The PCB industry is increasingly using high-frequency and high-speed plates to meet special requirements such as signal transmission speed, signal integrity, and impedance matching. Polytetrafluoroethylene is a widely used high-frequency material with excellent electrical properties. However, due to the characteristics of PTFE material itself, during the machining process of PCB slot holes, problems such as warping of the copper skin at the edge of the hole, deformation, and damage of the slot hole, and improper removal of fiber filaments at the edge of the corner hole are prone to occur. This article focuses on the optimization of the processing parameters of the milling machine and the optimization of the processing parameters of the drilling machine, choosing the most optimized processing method and processing parameters, and expecting to improve the processing quality of the PTFE series plate slot holes.
In PCB design, the high-frequency materials currently used are all made of PTFE as the main body, mixed with other different fillers such as glass fiber cloth or ceramics, so that the dielectric constant and dielectric loss of the material can meet the high-frequency design requirements. However, due to the difference in the characteristics of PTFE materials, it is a huge challenge for PCB machining. Although material suppliers have tried their best to improve the filler formula and reduce the difficulty of machining, the difficulty and cost of processing are still higher than that of ordinary FR. -4 materials. Major PCB suppliers are also committed to the innovation of PTFE material machining technology, hoping to reduce machining costs and improve machining efficiency, but most of them are not ideal. Slot holes are common tool holes in printed circuit board machining. It is inevitable to encounter troubles when processing high-frequency materials.
Slot type
- Slot length classification: slots have short slots and long slots. Slots whose length is less than twice the width are short slots, and the above slots are long slots;
- Slot shape classification: round slot and right angle slot.
- Slot attribute classification: NPTH slot and PTH slot.
The slot holes discussed in this article are round-corner slot holes and right-angle slot holes with PTH attributes.
Improvement direction
The conventional production method is to complete slot machining by drilling or milling slot holes. Whether drilling or milling slot holes, slot holes are prone to edge burrs. The copper plating layer adheres to the PTFE burrs when the copper plate is electrified. At the same time, copper thorns grow rapidly, which can seriously or completely block the slot holes and affect their function of the slot holes. In this thesis, through comparative experiments on drilling slot holes or milling slot holes, looking for suitable operation methods and operating parameters, to improve the quality and efficiency of slot hole processing.
To drill a slot hole is to use a slotted knife with a higher hardness than the drill bit to form a slot hole according to the required slot width and length. The direction of drilling through optimization starts from two aspects: optimization of drilling through operating parameters and optimization of drilling through engineering data. Through the optimization of drilling operation parameters, the burrs in the hole and on the edge of the hole are reduced during each drilling; through the optimization of engineering data, it is expected that the burr residue at the junction of the hole and the hole can be reduced or even removed by changing the drilling path.
Slot milling is to use a milling cutter with a milling machine to cut a slot according to a certain path. The optimization direction of slot milling is mainly based on the selection of cutter types, milling machine operating parameters, and engineering data to reduce slot burrs and meet quality requirements.
Drilling process optimization
1. Current status
In the early production process, the slot holes often have burrs and plug holes, which require manual repairs, which not only affect the product delivery date but also affect the quality. It is easy to cause the holes to be free of copper due to poor repairs, which affects the customer’s component plug-in and welding; even if the customer gives in After use, it also has a certain impact on the debugging of antenna products.
2. Causes of burrs in drilling slots:
In the existing PTFE sheet, the unshredded fiber filaments will be entangled on the drill when drilling, which will reduce the chip removal performance of the drill, which will cause more filaments to wrap around the knife, resulting in rough hole walls and draped edges. Undesirable phenomena such as severe frontal. Therefore, the purpose of optimizing the drilling parameters is to ensure that the fiber filaments in the plate can be cut to the maximum and to ensure that the drill is not entangled by the fiber filaments during the production process.
3. Optimization of drilling parameters
Optimize the drilling parameters, select the commonly used PTFE materials of 0.3mm, 0.8mm, 1.2mm, 2.4mm, 3.2mm for parameter comparison tests. After the immersion copper plate is sliced to confirm the roughness of the hole wall, to confirm the correctness of the drill when working Whether the cutting of the sheet can meet the quality requirements. Test material: 2.0mm 1/1 from a certain supplier, dielectric constant 2.65.
Through the above comparison experiments of drilling round holes, it can be found that the new drilling parameters have better processing results than the original parameters. The quality of the hole wall has been increased from the original parameter of 1.16 mill to 0.56 mill, which can achieve good processing results.
New parameters: The spindle speed is reduced by 10% during slot machining, the minimum speed is not less than 20krpm, the feed speed is reduced by 40%, and the tool retreat speed is reduced by 20%, new tools are used, and the tool life is adjusted to 80% of the original parameter life.
Slot milling process optimization
1. Current status
The corner hole and the milling groove in the right-angle slot are most prone to burrs. Manual scraping is not only inefficient but also easy to cause holes without copper and affect the welding of customer devices. It is necessary to conduct process tests on the production of right-angle slots to avoid such complaints.
2. Causes of corner hole burrs
Since the drill and milling cutter are both rounds, it is impossible to produce absolute right-angled slot holes. To avoid the bad influence of non-right-angled slots on the plug-in, the usual method is to drill the corner holes at the four corners of the slot first and then perform CNC Slot milling. However, PTFE is too soft, and the material at the edge of the corner hole moves during CNC milling, making the milling cutter unable to remove the burrs at the junction of the corner hole and the milling cutter.
3. Improvement direction
1. Optimize CNC engineering data: remove the corner holes and integrate the corner holes in the CNC milling groove path, thereby removing the cutting blind spots caused by the material deformation of the milling cutter.
2. Optimize CNC parameters: due to improper parameter settings, it is difficult to discharge the cut fiber filaments, and the milling cutter is entangled by the fiber filaments and loses the cutting ability, causing the tool to break at the beginning of the tool. In order to prevent the milling cutter from being entangled by filaments and breaking the cutter, the main improvements are:
① Optimize the spindle speed, improve the cutting ability of the tool, and thoroughly chop the fiber. The main way to increase the cutting capacity is to increase the linear velocity of the tool edge. The greater the linear velocity, the higher the cutting ability. Of course, the linear velocity is not as fast as possible, and heat dissipation needs to be considered in real-time.
② Use a spiral milling cutter to reduce the speed of the cutter and increase the chip removal ability of the milling cutter. Use spiral milling cutters with stronger chip removal capabilities to replace ordinary milling cutters, reduce the cutting speed of the milling cutters, and allow the milling cutters ample time for chip removal
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