1. The pre-bake process before assembling a flexible printed circuit board (PCB) is an industry-standard requirement outlined in IPC-2232 Section 5.3.5 and IPC-FA-251 Section. This applies to all flexible and rigid-flex designs made from polyimide materials. But why is pre-baking necessary before assembly, rather than during the earlier stages of PCB manufacturing?
2. Most flexible circuit designs that require component assembly are made from polyimide materials. Polyimide is commonly used for flexible cores, coatings, and in many cases, reinforcing ribs. A key inherent property of polyimide is its ability to absorb moisture. At 20°C and 50% relative humidity, polyimide will absorb approximately 2% of its weight in water. In higher temperature and humidity environments, this moisture absorption may increase. This behavior is consistent across all polyimide materials from any supplier and is not something that the flexible PCB manufacturer can influence or alter.
3. The hygroscopic nature of polyimide materials is an important factor to consider during PCB manufacturing and assembly.
Although hygroscopicity does not directly affect the mechanical or electrical properties of polyimide materials, it becomes a concern when components are exposed to high temperatures during the assembly reflow process. Specifically, in most cases, the absorbed moisture is converted into steam during the reflow process. As moisture transitions from liquid to gas, it expands, which can lead to delamination between layers. This issue has become more prominent as the temperature requirements for RoHS-compliant processes have increased. To prevent delamination during assembly, the most effective solution is to pre-bake flexible or rigid-flex parts before assembly, ensuring that they are completely moisture-free. Delamination between the FR4 stiffener and flexible circuits often occurs when moisture is not fully removed, which is the primary cause of delamination in cover layers, internal layers, and the steel layers.
### Flexible PCB Pre-Baking Process
PCB pre-baking is typically carried out at 120°C for 2-10 hours, with the specific duration depending on the part design. Factors such as the number of layers, ribs, and structural elements can extend the required pre-drying time. Additionally, the parts should be arranged in the oven to allow adequate airflow around each component. After pre-baking, it is advisable to remove the components from the oven and allow them to cool to room temperature before assembly. Any significant delay can result in the components re-absorbing moisture. For flexible PCBs that undergo multiple assembly cycles, if the interval between cycles is prolonged, a second pre-baking may be necessary.
### 2. Common PCB Manufacturing Issues: Electroplating Voids
Plated through holes (PTHs) are copper-plated holes in a printed circuit board (PCB) that provide electrical connections between the different layers of the circuit board. In multi-layer PCB designs, PTHs serve as interconnections that allow signals to pass from one layer to another. During PCB manufacturing, holes are drilled through the laminate and foil on both sides of the board. The hole walls are then electroplated to facilitate signal transmission between layers. To prepare for electroplating, the board is coated with a thin layer of electroless copper, which adheres to the hole’s interior surface and the edges of the board. This process is known as copper deposition.
After the deposition, a circuit image is applied and developed, and the exposed areas are plated with a thicker layer of copper, typically around 0.001 inches (0.025 mm). This layer covers both the hole and the circuit, building up to the final required thickness. The PCB continues through the manufacturing process from this point until completion.
### Deposition Issues
Problems during the copper deposition process can affect the integrity of the interconnection within the hole wall, potentially leading to PCB failure. One of the most common defects is the appearance of plating gaps along the copper-plated hole wall. If the hole wall is not smooth or the coating is incomplete, electrical current cannot flow through the hole properly. The image below illustrates a cross-section of a PTH where the copper on the wall is too thin, likely due to poor deposition or electroplating.
During deposition, non-uniform copper coating can occur, leading to gaps in the plating. This issue may arise from contamination, air bubbles on the hole’s surface, or rough drilling. Any of these factors can create uneven surfaces on the hole walls, making it difficult to achieve a smooth, continuous copper plating.
### Preventing Plating Voids in PCB Manufacturing
To avoid plating voids caused by rough drilling, it is essential to follow the manufacturer’s guidelines carefully. Manufacturers typically provide recommendations regarding the optimal drill bit sizes, feed rates, and speeds. Using drill bits with a low rate of penetration (ROP) can result in rough surfaces that are difficult to coat evenly during the deposition and electroplating processes. If the ROP is too low, drill bit contamination may occur, but this can be corrected during the cleaning process.
If you have any PCB manufacturing needs, please do not hesitate to contact me.Contact me
2. Most flexible circuit designs that require component assembly are made from polyimide materials. Polyimide is commonly used for flexible cores, coatings, and in many cases, reinforcing ribs. A key inherent property of polyimide is its ability to absorb moisture. At 20°C and 50% relative humidity, polyimide will absorb approximately 2% of its weight in water. In higher temperature and humidity environments, this moisture absorption may increase. This behavior is consistent across all polyimide materials from any supplier and is not something that the flexible PCB manufacturer can influence or alter.
3. The hygroscopic nature of polyimide materials is an important factor to consider during PCB manufacturing and assembly.
Although hygroscopicity does not directly affect the mechanical or electrical properties of polyimide materials, it becomes a concern when components are exposed to high temperatures during the assembly reflow process. Specifically, in most cases, the absorbed moisture is converted into steam during the reflow process. As moisture transitions from liquid to gas, it expands, which can lead to delamination between layers. This issue has become more prominent as the temperature requirements for RoHS-compliant processes have increased. To prevent delamination during assembly, the most effective solution is to pre-bake flexible or rigid-flex parts before assembly, ensuring that they are completely moisture-free. Delamination between the FR4 stiffener and flexible circuits often occurs when moisture is not fully removed, which is the primary cause of delamination in cover layers, internal layers, and the steel layers.
### Flexible PCB Pre-Baking Process
PCB pre-baking is typically carried out at 120°C for 2-10 hours, with the specific duration depending on the part design. Factors such as the number of layers, ribs, and structural elements can extend the required pre-drying time. Additionally, the parts should be arranged in the oven to allow adequate airflow around each component. After pre-baking, it is advisable to remove the components from the oven and allow them to cool to room temperature before assembly. Any significant delay can result in the components re-absorbing moisture. For flexible PCBs that undergo multiple assembly cycles, if the interval between cycles is prolonged, a second pre-baking may be necessary.
### 2. Common PCB Manufacturing Issues: Electroplating Voids
Plated through holes (PTHs) are copper-plated holes in a printed circuit board (PCB) that provide electrical connections between the different layers of the circuit board. In multi-layer PCB designs, PTHs serve as interconnections that allow signals to pass from one layer to another. During PCB manufacturing, holes are drilled through the laminate and foil on both sides of the board. The hole walls are then electroplated to facilitate signal transmission between layers. To prepare for electroplating, the board is coated with a thin layer of electroless copper, which adheres to the hole’s interior surface and the edges of the board. This process is known as copper deposition.
After the deposition, a circuit image is applied and developed, and the exposed areas are plated with a thicker layer of copper, typically around 0.001 inches (0.025 mm). This layer covers both the hole and the circuit, building up to the final required thickness. The PCB continues through the manufacturing process from this point until completion.
### Deposition Issues
Problems during the copper deposition process can affect the integrity of the interconnection within the hole wall, potentially leading to PCB failure. One of the most common defects is the appearance of plating gaps along the copper-plated hole wall. If the hole wall is not smooth or the coating is incomplete, electrical current cannot flow through the hole properly. The image below illustrates a cross-section of a PTH where the copper on the wall is too thin, likely due to poor deposition or electroplating.
During deposition, non-uniform copper coating can occur, leading to gaps in the plating. This issue may arise from contamination, air bubbles on the hole’s surface, or rough drilling. Any of these factors can create uneven surfaces on the hole walls, making it difficult to achieve a smooth, continuous copper plating.
### Preventing Plating Voids in PCB Manufacturing
To avoid plating voids caused by rough drilling, it is essential to follow the manufacturer’s guidelines carefully. Manufacturers typically provide recommendations regarding the optimal drill bit sizes, feed rates, and speeds. Using drill bits with a low rate of penetration (ROP) can result in rough surfaces that are difficult to coat evenly during the deposition and electroplating processes. If the ROP is too low, drill bit contamination may occur, but this can be corrected during the cleaning process.
If you have any PCB manufacturing needs, please do not hesitate to contact me.Contact me
