The OSP (Organic Solderability Preservative) process, with its advantages of being lead-free, environmentally friendly, and offering high flatness, has become the mainstream surface treatment method for high-density PCBs and high-frequency PCBs. The thickness of the OSP film directly determines the solderability and corrosion resistance of the PCB. If the film is too thin, it cannot effectively protect the copper surface; if it is too thick, it can lead to soldering defects. Traditional offline film thickness measurement methods, such as gravimetric analysis and ellipsometry, suffer from low efficiency and destructiveness. In contrast, the application of online infrared thickness measurement systems has made precise control of OSP film thickness both simple and efficient.

As PCB engineers, we are well aware of the "sensitivity" of OSP film thickness. The ideal thickness range for OSP film is 0.2–0.5 μm. Within this range, the film can effectively isolate the copper surface from air to prevent oxidation while also decomposing quickly during soldering without affecting solder joint quality. If the film thickness deviates from this range, issues such as copper surface oxidation/discoloration or soldering problems like "cold solder joints" or "poor pad wetting" may occur.

The working principle of the online infrared thickness measurement system is based on the absorption characteristics of infrared spectra. OSP films of different thicknesses exhibit varying absorption rates for infrared light at specific wavelengths. The system emits infrared light of a specific wavelength onto the OSP film on the PCB surface, receives the reflected infrared signal, and then calculates the OSP film thickness based on the Lambert-Beer Law. The entire detection process is non-contact and non-destructive, with a detection time of only 0.1 seconds per point, enabling full-area scanning of the PCB surface.

The core advantages of the online infrared thickness measurement system lie in its "online" and "real-time" capabilities. It is typically integrated after the drying section of the OSP production line. After drying, the PCB directly enters the thickness measurement area. The system's detection probe automatically scans every area of the PCB, including pads, traces, and substrates, recording the film thickness data for each measurement point and transmitting it in real time to the control system. If the detected film thickness exceeds the preset threshold, the system automatically triggers an alarm and adjusts the parameters of the OSP coating tank. For instance, if the film is too thick, it reduces the coating speed or dilutes the OSP solution concentration; if the film is too thin, it increases the coating speed or replenishes the OSP stock solution.

Compared to traditional offline detection methods, the advantages of the online infrared thickness measurement system are significant. Traditional gravimetric methods require sampling PCBs from the production line, stripping the OSP film, and calculating thickness by weighing. This process is time-consuming (approximately 30 minutes per test) and destroys the sample. In contrast, the online infrared system enables 100% full inspection, improving detection efficiency by hundreds of times without generating any waste.

In practical applications, the calibration of the online infrared thickness measurement system is critical. Since different brands of OSP solutions have varying compositions and infrared absorption characteristics, the system must be recalibrated using standard film thickness samples whenever the OSP solution is changed. The standard samples must cover the entire thickness range of 0.2–0.5 μm to ensure the system's measurement accuracy across this interval.

We once encountered a case where a batch of high-frequency PCBs had excessively thick OSP films (averaging 0.6 μm), leading to batch soldering defects for the customer. Tracing the issue with the online infrared thickness measurement system revealed that the OSP coating tank concentration was too high due to a malfunction in the automatic dosing system. After promptly recalibrating the dosing system, subsequent batches maintained stable OSP film thickness around 0.35 μm, reducing the soldering defect rate from 12% to 0.3%.

Furthermore, the detection data from the online infrared thickness measurement system can be used to generate quality traceability reports, documenting the film thickness distribution for each PCB. This not only facilitates customer queries but also provides data support for process optimization.