Manufacturing process of printed circuit boards, Physics

The copper clad laminate manufacturing process is formed in heated press which has highly polished stainless steel press plates. The manufacturing process starts by placing a sheet of copper foil onto the lower press plate. The copper sheet forms the surface of copper clad board and forms a base on to which the board structure is built. THE copper sheet is produced by rolling out a piece of copper or by an electroplating process. Electroplating is preferred as it produced a more uniform and controllable film thickness and granularity. Layer of base material impregnated with resin are built up on the top of the copper foil until required board thickness is achieved. Number of layers of base material should be determined to produce board of copper thickness after taking tolerances into account. Once the resin impregnated base material layers have been built up, a final layer is added. This final layer can be a second sheet of copper foil for double sided boards or a material which acts as a release film for single sided boards. The entire assembly is placed in a steam heated hydraulic press which bonds the different layers into a completed board. The pressing operation precedes by applying pressure and steam heating the plates to the curing temperature of the resin. After required curing time, the plates are then water cooled to 250C and the boards are removed from the press. After pressing, the boards the trimmed to remove any extruded resin. The sequence of manufacturing steps involved in making clad laminate are as under.

PCB MANUFACTURING PROCESSES: There are many technique for making PCBs, some of them are more suitable low volume manufacturing and other are better for high volume manufacturing in factories. PCBs are great when mass-producing a device or when the circuit complexity makes point-to –point wiring impractical.

These processes are:-

(1)         PREPROCESSING

(2)         PHOTOLTHOGRAPHY

(3)         ETCHING

(4)         DRILLING

(5)         ELECTROPLATING

(6)         BOARD TESTING

(7)         BOARD FINISHING

PREPROCESSING: This consists of initial preparation of a copper laminate ready for subsequent processing. Copper clad laminate are available in a fixed range of sized. These oversized copper clad laminates are cut into the required PCBs size. Once a piece of copper clad laminate is selected, and is cut to size, the next stage is to drill tooling holes and special features into the board. Tooling holes are required to achieve registration between processes. Additional tooling holes may be required to provide mechanical support in order to prevent wrapping during soldering. The copper surface of copper clad laminate needs to be thoroughly cleaned before actual processing. Cleaning operation is performed by passing a bound through rollers in abrasive slurry. When very fine tracks are required a PCB it is very important to ensure that the board and processing environments are clean.

PHOTOLITHOGRAPHY: Copper tracks and land patterns are defined on a copper clad board by means of photolithography process. This process for PCBs involves the exposure of a photo resist material to light through mask. A pre-processed board is laminated with a photo sensitive resist material. The photo resist can be applied either in liquid from or as a dry film. Liquid Resists are applied by dip coating, spray coating or a roller application process. Dry film resist materials take the form of a photo sensitive polymer material sandwiched between polymer films. The dry film is applied to the adhere directly to the board surface. The second cover of the film can be removed after application to the board or at later stage of processing. The exposure part of photolithography process involves exposure of the film coated copper clad board to UV light through an artwork mass. The mask can take the form of either a photographic positive image or a photographic negative image of the required track layout. The image should be correctly aligned with the board’s geometry especially in case of double sided and multi layer boards. After exposure the image needs to be developed. The development stage involves removal of less soluble area of resist material. This corresponds to the unexposed areas of resist material. This corresponds to the unexposed areas of resist material with negatively acting photo resist material. In case of dry film resistors and depending on the actual materials used in the resist, this removal process uses either Ethylene chloride solvent at a temperature of about 200C or a solution containing sodium Hydroxide at temperature of about 600C.

ETCHING: This process is used for removal of a PCBs copper surface which is not protected by an etch resist material. Thus final copper pattern is formed after etching. The etching process is performed by exposing the surface of the board to an etchant solution which dissolves away the exposed copper areas, thereby leaving the desired conductor pattern on the board.

The different types of etchant solutions used are:

(a)          FERRIC CHLORIDE

(b)         CUPRIC CHLORIDE

(c)           CHROMATIC ACID

(d)         ALKALINE AMMONIA

The most commonly used circuit board etching chemical is ferric chloride. FeCl method is easy and cheap, but a little messy. The choice of compatibility with each resist, etching speed, copper solving capacity, etchant price and pollution charactistics. Etching of PCBs is done in spray type etching machines. In spray etching, the etchant is pumped under pressure from the pump via a pipe network to the nozzles and from there gets splashed on to the boards. Spray etching machines offer high etching uniformly and is achieved by spraying through a full number of equally distributed nozzles in ring supply. High etching rate results from fresh flow of the etchant over the boards. However it is important to control the etching process. Insufficient etching will result in excess copper being the etching process being present on the board (under etching) and cause short circuiting between the tracks. Over-etching results in etchant under cutting the track area defined by the etch resist layer and can result into broken tracks or thin tracks. After completion of the etching process, the etch resist material is removed chemically. This leaves the PCB with desired copper pattern.

DRILLING: Drilling of component mounting holes into PCB is important mechanical operation in production processes. Thus drilling is used to create the component lead holes and through holes in a PCB. These holes pass through land areas and should be positioned properly. In addition to component holes and via hoes, holes are also required for providing mechanical connections. Holes may also be required for heat sink assembly or connectors connection to the board. The drilling can be done before or after the track areas have been defined depending upon the manufacturing process. The drilling process can be performed by using manually operated drilling machines or by using CNC drilling machines. Many boards can be drilled in single drilling operation.

SOLDER MASKING: Solder masking is the process of applying organic coatings selectively to those areas where no solder wetting is needed. Thus in a PCB with solder mask, only such areas to which components to be soldered are exposed. The solder mask is applied by screen printing using artwork which enables entire board to be converted except holes, pads and contact fingers.

The advantages of solar mask are:

(a)          It avoids solder bridging during soldering.

(b)         It reduces contamination of the solder alloy during soldering.

(c)           It improves mechanical properties of the PCB.

(d)         It acts as a barrier between atmosphere and the board thus eliminates danger of corrosion and leakage currents.

METAL PLATING: Printed circuit board are stacked before being taken for final assembly of components. The PCB should retain its solder ability for long time so that reliable solder joint can be produced during assembly. The plating can be of three types:

IMMERSION PLATING: It is deposition of metallic coating on a substrate from a solution of salt of coating metal. This process is simple and requires minor capital expense.

ELECTROLESS COPPER PLATING: Electro less plating is a chemical process where Cu is deposited on the entire surface of the board. It deposits a sufficient amount of copper on inside of holes to make the entire board surface conductive. After this the copper layer can be built up to the desired thickness using electro plating process.

ELECTOPLATING: Electroplating is done to increase the thickness of copper, as applied by the electro less plating process. On the inside of holes to the required dimensions. A board or part of its which have been covered by a layer of copper deposited by electro less plating act as a cathode.

BARE BOARD TESTING: Finally each board need to ensure that the required connections exist, that there are no short circuits, and that drills holes are properly placed. The testing usually consists of visual inspection and continuity testing.

BARE BOARD TESTING: Finally solder mask layer is applied as discussed earlier. The legend layer consists of component positional information in the form of diagram of component’s outlines and identifications codes such as C1 for the first capacitor. The legend layer consists of applied by silk printing after the board has been decreases in a solvent bath.

STEP 1: FILM GENERATION

Generated from you design films, we create an exact film representation of your design. We will create one film per layer.

STEP 2: SHEAR RAW MATERIAL

Industry standard 0.059” thick, copper clad two sides. Panels will be sheared to accommodate many boards.

STEP 3: DRILL HOLES

Using NC machine and carbide drills.

 STEP 4:  ELECTRO LESS COPPER

Apply thin copper deposit in whole barrels.

STEP 5: APPLY IMAGE

Apply photosensitive dry film (plate resist) to panel. Use light source and film to expose panel. Develop selected area from panel.

STEP 6: PATTERN PLATE

Electrochemical process to built copper in the holes and on the trace area. Apply tin to surface.

STEP 7: STRIP AND ETCH

Remove dry film then etch exposed copper. The tin protects the copper circuitry from being etched.

STEP 8: SOLDER MASK

APPLY solder mask area to entire board with the exception of solder pads.

STEP 9: SOLDER COAT

Apply solder to pads by immersing into tank of solder. Hot air knives level the solder when removed from the tank.

STEP 10: NOMENCLATURE

Apply white letter marking using printing process.

Step 11: FABRICATION

Route the parameter of the board using NC equipment.

Posted Date: 7/9/2012 3:42:35 AM | Location : United States







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