Special Requirements for Surface Mount Printed Circuits

All text Copyright Bob Willis, EPS

The introduction of surface mount technology has had a major effect on the electronics industry with a change of materials, component type and size. It has also seen new processes which assembly engineers have had to become familiar with during the board population stage. Generally speaking the processes associated with the fabrication of printed circuit boards for surface mount technology have not changed, so the content of this section should not be read in isolation from the board manufacturing requirements.

The use of SMT has had and will continue to have a direct effect on printed board features, with reductions in track and gap size, plated through holes for both via and component mounting, this will of course effect the price and should be discussed with the manufacturer for possible alternative design considerations.

Although the basic manufacturing method for fabricating printed boards has not changed radically, the process that are used for the assembly stages have changed which may in some cases require changes to the methods of manufacture or finished quality of the basic board. The direct involvement of the printed board manufacturer, during each stage of the  project will allow a better understanding of the final bare board requirement.

The new assembly stages which board are required to pass through are screen printing of solder cream, component placement and reflow by vapour phase or infra red. The use of wave soldering. techniques for mixed technology boards will also be prominent for many years as this can be a very cost effective method of board assembly. The new process stages, the component mixture and the type of working environment have affected the requirement of the printed circuit board. To provide a better understanding of the particular requirements of modern assembly methods and to provide a better understanding of the processes, which may be new to many board manufacturers, it is essential that suppliers and users have close contact during each stage of a project.

PCB SUBSTRATE MATERIAL

The selection of a substrate for surface mount applications will depend on the electrical requirements, the component type and the operating environment that the final assembly will have to withstand. Standard laminates like glass epoxy FR4 are widely used for most applications in surface mount products, it is only were the use of specialised components like leadless ceramic chip carriers (lCCC), that further consideration needs to be given to the substrate.

 The use of this package in situation were the assembly is going to be subjected to either temperature or power cycling will require an alternative board material if failure of the solder joints is not to occur. The material used for LCCC packages has a low thermal coefficient of expansion (TCE), the glass-epoxy resin system has a TCE of approximately three times that of the ceramic causing a thermal expansion mismatch between the two surfaces. The temperature which the board may be subjected should be considered in detail, it is not only the operating environment which needs consideration, but the effects of power cycling , storage and transport. The mismatch will in time lead to failure of the solder joints formed between the LCCC and the base substrate.

In situations were alternative components can not be used the designer will insure a extra cost in the basic board for alternative substrates to overcome this problem. Two basic alternatives exist the use of a board with a compliant elastomeric layer, which may be bonded directly to a standard laminated like FR4, the layer forms a compliant surface between the laminate and solder joint connection, this will in certain environments absorb the stresses imposed. The second choice is the use of matched materials which will expand and contract at the same rate as the LCCC. Examples of substrates which are currently available are epoxy/polyimide kevlar, polyimide quartz and copper-invar-copper. The use of these materials should be considered in detail prior to their selection due to the cost involved and the difficulty in board fabrication.

ARTWORK AND DRAWINGS

The drawings and possibly the artwork are generally the first opportunity for a manufacturer to see the type of printed board he has been asked to produce, certain features on the circuit may change and often the tolerance requirements are different for SMT boards.

In many cases, on surface mount boards additional information and restrictions are required, which must be fully documented to prevent any confusion at a latter stage. In the case of special processes like solder levelling they should in the first case be discussed with the manufacturer and specified as a requirement for a finished coating.

The tolerance requirement on actual board size may require further documentation due to the requirements of automatic handling during assembly and in process transport. A tolerance is generally required on two opposite sides of the panel to provide parallel surface to aid smooth handling.

The artwork for the manufacture of any printed circuit board is the single most important tool to produce a quality circuit board, it is also the main area which may cause tolerance errors on the final product. To provide the optimum quality photo tooling is advisable to allow the manufacturer to produce his own master artwork directly from the customers CAD (computer aided design) system, the information can be supplied direct on magnetic tape or floppy disc. Information supplied in his format also allows the circuit to be step and repeated to provide the most economic to manufacture panel, the added benefit of this process also enables the printed board to be supplied, if required  in a multi-panel to provide ease of manufacture during assembly.

The use of direct photoploted artwork has now become a standard for new designs which require greater control of the basic tolerances. Surface mount processing has also the requirement for extra artwork, in the case of reflow were solder paste is to be applied as part of the assembly stage an artwork is required for the basic pad footprints to make the screen.

TRACK PATTERNS

To take further advantage of the reduction in size offered by surface mount technology the basic track sizes are continually under review, at present 0.006-0.008" is commonly the minimum track size requested for board tracking. As the design size decreases more considerations are required at the design stage to make manufacture of the basic board.

For close tolerance requirements the use of thinner foils, half ounce (18 microns) on the basics laminate become a necessity for fine lines, with the move to tracks less than 0.008" thinner foils, quarter ounce (9 micron) may well become common place, but at a cost.

The size of tracks and pads on surface mount boards is continuously under review in an attempt to improve the soldering yields and decrease the size of the design. The reduction of geometry on the basic board will effect its producability and should be discussed with the manufacturer as this may well effect the final price. It is often the case that savings on the product price may be obtained by early discussions with the circuit supplier.

HOLE SIZES AND VIA SIZES

The introduction of surface mount components has seen the requirement for smaller hole sizes, this has not been the case for mixed technology boards were the requirement for leaded components still limits the choice on final hole size. The actual conventional component termination does allow for hole size reduction, but as more companies are introducing automatic insertion this is the limiting factor. Due to the range of tolerances involved with boards, insertion equipment, components and the programming a average size increase of between 0.4-0.5mm to the lead size is required providing a range of finished hole sizes of 0.85-1.1mm. The pressure to reduce hole sizes is all in the area of via holes, in most cases the common sizes presently used are 0.5-0.6mm with the common requirement to reduce this figure down to 0.4mm and beyond.

The introduction to landless via hole has provided some area of improvement with the elimination of the pad area allowing more open area for track routing, this again needs consideration due to the tolerance capability of the process. The introduction of small holes does require consideration as the size and use of multilayer boards is now on the increase,  the aspect ratio of the hole size to finished package thickness will have a dramatic effect on the ability to drill and plate a reliable through hole interconnection. A general guide to the aspect ratio for manufacture is a 5/6-1 ratio of hole size to package build.

SOLDER RESIST MASKS

There are three main types of solder resist commonly in use today, the first is a two part epoxy system which is screen printed directly to the circuit leaving the required exposed apertures. This process is not particularly accurate for the close tolerance requirements of surface mount designs and does lead to resist bleed on to solderable areas of the circuit. If proper clearance tolerances are provided for the process it defeats one of the prime advantages of surface mount, that of size reduction.

The process which has been shown to provide the accuracy and reproducibility is the use of photo-imagable resist systems. The most popular being dry film resist which is vacuum laminated onto the board surface and then exposed through photographic artwork to ultra violet light. This selectively hardens the resist allowing subsequent removal of the unwanted areas by chemical removal.

Dry film resist is commonly available in three thicknesses, 50 75 100 microns thickness each being laminated to the surface of the board as previously stated. Each of the materials may be used on surface mount boards, the thinnest of these providing many advantages to the assembly operation.

The use of the thinner materials has shown to provide higher soldering yields on both flow solder and reflow solder during the production cycle. The use of these resists does however put limitations on the board designer and circuit manufacturer in the materials and processes that he may specify.

The third material which is now gaining popularity is the liquid photopolymers which may be coated to the board in two ways, by curtain coating or open screen printing. After application the coating is dried to allow further processing to be conducted.

The next steps are the same for any photoimagable system the surface is imaged using photographic artwork and developed to relieve the solder pads. The finished coating thickness may be in the area of 25 microns this is of course dependent on the design and the plating thickness on the board surface. The manufacturing requirement should be discussed with the circuit fabricator to provide a cost effective solution which meets the mechanical, electrical and assembly requirements.

As a guide the tolerance requirements for resist systems are for screen print  liquid systems 0.010" tolerance around the pad area to prevent resist encroachment. In the case of dry film or photoimagable liquid systems a 0.005" window should be left around the pad area this guide will prevent the solderable area being covered by resist.

A further point should also be made that this tolerance should also be left a around test pads, in the case were thick resists are used it is possible for the test pins during test to be deflected and course open circuits to be recorded. It is possible for this to occurs during bare board or final product test.

LEGEND MARKINGS

It is still a requirement of many companies to provide markings on the board to identify selected component positions to provide component identification. This may be used during inspection, repair and test it also in many cases provides the basis for a operator assembly guide. With the general reduction in board size limited board area is now available for markings, it is not possible using existing methods of screen printing to reduce the size of lines and characters to the available area. The use of characters etched into the copper base foil during etching the circuit may in some cases provide another alternative for some applications, it should however be noted that it may effect the operation of the completed circuit.

Close co-operation with the supplier is important as the selection of resists may effect the ease of screen printing legends, the close proximity to solderable area can lead to contaminated pads. Although space is often available under certain components this in many cases leads to contaminated solder pads.

SOLDER FINISHES

Traditionally in the fabrication of printed circuit boards solder resists are placed over the tin/lead finish, either plated or after reflow. This has always been an area of concern to the assembler mainly from the cosmetic point of view. During the assembly operation on flow solder the tin/lead finish would reflow under the resist and cause the protective coating to crinkle, in the worst cases fall off the board surface. With the processes commonly used for surface mount assembly it is common for the hole board surface to be subjected to temperatures above the reflow point of tin/lead.

The popular alternative finish for surface mount boards is to have a selective coating of tin/lead on the plated through holes and pad areas only, this is generally applied by solder levelling, which has become an industry standard. The process provides a coating thickness which is inconsistent across the board surface, but does provide a reliable coating to provide a finish which provide a good solderable coating.

The present requirements for board finishes can be met by solder levelled finishes, but as the electronics industry move away from the common 0.050" component pitch to 0.025" the finish provided by solder levelling will not provide the ideal consistent level coating. As the size of component termination's is decreased the height of the solderable area will become more critical to the assembly operation.

During the screen printing of solder paste the deposit will depend on the height of the tinned pad areas, this in turn will effect the height of placement which may in turn lead to open circuit joints in non contact reflow methods.

Alternative coatings like the use of gold flash over a nickel coating or electroless gold flash provide advantages to the assembler of more consistent coating across the board surface. The finished provides a flat surface to screen print solder paste and place components, it provides a solderable coating which will last during storage and a coating thickness of less than 0.2microns. The thin coating of less than 0.2microns prevents any problems of brittle intermetallic being formed at the solder joint interface leading to joint failure.

A further finish which is used commonly on some products is a flux lacquer or oxide inhibitor which is applied directly to the copper surface tracking to provide a protective coating to maintain solderability. Flux lacquers are one of the cheapest coating that exist it does, however only provide a limited self life to maintain a solderable surface often less than 3 months.

REGISTRATION TOOLING HOLES

With the use of automatic component insertion equipment for conventional printed boards the most important factor is the registration of tooling holes to component holes. Generally holes up to 5-6mm are all drilled in one operation/setting of the drilling machine during the fabrication stage of the board, in this case the hole to hole tolerance is going to be good. The factors which effect the tolerances experienced during assembly are generally due to the processing steps during board fabrication and the instability of the base laminate.

The requirement for surface mount boards is different generally they are smaller than conventional boards although being manufactured in multi formats the dimensional instability will be smaller. The automatic component onserter requires  good registration between the tooling holes and the copper pads, with hole drilling being one operation and photoimaging being another it is common for this to be an area of dimensional inaccuracy. The registration of a second artwork for resist application may also lead to a build up in the registration error. These examples show the possible errors which can occur which require consideration during the design to provide suitable tolerances to be included to the board layout.

For surface mount assembly there are requirements for further tooling holes too be included with in the board, these are required for registration during screen printing and component placement each must be specified for its size and be unplated. Generally the same requirements exist for tooling holes for conventional assembly during automatic insertion.

BOARD SIZE AND MULTI PANELS

The actual board size and shape are determined by the final equipment and its application, due to the considerable variety most companies are adopting standard board sizes due to the high cost of tooling for individual boards. Traditionally each board was designed as a single part with the artwork supplied to the printed circuit manufacture who would produce artwork and drilling tapes to make the board in a multiple panel. The circuit would be laid out to make the most cost effective use of his standard panel size, a common one being 18"x24".

The common approach in order that time can be saved during assembly is to take a multi panel direct from the circuit supplier with the individual board shapes routed in the panel, but still held in place by tabs of the base material. The finished board after it has been fully assembled and tested can be easily removed, this approach eliminates the handling of difficult sized boards and reduces the tooling requirements of assembly. The design of the multi panel should be discussed in detail, as the optimum layout for board manufacture may not be ideal for the assembly process.

BARE BOARD TESTING

Modern printed boards justify that circuit testing be conducted at the earliest stage in the manufacture cycle the increasing density of printed circuit boards and the move to surface mount assemblies has proved that the final test of individual boards is a necessary requirement. As the density increases the difficulty to detect failure points on the finished assembly becomes more evident, if the cause of failure is a fault on the basic board then it may be impossible to detect. It is now imperative that any defect which may be present on the circuit must be detected, the board must be confirmed to be satisfactory before further assembly processing.

In the case of specifying bare board test it is important that it be discussed in detail so that the individual requirements are understood, 100% bare board testing does not necessarily mean that full circuit tracking have been 100% tested.

For further information on EPS services Email Bob Willis or visit his Internet Home Page.

Email: eps@bobwillis.co.uk
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