The quality of PCB copper foil substrates is becoming increasingly stringent due to the trends in electronic systems that demand lighter, thinner, shorter, more functional, denser, and more reliable products. The manufacturing and inspection specifications for PCB copper foil substrates encompass various factors, including the raw material glass fiber cloth, film baking conditions, glue content, glue flow, gelling time, conversion degree, and storage conditions. Additionally, the parameters set for substrate pressing conditions significantly influence the thickness quality of PCB copper foil substrates. To ensure thickness quality control, it’s essential to review all manufacturing processes and implement improvements in process capability, rather than simply increasing costs without purpose. Currently, PCB copper foil substrate manufacturing facilities are gradually transitioning to non-contact laser thickness measurement systems, moving away from manual inspections using sub-centimeter cards. The design of these systems features unique characteristics; most laser thickness gauge sensor mechanisms require integration with on-site design and construction. Testing methods vary, and any maintenance or introduction of new functions must involve the equipment manufacturer.
1. Origin
The PCB copper foil substrate serves as a foundation for the installation and interconnection of electronic components. As the industry shifts toward lightweight, thin electronic systems with high functionality, density, and reliability, the quality of the PCB copper foil substrate directly impacts the reliability of electronic products. In the manufacturing process of PCB copper foil substrates, significant attention is given to thickness quality control. Generally, this includes quality control of film semi-finished products and the alignment of pressing conditions, making the thickness result a comprehensive outcome of all process controls.
1. In the past, PCB manufacturers required only that the substrate thickness reach IPC-4101[1] CLASS B. However, since 2000, they have shifted to demanding CLASS C or higher to align with the market trends for advanced and high-density printed circuit boards. The PCB industry now feels that these requirements are insufficient, prompting the adoption of Statistical Process Control (SPC) [2]. The most commonly utilized metrics include process accuracy (Ca, with values closer to 0 being preferable) and process capability index (Cpk, where higher values indicate better performance). The formulas for these calculations are:
Ca = (measured average value – specification center value) / (half of specification tolerance) * 100%
Cpk = Min (upper specification limit – average value, average value – lower specification limit) / (3 standard deviations)
Statistical process control not only ensures products remain within specification limits but also emphasizes the importance of the central value of specifications. However, this method is primarily aimed at improving in-plant processes. If Cpk is inappropriately required to reach excessively high levels without regard for specification limits, it may lead to costly errors or the need for re-selection of products. For instance, consider a 6mil 1/1 PCB copper foil substrate with a copper thickness of 8.5mil as the median value, following a normal distribution. Here, the measured average is 8.5 (Ca=0), with a thickness distribution between 8.08 and 8.92, fitting within Class C specifications, yielding a Cpk of 1.67, while a drop to Class D results in a Cpk of 1.33. Thus, negotiation of Cpk and specifications between suppliers and manufacturers is essential.
Cpk is derived from data across an entire batch; theoretically, if Cpk is substandard, the entire batch would be rejected. This seems unreasonable since it penalizes products meeting specifications. Therefore, it’s vital to understand that even if all products are qualified, a low Cpk may arise from unstable processes or a non-central average value, indicating room for process improvement. To achieve a qualified Cpk, screening tests can be employed to remove products close to specification limits, which will improve Cpk but may decrease yield. In factories with yield-based bonus systems, this could lead to pushback from staff.
2. Method
Originally, board edges were measured manually with micrometers, but complete inspection was often challenging. Thus, a laser thickness gauge utilizing a non-contact laser displacement sensor was developed. Classifications should comply with IPC regulations, employing a labeling machine: Class A uses red labels and Class B blue. For customers with stricter requirements, sub-station processing can categorize products into four levels and four stacks.
3. Architecture
The thickness gauge designed with a laser displacement sensor integrates optical and mechanical components. The optical design has been developed as a standalone part of the sensor, allowing for simple electromechanical integration, while software expansion is necessary. Figure 3 illustrates the architectural flow of the thickness gauge. The selection and interconnection of various components are critical, as errors and instability will arise if not properly managed.
3.1 Displacement Sensor
Selecting a displacement sensor requires consideration of the PCB copper foil substrate’s characteristics and allowable tolerance resolution. Measurement distance, resolution, linearity, and sampling period should be compared. The measurement distance must encompass the thickness of all PCB substrates being tested; resolution must align with the sensor specifications. Higher resolution with fewer samples indicates better performance; lower linearity is preferable. For example, with a measurement distance of +/-5mm, linearity of 1% FS vs. 0.1% FS results in maximum errors of 0.1mm and 0.01mm, respectively. A slower sampling period will yield smaller fluctuations.
3.2 Analog-to-Digital Conversion Card
When choosing an analog-to-digital conversion card (ADC card), focus on resolution; in the thin-plate market, a 16-bit resolution is essential, as 12-bit resolution is inadequate. Next, consider the input channel and voltage range. Typically, industry designs require six input channels for three profiles, necessitating six displacement sensors. Most ADC cards can handle up to 16 channels, and the output signals from displacement sensors include voltage and current, typically ranging from -5V to +5V and 4 to 20 mA, respectively. Current signals can be converted to voltage (within +/-10V) with appropriate resistance for ADC input.
3.3 Digital Card
Digital I/O cards operate with binary values of 0 and 1, where 0V represents low potential (0) and 5V indicates high potential (1). Digital signal inputs (DI), including counters and photoelectric switches, can signal whether the PCB copper foil substrate has passed and display peripheral conditions. Digital signal outputs (DO) facilitate control or alarms, including displaying quality analysis results, often via a computer screen indicating OK/NG status or alarms linked back to a programmable logic controller (PLC). The analog-to-digital conversion and digital card functions are often combined into a multifunction I/O card, which suffices unless an excess of digital signals exists.
4. Theory
Over time, thickness measurement equipment may experience aging, damage, or changes in customer requirements, necessitating ongoing maintenance and improvements. At this stage, it’s crucial to analyze theoretical aspects to identify the causes of abnormalities.
5. Conclusion
The entire PCB copper foil substrate laser thickness measurement system integrates the laser displacement sensor (optical), mechanical design (machine), circuitry, photoelectric switches, wiring (electric), and software. Each component’s performance is interrelated; a lack of understanding of the architectural process may hinder timely fault resolution, resulting in a loss of trust from on-site personnel and rendering the system a burden, compromising quality control. Hence, caution and thorough understanding are imperative when utilizing this equipment.
1. Origin
The PCB copper foil substrate serves as a foundation for the installation and interconnection of electronic components. As the industry shifts toward lightweight, thin electronic systems with high functionality, density, and reliability, the quality of the PCB copper foil substrate directly impacts the reliability of electronic products. In the manufacturing process of PCB copper foil substrates, significant attention is given to thickness quality control. Generally, this includes quality control of film semi-finished products and the alignment of pressing conditions, making the thickness result a comprehensive outcome of all process controls.
1. In the past, PCB manufacturers required only that the substrate thickness reach IPC-4101[1] CLASS B. However, since 2000, they have shifted to demanding CLASS C or higher to align with the market trends for advanced and high-density printed circuit boards. The PCB industry now feels that these requirements are insufficient, prompting the adoption of Statistical Process Control (SPC) [2]. The most commonly utilized metrics include process accuracy (Ca, with values closer to 0 being preferable) and process capability index (Cpk, where higher values indicate better performance). The formulas for these calculations are:
Ca = (measured average value – specification center value) / (half of specification tolerance) * 100%
Cpk = Min (upper specification limit – average value, average value – lower specification limit) / (3 standard deviations)
Statistical process control not only ensures products remain within specification limits but also emphasizes the importance of the central value of specifications. However, this method is primarily aimed at improving in-plant processes. If Cpk is inappropriately required to reach excessively high levels without regard for specification limits, it may lead to costly errors or the need for re-selection of products. For instance, consider a 6mil 1/1 PCB copper foil substrate with a copper thickness of 8.5mil as the median value, following a normal distribution. Here, the measured average is 8.5 (Ca=0), with a thickness distribution between 8.08 and 8.92, fitting within Class C specifications, yielding a Cpk of 1.67, while a drop to Class D results in a Cpk of 1.33. Thus, negotiation of Cpk and specifications between suppliers and manufacturers is essential.
Cpk is derived from data across an entire batch; theoretically, if Cpk is substandard, the entire batch would be rejected. This seems unreasonable since it penalizes products meeting specifications. Therefore, it’s vital to understand that even if all products are qualified, a low Cpk may arise from unstable processes or a non-central average value, indicating room for process improvement. To achieve a qualified Cpk, screening tests can be employed to remove products close to specification limits, which will improve Cpk but may decrease yield. In factories with yield-based bonus systems, this could lead to pushback from staff.
2. Method
Originally, board edges were measured manually with micrometers, but complete inspection was often challenging. Thus, a laser thickness gauge utilizing a non-contact laser displacement sensor was developed. Classifications should comply with IPC regulations, employing a labeling machine: Class A uses red labels and Class B blue. For customers with stricter requirements, sub-station processing can categorize products into four levels and four stacks.
3. Architecture
The thickness gauge designed with a laser displacement sensor integrates optical and mechanical components. The optical design has been developed as a standalone part of the sensor, allowing for simple electromechanical integration, while software expansion is necessary. Figure 3 illustrates the architectural flow of the thickness gauge. The selection and interconnection of various components are critical, as errors and instability will arise if not properly managed.
3.1 Displacement Sensor
Selecting a displacement sensor requires consideration of the PCB copper foil substrate’s characteristics and allowable tolerance resolution. Measurement distance, resolution, linearity, and sampling period should be compared. The measurement distance must encompass the thickness of all PCB substrates being tested; resolution must align with the sensor specifications. Higher resolution with fewer samples indicates better performance; lower linearity is preferable. For example, with a measurement distance of +/-5mm, linearity of 1% FS vs. 0.1% FS results in maximum errors of 0.1mm and 0.01mm, respectively. A slower sampling period will yield smaller fluctuations.
3.2 Analog-to-Digital Conversion Card
When choosing an analog-to-digital conversion card (ADC card), focus on resolution; in the thin-plate market, a 16-bit resolution is essential, as 12-bit resolution is inadequate. Next, consider the input channel and voltage range. Typically, industry designs require six input channels for three profiles, necessitating six displacement sensors. Most ADC cards can handle up to 16 channels, and the output signals from displacement sensors include voltage and current, typically ranging from -5V to +5V and 4 to 20 mA, respectively. Current signals can be converted to voltage (within +/-10V) with appropriate resistance for ADC input.
3.3 Digital Card
Digital I/O cards operate with binary values of 0 and 1, where 0V represents low potential (0) and 5V indicates high potential (1). Digital signal inputs (DI), including counters and photoelectric switches, can signal whether the PCB copper foil substrate has passed and display peripheral conditions. Digital signal outputs (DO) facilitate control or alarms, including displaying quality analysis results, often via a computer screen indicating OK/NG status or alarms linked back to a programmable logic controller (PLC). The analog-to-digital conversion and digital card functions are often combined into a multifunction I/O card, which suffices unless an excess of digital signals exists.
4. Theory
Over time, thickness measurement equipment may experience aging, damage, or changes in customer requirements, necessitating ongoing maintenance and improvements. At this stage, it’s crucial to analyze theoretical aspects to identify the causes of abnormalities.
5. Conclusion
The entire PCB copper foil substrate laser thickness measurement system integrates the laser displacement sensor (optical), mechanical design (machine), circuitry, photoelectric switches, wiring (electric), and software. Each component’s performance is interrelated; a lack of understanding of the architectural process may hinder timely fault resolution, resulting in a loss of trust from on-site personnel and rendering the system a burden, compromising quality control. Hence, caution and thorough understanding are imperative when utilizing this equipment.
