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Source:互联网Author:qyadmin Addtime:2019-11-13 16:17 Click:

pcb drilling process


Printed circuit board drilling

 
      Printed Circuit Boards (PCBs) form the backbone of all major electronics. These miraculous inventions pop up in all computational electronics. including simpler devices like digital clocks, calculators etc. For the uninitiated. a PCB routes electrical signals through electronics. which satisfies the device's electrical and mechanical circuit requirements. In short, PCBs tell the electricity where to go, bringing your electronics to life.
 
       The basic materials forming a board are an epoxy glass mix. made by impregnating rolls of woven glass cloth with resin. and sheets of copper foil which =used to make the electrical connections . A typical board may have many alternate layers of epoxy glass and copper foil. manufactured by squeezing in a large hydraulic press. The board will not become a circuit. until connections made between the different copper layers. and when the discrete electrical components soldered or welded in place. Thus, through hole drilling is one of the fundamental processes. required in the manufacture to produce an electrical connection. between the circuits on the board.
 
      Drilling is a particularly complicated machining process. and it becomes much more complicated when the work piece is PCB. PCB is composite materials with anisotropy. Tool trends to wear and break. The accuracy of hole location, the presence of entry and exit burrs, smear of resin on the sides of holes. delimitation and so on may affect quality of boards. Only a small one of these defects may cause great losses.

       To overcome these problems, both the drilling process and PCB structure design. have researched by many scholars. But the investigations into the drilling processes of PCB are not systematic. The present review article address the report. about tool materials and geometrics, cutting force. cutting temperature, radial run-out and damages occurred in drilling processes. And as a conclusion, some of these critical issues proposed to meet the. challenges in analysis and optimization for PCB drilling.


pcb drilling process


pcb drilling process


      Circuit boards should be compatible with. a PCB layout created by the designer using PCB design software. Commonly-used PCB design software includes Altium Designer. OrCAD, Pads, KiCad, Eagle etc. NOTE: Before PCB fabrication. designers should inform their contract manufacturer. about the PCB design software version used to. design the circuit since it helps avoid issues caused by discrepancies.
 
      The PCB industry birthed extended Gerber as the perfect output format. Different PCB design software calls for different Gerber file generation steps. they all encode comprehensive vital information including copper tracking layers. drill drawing, apertures, component notations and other options. All aspects of the PCB design undergo checks at this point. The software performs oversight algorithms on the design. to ensure that no errors go undetected. Designers also examine the plan with regard to elements relating to track width. board edge spacing. trace and hole spacing and hole size.
 
       PCB printing begins after designers output the PCB schematic files . and manufacturers conduct a DFM check. Manufacturers use a special printer called a plotter. which makes photo films of the PCBs, to print circuit boards. Manufacturers will use the films to image the PCBs. Although it's a laser printer, it isn't a standard laser jet printer. Plotters use precise printing technology. to provide a detailed film of the PCB design.
 
     The final product results in a plastic sheet with a photo negative of the PCB in black ink. For the inner layers of PCB, black ink represents the conductive copper parts of the PCB. The remaining clear part of the image denotes the areas of non-conductive material. The outer layers follow the opposite pattern: clear for copper. but black refers to the area that'll etched away. The plotter develops the film, and the film stored to prevent any unwanted contact.
 
     Each layer of PCB and solder mask receives its own clear and black film sheet. In total, a two-layer PCB needs four sheets: two for the layers and two for the solder mask. all the films have to correspond to each other. When used in harmony, they map out the PCB alignment.
 
      To achieve perfect alignment of all films. registration holes should punched through all films. The exactness of the hole occurs by adjusting the table on which the film sits. When the tiny calibrations of the table lead to an optimal match, the hole punched. The holes will fit into the registration pins in the next step of the imaging process.
 
     The creation of films in previous step aims to map out a figure of copper path. Now it's time to print the figure on the film onto a copper foil.
 
       This step in PCB manufacturing prepares to make actual PCB. The basic form of PCB comprises a laminate board. whose core material is epoxy resin and glass fiber. that are also called substrate material. Laminate serves as an ideal body for receiving the copper that structures the PCB. Substrate material provides a sturdy and dust-resistant starting point for the PCB. Copper is pre-bonded on both sides. The process involves whittling away the copper to reveal the design from the films.
 
      In PCB construction, cleanliness does matter. The copper-sided laminate cleaned and passed into a decontaminated environment. During this stage, it's vital that no dust particles settle on the laminate. An errant speck of dirt might otherwise cause a circuit to be short or remain open.
 
      Next, the clean panel receives a layer of photo-sensitive film called photo resist. The photo resist comprises a layer of photo reactive chemicals that harden. after exposure to ultra violet light. This ensures an exact match from the photo films to the photo resist. The films fit onto pins that hold them in place over the laminate panel.
 
      The film and board line up and receive a blast of UV light. The light passes through the clear parts of the film. hardening the photo resist on the copper underneath. The black ink from the plotter prevents the light from reaching the areas not meant to harden. and they slated for removal.
 
     After the board becomes prepared. it washed with an alkaline solution that removes any photo resist left unhardened. A final pressure wash removes anything else left on the surface. The board is then dried.
 
    The product emerges with resist covering the copper areas meant to remain in the final form. A technician examines the boards to ensure that no errors occur during this stage. All the resist present at this point denotes the copper that will emerge in the finished PCB.
 
    This step only applies to boards with more than two layers. Simple two-layer boards skip ahead to drilling. Multiple-layer boards need more steps.
 
      With the photo resist removed and the hardened resist. covering the copper we wish to keep, the board proceeds to the next stage: unwanted copper removal. as the alkaline solution removed the resist, a more powerful chemical preparation eats. away the excess copper. The copper solvent solution bath removes all the exposed copper. Meanwhile, the desired copper remains protected beneath the hardened layer of photo resist.
 
    Not all copper boards created equal. Some heavier boards need larger amounts of copper solvent. and varying lengths of exposure. As a side note, heavier copper boards need extra attention for track spacing. Most standard PCBs rely on similar specification.
 
    Now that the solvent removed the unwanted copper. the hardened resist protecting the preferred copper needs washing off. Another solvent accomplishes this task. The board now glistens with only the copper substrate necessary for the PCB.
 
     With all the layers clean and ready, the layers need alignment punches to ensure they all line up. The registration holes align the inner layers to the outer ones. The technician places the layers into a machine called the optical punch. which permits an exact correspondence so the registration holes punched.
 
    Once the layers placed together. it's impossible to correct any errors occurring on the inner layers. Another machine performs an automatic optical inspection of the panels to. confirm a total absence of defects. The original design from Gerber, which the manufacturer received, serves as the model. The machine scans the layers using a laser sensor. and proceeds to compare the digital image with the original Gerber file.
 
    If the machine finds inconsistency, the comparison. displayed on a check for the technician to assess. Once the layer passes inspection, it moves to the final stages of PCB production.
 
In this stage, the circuit board takes shape. All the separate layers await their union. With the layers ready and confirmed, they need to fuse together. Outer layers must join with the substrate. The process happens in two steps: layer-up and bonding.
 
    The outer layer material consists of sheets of fiber glass. pre-impregnated with epoxy resin. The shorthand for this called prepreg. A thin copper foil also covers the top. and bottom of the original substrate, which contains the copper trace etchings. Now, it's time to sandwich them together.
 
    The bonding occurs on a heavy steel table with metal clamps. The layers fit into pins attached to the table. Everything must fit to prevent shifting during the alignment.
 
     A technician begins by placing a prepreg layer over alignment basin. The substrate layer fits over the prepreg before the copper sheet placed. Further sheets of prepreg sit on top of the copper layer. Finally, an aluminum foil and copper press plate complete the stack. Now it's prepped for pressing.
 
     The entire operation undergoes an automatic routine run by the bonding press computer. The computer orchestrates the process of heating up the stack. the point in which to apply pressure. and when to allow the stack to cool at a controlled rate.
 
   Next, a certain amount of unpacking occurs. With all the layers molded together in a super sandwich of PCB glory. the technician unpacks the multi-layer PCB product. It's a simple matter of removing the restraining pins. and discarding the top pressure plate. The PCB goodness emerges victorious from within its shell of aluminum press plates. The copper foil, included in the process, remains to comprise the outer layers of the PCB.
 
   Finally, holes bored into the stack board. All components slated to come later, such as copper-linking via holes. and leaded aspects, rely on the exactness of precision drill holes. The holes drilled to a hairs-width - the drill achieves 100 microns in diameter. while hair averages at 150 microns.
 
   To find the location of the drill targets. an x-ray locator identifies the proper drill target spots. Then, proper registration holes bored to . secure the stack for the series of more specific holes.
 
   Before drilling. the technician places a board of buffer material beneath. the drill target to ensure a clean bore enacted. The exit-material prevents any unnecessary tearing upon the drill's exits.
 
    A computer controls every micro-movement of the drill - it's only natural. that a product that determines the behavior of machines would rely on computers. The computer-driven machine uses the drilling file from the original. design to identify the proper spots to bore.
 
    The drills use air-driven spindles that turn at 150,000 rpm. At this speed, you might think that drilling happens in a flash, but there are many holes to bore. An average PCB contains well over one hundred bore intact points. During drilling, each needs its own special moment with the drill, so it takes time. The holes later house the vias and mechanical mounting holes for the PCB. The final affixation of these parts occurs later, after plating.
 
   After the drilling completes itself. the more copper that lines the edges of the production panel undergoes removal. by a profiling tool.
 
    After drilling, the panel moves onto plating. The process fuses the different layers together using chemical deposition. After a thorough cleaning, the panel undergoes a series of chemical baths. During the baths, a chemical deposition process deposits a thin layer . about one micron thick - of copper over the surface of the panel. The copper goes into the recently drilled holes.
 
 
    before  this step. the interior surface of the holes exposes the fiber glass material. that comprises the interior of the panel. The copper baths completely cover, or plate, the walls of the holes. the entire panel receives a new layer of copper. Most , the new holes covered. Computers control the entire process of dipping, removal and procession.
 
    we applied photo resist to the panel. In this step, we do it again - except this time, we image the outer layers of the panel with PCB design. We begin with the layers. in a sterile room to prevent any contaminants from sticking to the layer surface. then apply a layer of photo resist to the panel. The prepped panel passes into the yellow room. UV lights affect photo resist. Yellow light wavelengths don't carry UV levels to affect the photo resist.
  
    Black ink transparencies secured by pins to prevent misalignment with the panel. With panel and stencil in contact, a generator blasts them with high UV light.. which hardens the photo resist. The panel then passes into a machine that removes the  unhardened resist.  protected by the black ink opacity.
 
    The process stands as an inversion to that of the inner layers. Finally, the outer plates undergo inspection to ensure. all the undesired photo resist removed during the previous stage.
 
    We return to the plating room. As we did in Step 8, we electroplate the panel with a thin layer of copper. The exposed sections of the panel from the outer layer photo resist stage. receive the copper electro-plating. Following the initial copper plating baths, the panel usually receives tin plating. which permits the removal of all the copper left on the board slated for removal. The tin guards the section of the panel meant to remain. covered with copper during the next etching stage. Etching removes the unwanted copper foil from the panel.
 
     The tin protects the desired copper during this stage. The unwanted exposed copper and copper beneath the remaining resist layer undergo removal. Again, chemical solutions applied to remove the excess copper. Meanwhile, the tin protects the valued copper during this stage.
 
    The conducting areas and connections are now established.
 
    Before the solder mask applied to both sides of the board, the panels cleaned. and covered with an epoxy solder mask ink. The boards receive a blast of UV light, which passes through a solder mask photo film. The covered portions remain unhardened and will undergo removal.
 
    To add extra solder-ability to the PCB, we plate them with gold or silver. Some PCBs also receive hot air-leveled pads during this stage. The hot air leveling results in uniform pads. That process leads to the generation of surface finish. PCBCart can process many types of surface finish according to customers' specific demands.
 
    The completed board receives ink-jet writing on its surface. used to say all vital information of the PCB. The PCB finally passes onto the last coating and curing stage.
 
     As a final precaution, a technician performs electrical tests on the PCB. The automated procedure confirms the functionality of the PCB. and its conformity to the original design. At PCBCart, we offer an advanced version of electrical testing. called Flying Probe Testing. which depends on moving probes to test electrical performance. of each net on a bare circuit board.
 
    Now we've come to the last step: cutting. Different boards cut from the original panel. The method employed either centers on using a router or a v-groove. A router leaves small tabs along the board edges. while the v-groove cuts diagonal channels along both sides of the board. Both ways permit the boards to pop out from the panel.

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