**Why is the PCB Board Deformed and How to Prevent It?**
**2.2 Deformation Caused During PCB Processing**
The causes of deformation during PCB processing are multifaceted and can be broadly categorized into two types of stress: thermal stress and mechanical stress. Thermal stress primarily arises during the pressing process, while mechanical stress typically occurs during stacking, handling, and baking of the boards. Below is a brief discussion of these causes, presented in the sequence of the processing steps.
**Incoming Copper-Clad Laminate:**
Copper-clad laminates are typically double-sided, with a symmetrical structure and no graphics. Since the coefficient of thermal expansion (CTE) of the copper foil and the glass cloth are nearly identical, there is minimal deformation caused by the CTE mismatch during the pressing process. However, the press used for copper-clad laminates is quite large, and there are temperature variations across different areas of the hot plate. This can result in slight differences in the curing speed and resin curing degree across these areas during the pressing process. Additionally, the dynamic viscosity of the resin changes significantly at different heating rates, which can lead to local stress due to variations in the curing process. Generally, this type of stress reaches equilibrium after pressing but can gradually release and cause deformation during subsequent processing steps.
**Pressing:** The PCB pressing process is the primary step that generates thermal stress, with deformation arising from material and structural differences, as discussed in the previous section. Similar to the pressing of copper-clad laminates, local stresses caused by variations in the curing process also occur. PCBs experience more thermal stress than copper-clad laminates due to their greater thickness, varied pattern distribution, and higher amounts of prepreg material. The stress in the PCB is relieved during subsequent processes like drilling, shaping, or grilling, which can result in board deformation.
**Baking Process of Solder Mask, Characters, etc.:** Solder mask inks cannot be stacked during curing, so PCBs are placed in racks for curing. The solder mask is typically cured at a temperature of around 150°C, which slightly exceeds the Tg (glass transition) point of medium and low Tg materials. Above this temperature, the resin becomes highly elastic, making the board prone to deformation under its own weight or due to the strong airflow in the oven.
**Hot-Air Solder Leveling:** The temperature of the solder bath is typically between 225°C and 265°C, with a processing time of 3 to 6 seconds for standard boards. The hot air temperature ranges from 280°C to 300°C. During solder leveling, the board is placed into the tin bath at room temperature, and post-treatment water washing is performed at room temperature within two minutes of removing the board from the furnace. This entire hot-air solder leveling process involves a sudden heating and cooling cycle. Due to the varying materials and uneven structure of the circuit board, thermal stresses inevitably develop during the heating and cooling stages, leading to microscopic strain and overall deformation or warping.
**Storage:** During the semi-finished product stage, PCBs are usually firmly inserted into racks. If the rack’s tightness is improperly adjusted or if the boards are stacked improperly during storage, mechanical deformation may occur. This is especially significant for thinner boards (less than 2.0mm), where the impact is more pronounced.
Apart from the above factors, there are numerous other variables that can contribute to PCB board deformation.
### 3. Prevention of PCB Warpage and Deformation
PCB warpage significantly impacts the production process of printed circuit boards. Warpage is a common and critical issue. When the board is bent after soldering, the component leads may become misaligned, and the board may not fit properly into the chassis or socket. This can disrupt the normal operation of subsequent processes. With the advent of surface-mount technology (SMT) and chip-on-board mounting, the industry’s tolerance for warpage continues to decrease. Therefore, understanding the causes of warping is crucial.
1. **Engineering Design:** Attention must be paid to PCB design to minimize warpage:
– A. The interlayer prepregs should be symmetrically arranged. For example, in a six-layer board, the thickness between layers 1–2 and 5–6, as well as the number of prepregs, should be the same. Otherwise, warpage is likely to occur after pressing.
– B. Both the multi-layer core and prepregs should come from the same supplier.
– C. The circuit pattern area on the A-side and B-side of the outer layer should be as similar as possible. If the A-side has a large copper surface and the B-side contains only a few traces, warping may occur after etching. To balance, independent grids may be added to the thinner side if the circuit area on the two sides is too different.
2. **Drying the Board Before Cutting:** Drying the copper-clad laminate before cutting (at 150°C for 8±2 hours) removes moisture and ensures the resin in the board fully cures, eliminating residual stress. This step helps prevent warping. Many double-sided and multi-layer boards still follow this baking step before or after cutting. However, drying times can vary, typically ranging from 4 to 10 hours. The drying time should be adjusted based on the grade of the PCB and the customer’s warpage tolerance. It is generally recommended to bake the board after cutting, but baking before cutting is also an option. Additionally, the inner layers should also undergo baking.
3. **Warp and Weft Direction of the Prepreg:** After lamination, the prepreg may shrink differently along the warp and weft directions. It is essential to distinguish these directions during blanking and lamination. Failing to do so can lead to warping of the finished board, which is difficult to correct even with post-pressing. A common cause of warpage in multi-layer boards is when prepregs are not correctly oriented during lamination. To differentiate the directions, the direction of the rolled prepreg is the warp direction, and the width direction is the weft direction. For copper foil boards, the long side is the weft direction, and the short side is the warp direction. Always verify this with the supplier.
4. **Stress Relief After Lamination:** After hot pressing and cold pressing, the multi-layer board should be cut or milled to remove burrs. It should then be placed flat in an oven at 150°C for 4 hours to gradually release internal stresses and ensure complete curing of the resin. This step is essential and should never be skipped.
5. **Straightening Thin Boards During Electroplating:** Ultra-thin multi-layer boards (0.4–0.6mm) should be processed with specialized nip rollers for surface and pattern electroplating. After the thin board is clamped onto the fly bus of the automatic electroplating line, the nip rollers should be applied to the entire fly bus to straighten all the plates. Without this step, the board may warp after plating, especially after a 20–30 micron copper layer is deposited, and it becomes difficult to rectify.
6. **Cooling the Board After Hot-Air Leveling:** The board undergoes high-temperature exposure (about 250°C) during hot-air leveling. After removal, it should be placed on a flat marble or steel plate for natural cooling before moving to the post-processing machine for cleaning. This process helps prevent warping. Some factories cool the board by immersing it immediately in cold water after hot-air leveling, then performing post-processing. This rapid hot-cold shock can cause warping, twisting, delaminating, or blistering. An air flotation bed can be used during cooling to help prevent such issues.
7. **Treatment of Warped Boards:** In well-managed factories, printed boards are inspected for flatness at the final inspection stage. Any unqualified boards are removed and placed in an oven at 150°C under heavy pressure for 3–6 hours. After cooling naturally under pressure, the boards are checked for flatness again. Some boards may need to undergo the baking and pressing process two or three times to become flat. If these anti-warping measures are not followed, some boards may be deemed unusable and will be scrapped.
### 4. PCB Board Warpage Standards
For PCB warpage standards, refer to IPC-A-600G, Section 2.11 (Flatness Standard). For printed boards with surface-mounted components (such as SMT), the distortion or bowing should not exceed 0.75%, while other types of boards should not exceed 1.5%. The testing method is outlined in IPC-TM-6502.4.22.
**2.2 Deformation Caused During PCB Processing**
The causes of deformation during PCB processing are multifaceted and can be broadly categorized into two types of stress: thermal stress and mechanical stress. Thermal stress primarily arises during the pressing process, while mechanical stress typically occurs during stacking, handling, and baking of the boards. Below is a brief discussion of these causes, presented in the sequence of the processing steps.
**Incoming Copper-Clad Laminate:**
Copper-clad laminates are typically double-sided, with a symmetrical structure and no graphics. Since the coefficient of thermal expansion (CTE) of the copper foil and the glass cloth are nearly identical, there is minimal deformation caused by the CTE mismatch during the pressing process. However, the press used for copper-clad laminates is quite large, and there are temperature variations across different areas of the hot plate. This can result in slight differences in the curing speed and resin curing degree across these areas during the pressing process. Additionally, the dynamic viscosity of the resin changes significantly at different heating rates, which can lead to local stress due to variations in the curing process. Generally, this type of stress reaches equilibrium after pressing but can gradually release and cause deformation during subsequent processing steps.
**Pressing:** The PCB pressing process is the primary step that generates thermal stress, with deformation arising from material and structural differences, as discussed in the previous section. Similar to the pressing of copper-clad laminates, local stresses caused by variations in the curing process also occur. PCBs experience more thermal stress than copper-clad laminates due to their greater thickness, varied pattern distribution, and higher amounts of prepreg material. The stress in the PCB is relieved during subsequent processes like drilling, shaping, or grilling, which can result in board deformation.
**Baking Process of Solder Mask, Characters, etc.:** Solder mask inks cannot be stacked during curing, so PCBs are placed in racks for curing. The solder mask is typically cured at a temperature of around 150°C, which slightly exceeds the Tg (glass transition) point of medium and low Tg materials. Above this temperature, the resin becomes highly elastic, making the board prone to deformation under its own weight or due to the strong airflow in the oven.
**Hot-Air Solder Leveling:** The temperature of the solder bath is typically between 225°C and 265°C, with a processing time of 3 to 6 seconds for standard boards. The hot air temperature ranges from 280°C to 300°C. During solder leveling, the board is placed into the tin bath at room temperature, and post-treatment water washing is performed at room temperature within two minutes of removing the board from the furnace. This entire hot-air solder leveling process involves a sudden heating and cooling cycle. Due to the varying materials and uneven structure of the circuit board, thermal stresses inevitably develop during the heating and cooling stages, leading to microscopic strain and overall deformation or warping.
**Storage:** During the semi-finished product stage, PCBs are usually firmly inserted into racks. If the rack’s tightness is improperly adjusted or if the boards are stacked improperly during storage, mechanical deformation may occur. This is especially significant for thinner boards (less than 2.0mm), where the impact is more pronounced.
Apart from the above factors, there are numerous other variables that can contribute to PCB board deformation.
### 3. Prevention of PCB Warpage and Deformation
PCB warpage significantly impacts the production process of printed circuit boards. Warpage is a common and critical issue. When the board is bent after soldering, the component leads may become misaligned, and the board may not fit properly into the chassis or socket. This can disrupt the normal operation of subsequent processes. With the advent of surface-mount technology (SMT) and chip-on-board mounting, the industry’s tolerance for warpage continues to decrease. Therefore, understanding the causes of warping is crucial.
1. **Engineering Design:** Attention must be paid to PCB design to minimize warpage:
– A. The interlayer prepregs should be symmetrically arranged. For example, in a six-layer board, the thickness between layers 1–2 and 5–6, as well as the number of prepregs, should be the same. Otherwise, warpage is likely to occur after pressing.
– B. Both the multi-layer core and prepregs should come from the same supplier.
– C. The circuit pattern area on the A-side and B-side of the outer layer should be as similar as possible. If the A-side has a large copper surface and the B-side contains only a few traces, warping may occur after etching. To balance, independent grids may be added to the thinner side if the circuit area on the two sides is too different.
2. **Drying the Board Before Cutting:** Drying the copper-clad laminate before cutting (at 150°C for 8±2 hours) removes moisture and ensures the resin in the board fully cures, eliminating residual stress. This step helps prevent warping. Many double-sided and multi-layer boards still follow this baking step before or after cutting. However, drying times can vary, typically ranging from 4 to 10 hours. The drying time should be adjusted based on the grade of the PCB and the customer’s warpage tolerance. It is generally recommended to bake the board after cutting, but baking before cutting is also an option. Additionally, the inner layers should also undergo baking.
3. **Warp and Weft Direction of the Prepreg:** After lamination, the prepreg may shrink differently along the warp and weft directions. It is essential to distinguish these directions during blanking and lamination. Failing to do so can lead to warping of the finished board, which is difficult to correct even with post-pressing. A common cause of warpage in multi-layer boards is when prepregs are not correctly oriented during lamination. To differentiate the directions, the direction of the rolled prepreg is the warp direction, and the width direction is the weft direction. For copper foil boards, the long side is the weft direction, and the short side is the warp direction. Always verify this with the supplier.
4. **Stress Relief After Lamination:** After hot pressing and cold pressing, the multi-layer board should be cut or milled to remove burrs. It should then be placed flat in an oven at 150°C for 4 hours to gradually release internal stresses and ensure complete curing of the resin. This step is essential and should never be skipped.
5. **Straightening Thin Boards During Electroplating:** Ultra-thin multi-layer boards (0.4–0.6mm) should be processed with specialized nip rollers for surface and pattern electroplating. After the thin board is clamped onto the fly bus of the automatic electroplating line, the nip rollers should be applied to the entire fly bus to straighten all the plates. Without this step, the board may warp after plating, especially after a 20–30 micron copper layer is deposited, and it becomes difficult to rectify.
6. **Cooling the Board After Hot-Air Leveling:** The board undergoes high-temperature exposure (about 250°C) during hot-air leveling. After removal, it should be placed on a flat marble or steel plate for natural cooling before moving to the post-processing machine for cleaning. This process helps prevent warping. Some factories cool the board by immersing it immediately in cold water after hot-air leveling, then performing post-processing. This rapid hot-cold shock can cause warping, twisting, delaminating, or blistering. An air flotation bed can be used during cooling to help prevent such issues.
7. **Treatment of Warped Boards:** In well-managed factories, printed boards are inspected for flatness at the final inspection stage. Any unqualified boards are removed and placed in an oven at 150°C under heavy pressure for 3–6 hours. After cooling naturally under pressure, the boards are checked for flatness again. Some boards may need to undergo the baking and pressing process two or three times to become flat. If these anti-warping measures are not followed, some boards may be deemed unusable and will be scrapped.
### 4. PCB Board Warpage Standards
For PCB warpage standards, refer to IPC-A-600G, Section 2.11 (Flatness Standard). For printed boards with surface-mounted components (such as SMT), the distortion or bowing should not exceed 0.75%, while other types of boards should not exceed 1.5%. The testing method is outlined in IPC-TM-6502.4.22.
