materials, Structures, and Patterns on Deformation**
The PCB board is formed by pressing together a core board, prepreg, and outer copper foil. When the core and copper foil are pressed, both materials heat up and deform. The extent of this deformation is determined by the coefficient of thermal expansion (CTE) of each material.
– The CTE of copper foil is approximately 17 × 10⁻⁶.
– The Z-direction CTE of a typical FR-4 substrate at the Tg point ranges from (50–70) × 10⁻⁶.
– Above the Tg point, the CTE increases to (250–350) × 10⁻⁶, while the X-direction CTE typically mirrors that of copper foil, due to the presence of glass cloth.
**Tg Point Considerations:**
At a specific temperature, known as the glass transition temperature (Tg), the substrate material of a PCB changes from a “glass state” to a “rubber state,” losing its rigidity. This temperature, which varies by material, represents the highest temperature at which the substrate remains rigid. At this point, typical PCB substrates not only soften and deform but also experience a sharp decline in both mechanical and electrical properties.
– Standard Tg for a PCB substrate is above 130°C, while high Tg is usually above 170°C, and medium Tg is around 150°C.
– PCBs with Tg ≥ 170°C are classified as high-Tg boards.
Higher Tg values enhance heat resistance, moisture resistance, chemical resistance, and overall stability. This is particularly important in lead-free processes, where high-Tg PCBs are commonly used.
**5. High-Tg Circuit Boards:**
High-Tg materials are essential for the evolving electronics industry, especially for high-functionality and multi-layered designs. Advanced mounting technologies like SMT and CMT, which involve smaller apertures, finer wiring, and thinner boards, require substrates with high heat resistance to support these advancements.
The primary difference between standard FR-4 and high-Tg FR-4 lies in mechanical strength, dimensional stability, adhesion, water absorption, and thermal decomposition at high temperatures, particularly after moisture absorption. High-Tg materials offer superior performance in these areas.
When the core board and inner-layer patterns differ in material characteristics, thickness, or pattern distribution, deformation can occur. If the core board’s pattern distribution is uneven or the material thickness is not uniform, it can lead to deformation. Similarly, asymmetry in the laminate structure or pattern distribution causes significant differences in the CTE of various layers, leading to deformation during lamination. This can be explained using the following principles.
Consider two core boards with a substantial CTE difference. For instance, if core board A has a CTE of 1.5 × 10⁻⁵/°C and the core’s length is 1000mm, the prepreg bonds the two cores during the lamination process through softening, flowing, filling, and curing.
As shown in the analysis, the uniformity of the laminate structure and material type directly impacts the CTE difference between the core boards and copper foil. This difference in thermal expansion and contraction during the lamination process is retained in the prepreg sheet, eventually resulting in PCB deformation.
