The functionality of the LDI exposure machine depends on the coordinated operation of "laser generation - pattern control - precise scanning - stable support - quality monitoring". Its core systems can be broken down into the following 5 parts, each with a clear division of labor and mutual cooperation:
When choosing or evaluating an LDI exposure machine, the following technical parameters should be paid attention to, as they directly affect the imaging accuracy, production efficiency, and application scenarios of the PCB:
- Laser Spot Diameter: A core indicator determining the minimum line - width capability. The spot diameter of mainstream devices is 5 - 10μm, and high - end models can reach 3 - 5μm (suitable for ultra - fine circuit requirements such as IC substrates, Mini LED PCBs). The smaller the spot, the thinner the circuits that can be made (e.g., a 5μm spot can achieve a 20 - 30μm line - width / line - pitch).
- Alignment Accuracy: Crucial for the inter - layer alignment of multi - layer PCBs, relying on the visual positioning system. The alignment accuracy of mainstream devices is ±2 - 5μm, and some high - precision models can reach ±1 - 2μm (to avoid inter - layer short - circuits / open - circuits in multi - layer boards).
- Scanning Speed: Affects production efficiency, usually measured by "scanning area per hour" or "meters per second". The scanning speed of mainstream devices is 1 - 3m/s, and multi - beam models (2 - 4 laser heads) can be increased to over 5m/s (meeting the needs of high - volume PCB production).
- PCB - compatible Size: The maximum bearing size of the workbench. Common specifications are 500mm×600mm, 600mm×800mm, and some devices support 1200mm×1500mm large - size substrates (suitable for automotive PCBs, server PCBs, etc.).
- Laser Energy Stability: Directly affects the curing effect of photosensitive materials, requiring a laser power fluctuation of ≤ ±2% (if the fluctuation is too large, it may cause insufficient curing in some areas, and the pattern may fall off after development).
According to the different requirements of PCB manufacturing processes (inner - layer, outer - layer, solder - resist), LDI exposure machines can be divided into three categories, with differences in design focus and functional adaptation:
- Inner - layer LDI Exposure Machine: Suitable for the inner - layer core boards of multi - layer PCBs (with a thickness of 0.1 - 0.3mm, prone to deformation). The core feature is "fixed platform + galvanometer scanning" (to avoid deformation of thin core boards during movement), and it supports "positive - film imaging" (the laser - illuminated area is the circuit area, which is retained after curing). High alignment accuracy is required (±2 - 3μm) to ensure the alignment of inner - layer and outer - layer boards during subsequent lamination.
- Outer - layer LDI Exposure Machine: Used for the production of outer - layer circuits of PCBs (with copper foil on the surface, dry film needs to be covered). It supports the coordination of "platform movement + galvanometer scanning" (covering large - size PCBs) and can switch between "positive - film / negative - film imaging" (in negative - film imaging, the laser - illuminated area is the non - circuit area, which is removed after development). Some models are suitable for thick - copper PCBs (copper thickness > 100μm), and the "3D focusing compensation" is used to solve the problem of uneven spots caused by copper foil bumps.
- Solder - resist LDI Exposure Machine: Specifically used for the solder - resist layer (green - oil layer) windowing (exposing pads) of PCBs. It uses a 405nm wavelength laser (matching the photosensitive characteristics of solder - resist ink). High windowing accuracy is required (±5μm), suitable for micro - pads (such as pads of 0201, 01005 package components). Some models support "local exposure" (only scanning the areas that need windowing, improving efficiency).
In the actual production of PCBs, the operation of the LDI exposure machine needs to be connected with the previous (film - pasting / ink - printing) and subsequent (development) processes. The specific process is as follows:
- Pre - process Preparation: The PCB board undergoes "cleaning → film - pasting (dry film) / printing solder - resist ink → pre - baking" treatment to ensure that the surface photosensitive material is uniform and free of impurities (to avoid affecting laser imaging).
- Equipment Calibration: After startup, calibrate the laser energy (detected by an energy meter to ensure it meets the process requirements) and calibrate the CCD camera (to ensure accurate fiducial point recognition).
- PCB Loading: Place the processed PCB board on the workbench and start the vacuum adsorption to ensure the board is flat and fixed.
- Fiducial Point Positioning: The CCD camera automatically identifies 2 - 4 fiducial points (Mark points) on the PCB board, calculates the deviation between the actual position and the theoretical position of the PCB, and automatically adjusts the workbench position to complete the alignment.
- Laser Exposure: Import the corresponding Gerber pattern file, and the device starts laser scanning according to the set parameters (spot diameter, scanning speed, energy) to complete the pattern imaging of the entire PCB board.
- Post - exposure Inspection: After exposure, some devices use the vision system to quickly check whether there are obvious pattern defects (such as missed exposure, circuit breaks). If there are no problems, it is sent to the subsequent development process (rinsing off the uncured photosensitive material).
