Overview of a printed circuit board production
Industrial printed circuit board production is a complex and multi-layered process that requires a wide variety of processing steps. Mechanical, chemical and photosensitive processes alternate. Each PCB is manufactured individually - although there are some basic processes and time-defined sequences - depending on the structure, material and requirements, the sequence or duration of the individual work steps vary considerably.
Not every manufacturer is able to deliver the same PCB in the same quality or time - machinery and know-how play an essential role in choosing the right partner. In the following we present a highly simplified overview of the most important processes. Depending on the type of PCB, its structure and material, the actual process can vary greatly - work steps take place in a different order or frequency or take different lengths of time.
Depending on the end product, the PCBs required by our customers vary greatly in size and complexity. In addition, quantity and delivery date play an important role in the quotation or order processing. As a rule, our customers send so-called Gerber data or layout data. On this basis, the necessary data is created for production. The following parameters, for example, are queried as standard:
- Number of pieces
- Delivery date
- PCB thickness (standard is mostly 1.6 mm)
- Conductor board material (mostly FR4 or special materials)
- Copper thickness (layer thickness between 35 and 300 µm)
- Number of layers
- Desired surfaces (e.g. HAL, electrotin, OSP)
- Solder resist properties and color
Circuit board production is limited by process and machine park; these limits are also referred to as "design rules". A design rule check is performed when the circuit layout is created. During the data check, the customer's information is checked for completeness and production-relevant rules of PCB production. A number of preparatory measures are checked, such as the optimization of milling paths and drilling sequences, the provision of traces with etching allowances, or general plausibility checks such as gaps in the contour. In case of ambiguities, the customer is contacted to clarify remaining questions.
Data preparation includes so-called CAM processing and work plan generation. In CAM processing, the production-ready data is linked with machine data. One application in the program is the linking of drilling and milling data with fitting holes and the associated transcription into machine formats. Work plan generation contains all the necessary information on materials, layer structure, quantities, deadlines, provision of films, film plotting and work steps in production. Multilayers are printed circuit boards consisting of more than two layers. Plotting films is required for the exposure of solder masks, copper structures or other printed images. Plotting is the development of films in dark chambers, which are realized by laser plotters and thus enable a high-resolution image quality. The area of film production is gradually being replaced by modern LDI (Laser Direct Imaging) systems in order to achieve an even higher resolution and to produce in a more resource-saving way.
The components of mechanical processing are packaging, drilling and deburring. A commonly used base material for printed circuit boards is the so-called FR4, a glass fiber mat impregnated in epoxy resin. This FR4 and other base materials such as Teflon or ceramic are supplied by the manufacturers in large dimensions and must be cut to a suitable size. This cutting is usually done with impact shears. The blanks are stacked on top of each other, drilled on two sides and fixed with pins. Stacking makes it possible to drill several blanks at the same time during one drilling operation. When drilling the stacked PCB blanks, a distinction is made between drilling through holes, blind holes and buried holes. In general, the PCB blanks are fixed on a drilling machine and the drilling program is run. Here, the machine varies according to the hole diameter and regulates the drilling speed, swap-in speed and rotational speed. Following the drilling process, the drill burr must be removed. The pins are loosened, and brushing machines and grinding rollers deburr the plate blank. This, in addition to deburring, cleans and roughens the copper surface, which improves the adhesion of the photoresist.
The roughened copper surface is covered with a UV light-sensitive photographic film. The plotted film is aligned to match the drill image and exposed or fed to the LDI process. The exposure results in the photoresist curing in the exposed areas and exposing the copper in unexposed areas during the development process.
The galvanic process is the application of a galvanic copper layer. In this process, the circuit board is energized for a certain cycle time and a galvanic copper layer builds up in a hole.
A short tin is applied to areas where the copper conductive layer has been exposed, which protects the copper conductive layer of the conductive pattern during the subsequent etching process.
Etching is the removal of the excess copper. Here, the PCB passes through an etching solution and the excess copper layer is etched away.
The removal of the metal resist, also called stripping, is realized by a salt,- and nitric acid solution. The short tin is dissolved and the circuit board is technically functional for the first time.
The protective lacquer, also called solder resist, is applied to areas on the PCB where no solder is desired. Here, the tin can now no longer adhere and a possible formation of tin bridges is prevented.
The solder resist is usually green and gives the PCB its characteristic color. Since PCB assembly used to be done by hand and green is a color that is tolerable to the eye, this color scheme was established. Today, the PCB can be manufactured in many colors.
Surface plating includes the steps for coating the exposed copper surfaces to ensure optimal soldering.
Possible methods of plating include the HAL process or chemical methods using the elements tin, silver, nickel/gold and palladium.
The placement print has no electrical relevance and serves only as orientation. The standard colors are white and yellow, as these are easily visible on the typical green solder resist. The process for placement printing is usually realized via screen printing. A light-sensitive ink is applied to a screen, which is exposed via a film. The exposed area hardens, and the excess area is rinsed off with water. Any unexposed areas are printed so that the color pushes through white or yellow and becomes visible on the circuit board. The screen is aligned to match the PCB and the ink is forced through the screen meshes. Digitization also takes place in this area of assembly printing in the form of inkjet printers that produce more efficiently.
The electrical test can be performed via two options, adapter test or finger tester.
In the finger tester, the netlists are created based on the layout information. These netlists are checked for open spots, short circuits and conductivity.
In the adapter test, adapter plates are drilled and contact needles are mounted between the plates. The contact needles scan the PCB contacts according to the created netlists and check the PCBs for functionality.
The separation is made possible by three technical processes, punching, scribing, milling or combinations of these processes.
Die cutting involves punching printed circuit boards from the manufacturing blank. The disadvantages of this process are the high cost of the punching tool. The advantage is the high speed of separation.
In scribing, the printed circuit board is clamped on both sides between two scribing disks and scribed along the contour. To use this method is preferable for small PCBs, because between the small plates no spacing is needed and the area of the payload can be used completely.
Milling involves drilling non-contact holes and milling along the contour. This method of singulation is very precise, but also slow and costly.
Before shipment to the customer, the PCBs are inspected for scratches, contour deviations and defects in the solder resist as well as in the solder surface. Printed circuit boards that do not pass the inspection are sorted out and the raw materials such as tin and copper, as well as the epoxy resin that forms the base material for the printed circuit board, are stripped and recycled.
The circuit boards, which are checked and functional after the final inspection, are either stored or prepared for shipment to the customer.
Depending on the size, type and sensitivity of the PCB, it is either packed in bubble wrap or shrink-wrapped in airtight film.