PCB fabrication sequences


Introduction

So far in this module we have looked at the processes of board fabrication and assembly only in outline, and concentrated on giving you an understanding of the components and materials used within boards and assemblies. Now it is time to turn our attention to the issue of board fabrication.

Our focus is first to understand the sequence of processes by which boards are made, and then in the two next parts to look in more detail at the basic processes involved. Finally, we shall be considering some of the quality issues associated with boards.

In Fabrication and assembly process outline we described the basic processes, and encouraged you to carry out some web research to establish the difference between real processes (with all their process control) and our process skeleton. If you haven’t yet taken steps to get an insight into a fabrication environment, may we again stress the benefit of visiting a board fabricator, and looking at real processes and equipment – there is no substitute for practical experience. Be assured that most fabricators are very favourably disposed towards participating in training of this nature, realising that everything they can do to build bridges between designers and fabricators helps them in the longer run.

The sequences we consider relate to three types of board: single-sided, double-sided plated through-hole and multilayer. This is mostly because there are some similarities between all three types, but additional processes are used as the number of layers increases. The sequence also aligns with the historical introduction of the three types, starting in the 1940s with the single-sided board.





Manufacturing principles

The PCB manufacturing process starts with the ‘process blank’, a large sheet of laminate which will be divided after processing into individual circuits or panels containing multiples of circuits. In order to minimise the area of laminate which is wasted, care has to be taken in selecting the size of blank and the arrangement of the panels on the blank. This is a critical area for cost minimisation, and will be considered in more detail in Design for eXcellence.

There are two main board patterning options which may be used alone or in combination to create any type of PCB:

In a fully additive process, the base laminate would start without a copper covering, and the copper for the tracks be deposited only where needed. This gives, in theory at least, potential for considerable cost and environmental savings, but the processes are complex, and totally additive processes have not yet become common.

However, some additive element is needed to create connections between layers, as the inside of a drilled hole necessarily starts free of copper! Most boards are therefore made using what is referred to as a ‘semi-additive’ process, where the starting point is a thin copper-clad laminate, with holes which are plated internally (additive) and surface patterns which are etched (subtractive).

The remainder of this document looks at the way in which these two options are used to create PCBs. You will already have gained some insight into the materials used from previous study, and will get further information on the techniques involved in PCB fabrication processes.





Single-sided boards

Single-sided boards can be made by a very simple subtractive process, using clad copper laminate as the base material. Etch resist is applied to the board to protect the required pattern, following which an etchant is used to dissolve the copper that is not needed, leaving behind a track pattern and component lands firmly adhered to the surface of the base material.

The main elements of a typical process sequence for a simple single-sided board are:

  1. Screen print liquid etch resist and cure.

  2. Etch exposed copper areas (that is, those not protected by resist).

  3. Strip resist.

  4. Screen print solder mask.

  5. Screen print component identification (‘legend’ or ‘nomenclature’).

  6. Apply solderable finish.

  7. Drill or punch component holes.

  8. Visual inspection and (optional) electrical test.

Depending on the design requirements, Steps 4, 5 and 6 may be omitted.

The technique of applying etch resist is also called ‘print and etch’, and was the process which gave rise to the ‘printed’ in ‘printed circuit board’: the board produced is sometimes termed a ‘conventional’ board.

Etch resist prints are relatively thick in printing terms, and the process generally used is derived from silk screen printing, an art process first developed during the nineteenth century, and today used for printing tee-shirts as an alternative to applying transfers. Stencil printing, as used for solder paste application during assembly, is a very similar process, but is unable to produce other than relatively simple shapes. For example, creating a long L-shaped track would leave a section of the stencil totally unsupported. For the same reason, screen printing is also used in PCB manufacture for applying legend.

Screen printing generally produces boards of only limited accuracy and quality, so a photographic image transfer technique is used for high-density PCBs. The whole board surface is coated with a thin layer of photosensitive etch resist (‘photoresist’), either by a liquid process or as a ‘dry film’. This resist layer is exposed to ultra-violet light through a photomask, so that the areas protecting the required pattern are polymerised and hardened. Unhardened photoresist is removed by ‘developing’, the remaining resist is baked to increase its etch resistance, and the board is then etched.

The process route given in Table 1 describes the process both for ‘conventional’ boards, produced by screen printing, and the more accurate types produced by photoresist techniques– note that PCBs with finer features generally demand more inspection and test, even for single-sided boards.

Table 1: Alternative Plated Through-Hole (PTH) process flows
Non-critical circuits Cut, edge and clean panels
Cut, edge and clean panels
Screen print positive liquid etch resist and UV cure Laminate with dry film resist, image and develop
Etch exposed copper areas not protected by primary resist
Strip primary resist
Brush clean copper pattern
Screen print solder mask AOI scan
Apply, expose and develop photoimageable solder mask
Screen print component indent
Apply solderable finish (roller tin, HASL, nickel-gold)
NC drill or punch component holes
Trim board to final size required
Electrical test (if specified)
Final visual inspection

Subtractive processes can also be used to make double-sided boards, with a double application of resist, and a single etching step. However, in order to improve the alignment between the patterns on opposite sides, photoresist methods are generally preferred, with simultaneous exposure to ultra-violet light, placing the board between a pair of photomasks which have been accurately aligned to each other.

These double-sided boards are rarely a product in themselves, because they lead to assembly problems and have unreliability consequences: when pins or components leads are used to join the two sides, flux and air are trapped inside the hole, resulting in poor joints. However, such double-sided boards, made with a very thin laminate, form the ‘inner layers’ or ‘cores’ from which multilayer constructions are made.





Options for adding metal

Boards generally involve some processes which add copper, both to conductive areas and the inside of holes: more about the different plating processes involved will be discussed in PCB fabrication processes. Regardless of which process is used for plating, when patterning is intended, there are two alternative approaches referred to in the industry. These are:

Panel plating and pattern plating may be carried out either by electrolytic or electroless processes, depending on the material to be deposited and on whether any parts of the area are not already covered by a conductive layer.

As may be seen in Figure 1, there are obvious differences between the approaches in terms of the definition and accuracy of the copper pattern produced.

Figure 1: ‘Pattern’ and ‘panel’ plating compared

Figure 1: ‘Pattern’ and ‘panel’ plating compared

Note that the pattern plating process still needs an etch after patterning, in order to remove the base metallisation or foil.

Self Assessment Questions

Explain the way in which subtractive and additive processes can be combined in the manufacture of PTH boards.

Compare your answer with this one


Plated Through-Hole boards

The most common manufacturing route for plated through-hole boards uses a plating of tin-lead or (now more commonly) tin as a solder resist after electroless plating copper overall:

1 Cut, edge and clean panels
2 NC drill all holes
3 De-burr
4 De-smear and electroless1 copper plate all exposed surfaces
5 Clean board surfaces and laminate primary dry film resist
6 Expose and develop dry film
7 Electroplate approx. 25 µm of copper and 4-10 µm of tin or tin-lead
8 Strip primary resist
9 Etch exposed areas of copper
10 Strip tin/tin-lead deposit
11 Brush clean copper pattern
12 AOI scan if high density
13 Apply photoimageable solder mask and dry
14 Expose and develop solder mask
15 Screen print any component identification
16 Apply solderable finish (e.g. HASL, electroless nickel-gold)
17 Trim board to final size required
18 Electrical test
19 Final visual inspection

1 As discussed in PCB fabrication processes, the alternative of direct plating is becoming more common.

Depending on the design requirements, Step 15 may be omitted, and Steps 10 and 16 may be replaced by reflowing the tin-lead plating to form a solderable solder coat.




Multilayer boards

The final stages of processing multilayer boards are the same as for plated through-hole boards, but what is processed is a ‘sandwich’ of pre-patterned double-sided boards between two blank sheets of single-sided laminate or, more usually, two copper foils. The sandwich is held together, and the layers of foil insulated from each other, by interleaving sheets of ‘prepreg’. Prepreg consists of thin sheets of glass fabric impregnated with epoxy resin which has been only partially cured – so-called ‘B-stage’.

The inner layers, outer foils and prepreg are carefully assembled (a process referred to as lay-up) and heat and pressure are applied to create a single composite laminate, completing the curing process of the epoxy resin.

Inner layers are not visible from the surface, so that it is very important to make sure that these are correct before lay-up to form the multilayer assembly. This is always done visually, in order to avoid damage, usually involving Automated Optical Inspection (AOI) equipment. Depending on the quality constraints, it may be possible to carry out some degree of repair of defective inner layers, but only before lamination.

In order to get the best possible yield, these ‘inner layers’ have their metal surfaces treated in order to improve adhesion. A number of processes are used for this, the traditional ‘black copper’ process, which builds up a layer of oxide on the copper, now being under threat for environmental reasons. After the black copper process, the inner layers must be handled very carefully.

Figure 2: Creating a multilayer board

Figure 2: Creating a multilayer board

The multilayer process sequence is given in Table 2. Note that Steps 1 to 8 need to be repeated for each of the inner cores: a six-layer board will be made from two double-sided cores and two copper foils or outer boards, and involve a total of three double-sided patterning processes

Table 2: Multilayer process flow
1 Cut and clean inner layer cores
2 Laminate primary dry film resist
3 Expose and develop dry film
4 Etch exposed areas of copper (acid or ammoniacal etchant)
5 Strip primary resist2
6 Post etch punch registration slots
7 AOI scan inner layers
8 Surface treat the copper to enhance adhesion (‘black oxide’)
9 ‘Lay-up inner layers’ with prepreg and outer layer copper foils
10 Bond
11 Remove resin spew and clean panel edges
12 Post bond tool NC drill registration holes (located with reference to inner layers)
13 Process as PTH board

2 Wet film types have also been developed. These remain on the board after etching and react with the epoxy resin of the prepreg during lamination, saving several process steps.

The many process steps in making a multilayer board, and the increased opportunity for defects, mean that much attention has to be paid to in-process quality control. Even with this, a multilayer construction is substantially more expensive than double-sided PTH, and the cost increases dramatically with layer count, because of both complexity and reduced yield. Multilayer boards have been made with over thirty layers, but boards with four to twelve layers are most common.

Self Assessment Questions

Explain the way in which inner layers are inspected and treated, and the reasons for this.

Compare your answer with this one



The PCB fab

The PCB fabricator is typically an independent operation, providing a custom product for a range of discerning customers. The nature of the business is that quantities are very variable, from a few-off prototypes to high volume, but they share the fact that each design is specific to its application. This means that little preparatory work can be done, except procurement of materials, and an order is fulfilled by starting one or more batches and taking them through a production process.

The nature of the processes is that some use of conveyors between stages is possible, such as between exposure, development, etch and strip, but process blanks and panels are typically handled in batches using cassettes. Conveyors are typically of the brush or roller variety, and capable of handling a range of sizes. In contrast to those that you will see in an assembly environment, board conveyors are typically not involved in the alignment process, and do not have to be adjusted between batches.

Mechanisation within the plating areas is dictated by the need to make good electrical contact with the board. Plating processes involve immersion in a succession of baths, which is carried out by automated handlers with a fair degree of flexibility and intelligence.

The turn-round on the entire process will depend on volume, but may be as low as a few days at the prototype stage. In fact, some companies are able to produce even quite sophisticated boards in less than 72 hours. Commercially there is always a significant premium for fast turn-round batches, as these will take priority over other production throughout the process sequence.

The combined consequence of batch operation, of yields that are always less than 100 per cent, and of the customer’s need to have a defined number of boards, particularly at the prototype stage, is that most companies slightly over-make, and will retain surplus boards for subsequent sale. However, storage limitations frequently mean that surplus parts, and even master artwork, may only be retained for as little as six months.

The PCB factory is typically quite large, and has a number of processes with safety implications. Moving machinery, as in drills, are obvious hazards, but these are easily guarded, and the main safety issues relate to the chemical processes. Even copper plating involves sulphuric acid (a hazard to people) and a copper salt which is fairly poisonous, and the copper itself, (classed as a ‘heavy metal’) is hazardous waste. Be aware that major costs are incurred by the PCB manufacturing industry in meeting the requirements for effective and environmentally-friendly effluent disposal.

The starting points for the fabricator are laminated foil, prepreg, chemicals and photographic materials, all of which are purchased. You already have some information on laminate, foil and prepreg: photographic materials are a commodity item, but be aware that the chemicals used are not straightforward chemicals that one might buy from a laboratory supplier. A number of companies have made a very significant investment in producing groups of chemicals and other materials for the processes involved, and these proprietary items are not interchangeable – each board supplier you contact will have made different decisions about the specific materials used, and these may have significant implications for the quality and reliability of the end product. The most significant differences will be found between materials intended for electroless plating.

Activity

As was pointed out in Fabrication and assembly process outline, you need to have a very firm grasp of fabrication sequences for the main types of boards. If you have not done this earlier, we recommend that you do some browsing to establish the process flows used by typical manufacturers. As an example, you can take the plant tour at Proto Engineering3, which will show you the kind of equipment used. If you are looking for a starting point, there are some nice long lists of fabricators and assemblers at Surfinbox4.

We recommend that you don’t be seduced by the pretty pictures, but try and generate flow diagrams showing the sequence of process steps. Hopefully there will be many similarities between different companies! You should, however, expect there to be some differences, reflecting whether the company’s primary focus is in prototyping or in volume production. There will certainly be differences in the materials used.

Comparing our process flow sequences against real life should also illustrate that most companies introduce more QA stages than our simplified outline. This helps protect yield as well as ensuring satisfactory overall quality.

3 http://www.protoengineering.com/plant_tour.htm

4 http://www.surfinbox.com/dir.php/Top/Business/Industries/Electronics_and_Electrical/
Contract_Manufacturers/Printed_Circuit_Boards