Simply defined, a Printed Circuit Board (PCB) is a device that interconnects electronic components mechanically and electrically. These components are mounted through holes in the board or on the surface of the board and are held in place with solder. The electrical connections to the components are achieved by tracks which are etched on the boards, and through conductive plating in the holes, which carry the current from one layer to another.
The Cycle
Manufacturing PCBs consists of more than 100 individual steps, involving a variety of expensive raw materials, and employing mechanical, chemical, electrical and graphic imaging processes and skills. All major steps specific to manufacturing PCB are shown in the following flowchart:
The track sides, solder mask & component legend images are prepared on CAM from the CAD data supplied. Generally some of the work done includes:
- design rule check;
- minor design edits including enlargement of solder mask pads;
- step & repeating of images to fit within suitable size panel;
- addition of test coupons and plating thieving pattern.
Films are then laser photoplotted using 6.35um (1/4 mil) resolution on 175um (7 mil) thick film for stability. These films are duplicated on diazo film for use as phototools in imaging.
If images are supplied as taped artworks or penplots, a reduction film is made before Step & Repeating on a computerised camera.
(Refer to CID-013 as a guide to documentation requirements)
The copper-clad laminate sheets are cut into panels before being baked to remove stresses and moisture and to ensure curing.
The panels are then stacked and sandwiched in between a sheet of entry material on top of the stack and a piece of backup board on the bottom. These stacks are then pinned together with 2 pins located on opposite ends of the panels. The limiting factor to stack height is the smallest hole size and the laminate thickness. As a general rule a height to hole size aspect ratio of 5:1 is used with a maximum stack height of 5mm.
The entry material serves several purposes. It provides a soft material for the drill bit to enter before hitting the copper-clad panel thereby improving accuracy and preventing burrs. It also cools the drill bit and removes debris from it when retracting, improving the finish of the next hole. The backup board helps to prevent burrs from forming on the bottom panel and protects the drill table.
The stacks are positioned to the computer numerically controlled (CNC) drilling machine by locating the pins to the bushings built into the machine tables.
Drill data which includes the X-Y coordinates and hole size information is loaded to the drilling machine by punched paper tape, floppy disk or directly through a local area computer network connected to a CAM station.
Plated through holes (PTH) are drilled oversize allowing for the thickness of the plating. Non-plated holes are processed in several ways, depending on the number of holes:
- tenting holes with dry film resist.
- plugging holes with a rubber plug.
- secondary drilling after plating.
4/ Electroless Copper Plating (not for single-sided boards)
The drilled panels are then scrubbed to mechanically clean the copper surface, before being placed in racks and processed through a series of chemical baths. The chemicals remove organic contaminants and clean the copper, before activating the epoxy glass on the walls of the drilled holes so that a thin 2.5um coating of copper can be deposited.
This copper coating is not sufficiently thick to carry the electrical load, but it provides a metallised base upon which additional copper can be electrolytically deposited.
In imaging, the circuitry is transferred to the panels. The first step is to hot roll laminate an ultraviolet light sensitive dry photopolymer resist to the panels.
After lamination, the panels are placed in a UV printer frame with the diazo phototool positioned to the panel using tooling pins for location. The clear areas of the phototool allows the UV light to pass through and expose the resist whilst the red areas block out the light.
Exposed using Diazo Phototool Positive diazo phototools, with the circuitry in red, are used for double-sided and multilayer boards, whilst negative phototools with clear circuitry are used for single-sided boards. The panels are then placed in a conveyorised developing machine which sprays a solution onto the panels to dissolve the unpolymerised resist, that is the unexposed areas.
The photopolymer resist will act as a plating-resist (circuitry open) when using a positive phototool or as an etch-resist (circuitry covered) when using a negative phototool.
6/ Pattern Plating (not for single-sided boards)
The imaged panels are clamped in plating racks and immersed in a series of chemical baths to clean and prepare the copper pattern and holes.
Next, the panels are immersed in a copper plating solution. The solution and panels are electrolytically charged with opposite polarities, which causes the copper ions to migrate from the copper anodes to the uncoated copper circuitry and into the holes. Copper is plated to a thickness of 25um.
Next, the panels are immersed in a tin plating solution where the circuitry pattern and holes are again electroplated. Tin is plated to a thickness of 5-10um.
The plated panels are then immersed in a solution that strips away the photoresist, revealing the original copper.
After the resist is stripped, the panels follow on into a conveyorised spray etcher, where the sprayed chemical etchant attacks and dissolves the uncovered copper.
The tin which covers the tracks and holes acts as an etch resist protecting the circuitry from attack.
The tin is then chemically stripped from the copper, revealing the plated copper on the circuitry pattern.
Single-sided boards are etched first, before having the etch resist stripped. Having not been plated, they do not require solder stripping.
Solder mask is then applied to the panel surface. The mask covers the circuitry except pads and holes. It is applied directly to copper, hence the acronym SMOBC (Solder Mask Over Bare Copper).
There are three basic types of solder mask materials available:
- Screen-printable epoxy inks.
- Liquid photoimagable (LPSM) inks.
- Dry-film (DFSM) material.
Advantages and disadvantages together with methods of application for each type is presented in CID-022.
Some boards are designed with contacts that mate with an external connector. These contacts require to be plated with nickel and/or gold. Solder or bare copper do not provide reliable contact surfaces.
The first step in the gold plating process is to mask off all parts of the panel except the area(s) to be plated. A special tape which conforms to the geometry of the circuitry is used, to prevent plating solutions from creeping underneath. The copper is then chemically cleaned before being electroplated with nickel and/or gold. The thickness of each varies according to specific needs. After plating the tape is removed.
To facilitate soldering of components at the assembly stage, solder is applied to the exposed pads and holes.
Application of the solder is by dipping the pre-fluxed panels into molten solder. The solder coats all copper surfaces not covered with solder mask. The excess solder is removed with a blast of hot air, a method called hot air levelling (HAL).
The component legend image is made into a stencil which is applied to a screen. Epoxy ink is then printed to the component side(s) of the panels through the screen. After printing the ink is cured.
The legend, which is comprised of component outlines, symbols and numbers is an aid to the component assembler and to test or field service personnel.
The panels are now machined into individual boards using a CNC routing machine. The complete outline including slots, cutouts and notches are routed to the required dimensions.
Electrical testing for open and short circuits is the first inspection step for finished boards. To electrically test, a fixture is constructed by drilling two pieces of clear plexiglass with the same pattern as the drilled holes in the board, minus where possible via holes. For SMT boards, a pattern of the component mounting pads is also drilled. Spring loaded metal pins are then inserted in each of the holes of the test fixture to make contact with the corresponding points on the board to be tested. The opposite ends of the pins are connected via the electronics of the test equipment.
The boards are then subject to a visual inspection for overall appearance and other cosmetic factors. They are also checked for mechanical dimensions and hole sizes. Some boards are also subjected to destructive and non-destructive laboratory tests.
The inspected boards are then sealed, packed and delivered.
The following is a work-in-progress valuation table to be used as an approximate guide to the value of boards at any given stage.
After
Single-Sided
Double-Sided
Multilayers
Drilling
E'less Copper Plating
Imaging
Pattern Plating
Stripping & Etching
Solder Mask
Solder Coating
Component Legend
Machining
Final Inspection
38%
NA
64%
NA
80%
90%
92%
96%
99%
100%
34%
44%
64%
76%
82%
92%
94%
97%
99%
100%
48%
58%
74%
84%
86%
95%
96%
98%
99%
100%
Contact our Sales staff if you require further information or if you wish to visit our manufacturing facility.
