What are the production precautions for halogen-free PCBs?
During the production of halogen-free PCBs, adjustments to the heating rate and pressure are required during the lamination process to ensure the bonding strength of the resin. In the drilling process, the rigidity of the material and the glass transition temperature (Tg) value need to be considered, and parameters should be adjusted according to alkaline conditions to prevent damage to the substrate. The operation of halogen-free solder mask ink is similar to that of ordinary ink, but requires special attention.
With the enhancement of environmental awareness and the increasing requirements for the safety of electronic products, halogen-free PCBs have gained wider application in the electronics industry due to their environmental protection characteristics. However, the production process of halogen-free PCBs differs from that of traditional halogen-containing PCBs, and special attention needs to be paid to multiple aspects to ensure product quality and performance. The following are the key considerations in the production of halogen-free PCBs:
Halogen-free PCB substrate materials should be selected in compliance with halogen-free standards, that is, the bromine (Br) content should be less than 900 ppm, the chlorine (Cl) content should be less than 900 ppm, and the total amount of bromine + chlorine should be less than 1500 ppm. When choosing substrate materials, it is essential to focus on whether their electrical, thermal, and mechanical properties meet the product design requirements. For example, for high-frequency and high-speed electronic products, halogen-free substrate materials with low dielectric constant and low dissipation factor should be selected to reduce signal transmission losses and distortion.
The quality of copper foils directly affects the conductivity and reliability of PCBs. Copper foils with a smooth surface, appropriate roughness, and free from oxidation and impurities should be selected. During the inspection of copper foils, their thickness uniformity should be checked to ensure compliance with design requirements and to avoid unstable PCB performance due to uneven copper foil thickness.
Halogen-free solder mask inks are key materials for achieving surface insulation and protection of PCBs. It is crucial to select halogen-free solder mask inks that have good compatibility with substrate materials, strong adhesion, excellent chemical corrosion resistance, and heat resistance. During incoming inspection, indicators such as the shelf life, viscosity, and color of the inks should be checked to ensure stable quality.
The resin system of halogen-free substrate materials differs from that of traditional halogen-containing materials, and so do their curing reaction characteristics. During the lamination process, the heating rate needs to be appropriately adjusted. If the heating rate is too fast, the resin may not be able to flow and fill sufficiently, resulting in defects such as voids and delamination due to poor interlayer bonding. If the heating rate is too slow, it will prolong the production cycle and increase production costs. Generally, the optimal heating rate curve should be determined through experiments based on the characteristics of the substrate materials and the performance of the lamination equipment.
Appropriate pressure is a crucial factor in ensuring lamination quality. During lamination, it is necessary to ensure that the pressure is evenly applied to the PCB so that the resin can fully infiltrate the copper foils and the substrate, forming a good bonding interface. Insufficient pressure may prevent the resin from fully filling the interlayer gaps, leading to insufficient bonding strength. Excessive pressure may damage the substrate or deform the copper foils, affecting the dimensional accuracy and electrical performance of the PCB. Usually, a lower pressure is applied at the initial stage of lamination, and then the pressure is gradually increased to the set value after the resin starts to flow, and the pressure is maintained stable during the holding stage.
The rigidity and glass transition temperature (Tg value) of halogen-free substrate materials may differ from those of traditional materials. During drilling, parameters such as drill bit speed, feed rate, and number of drilling passes should be reasonably selected based on the material rigidity and Tg value. For materials with high rigidity and a high Tg value, the drill bit speed and feed rate should be appropriately increased to reduce friction heat during drilling and prevent rapid drill bit wear and substrate charring. Conversely, the speed and feed rate should be reduced to ensure drilling quality.
Halogen-free PCBs are usually drilled under alkaline conditions to reduce burrs and smear on the hole walls. Under alkaline conditions, drilling parameters such as the geometry of the drill bit, the pressure and flow rate of the coolant need to be adjusted. An appropriate drill bit geometry can improve chip evacuation performance and reduce damage to the hole walls. Sufficient coolant can promptly carry away the heat generated during drilling, lowering the temperature of the hole walls and preventing thermal decomposition of the substrate. Meanwhile, the coolant should be regularly inspected and replaced to ensure stable performance.
Before applying halogen-free solder mask inks, strict pretreatment of the PCB surface is required, including degreasing, cleaning, and roughening processes. Degreasing can remove oil stains and impurities from the PCB surface, ensuring good adhesion between the ink and the substrate. Cleaning can further remove residual substances on the surface, improving surface cleanliness. Roughening can increase the surface roughness of the substrate, enhancing the mechanical interlocking force between the ink and the substrate. The quality of pretreatment directly affects the adhesion and reliability of the solder mask ink.
There are various coating methods for halogen-free solder mask inks, such as screen printing and spraying. Different coating methods are suitable for different product requirements and production scales. When selecting a coating method, factors such as the viscosity of the ink, the shape, and size of the PCB should be considered. For example, spraying may be more suitable for PCBs with complex shapes and small sizes, while screen printing has the advantages of high efficiency and low cost for mass production. During the coating process, the uniformity of the ink thickness should be controlled to avoid uneven thickness, which can affect the solder mask effect and the appearance quality of the PCB.
The curing of halogen-free solder mask inks is a key step to ensure their stable performance. Curing temperature and time are important factors affecting the curing effect. If the curing temperature is too low or the time is too short, the ink may not be fully cured, resulting in problems such as reduced adhesion and insufficient hardness. If the curing temperature is too high or the time is too long, the ink may change color or become brittle, affecting the electrical performance and reliability of the PCB. Therefore, the curing temperature and time should be precisely controlled according to the characteristics of the ink and the performance of the production equipment to ensure complete curing of the ink.
In the production process of halogen-free PCBs, a comprehensive quality inspection system should be established to conduct strict quality inspections and control at each production stage. Advanced inspection equipment and technologies, such as X-ray fluorescence spectrometers (XRF) for detecting halogen content, optical microscopes for inspecting hole wall quality, and flying probe testers for testing electrical performance, should be used to promptly detect and resolve quality issues, ensuring that the produced halogen-free PCBs comply with relevant standards and customer requirements.
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