In other topics we have been looking at various types of component in terms of circuit function and construction, with quite a bit of information on the packages used. The purpose of this topic is to indicate the ways in which your choice of component affects the selection of assembly process, and consequently has an impact on both the board design and the specifications of both board and component.
The choice of assembly process has to be made on an individual basis, and depends most critically on the characteristics of components – their availability, cost and mechanical format and their ability to withstand processing. Some of the many possible process routes are shown in Figure 1 overleaf: the option selected will also be influenced by design constraints, the capabilities of the equipment and the requirement to balance the line.
The choice of route is primarily determined by whether or not there are any through-hole components. The choice is particularly straightforward if you have:
Where there are relatively few through-hole components, and the circuit is otherwise heavily populated with a wide variety of surface mount components, your choices lie between hand soldering, selective wave soldering or intrusive reflow. As Table 1 comments, these last two processes are not always available to you – you certainly need to consult your assembler to understand their preferences and recommendations.
|assembly process||hand soldering||wave soldering||reflow soldering|
|through-hole||can be used for all types of through-hole component||
all but those with very heavy leads can be wave soldered, but the parts need to be held down during soldering
|surface mount||difficult, especially for small components: used only for rework||feasible for smaller components, provided that they are first glued in place||preferred process, which can be carried out on both sides of the board|
|mixed technology||prototypes only||many Type III assemblies use reflow on the top side, and wave soldering on the underside|
|where through-hole components are few or large, use double -sided reflow, then hand soldering or selective wave soldering†|
† indicates processes that are not available from all assembly houses
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The board design is heavily influenced by the type of component, as indicated in Table 2. Generally through-hole components are confined to low-density applications and relatively simple boards, but the comments in Table 2 on traces and layer count would not necessarily apply if the through-hole components were complex connectors: in that case many layers of interconnect may be needed, purely in order to provide access to all the pins.
|board issue|| device
each lead has to be associated with large through-hole, but dimensions are critical
need clearance around lead/body for insertion (especially automatic)
radial types need allowance for lead bending1 radius
potential for connections from lead on every layer
generally low density Þ relatively simple board Þ wide tracks/gaps
|low density Þ relatively simple board leads can bridge over traces and connect between layers, further simplifying designs: single- layer can be feasible|
soldered area usually independent2 of through connect area of pad (and solder paste)
determines volume of solder joint, and thus dimensions are critical.
inter-layer connections made by vias, not leads
generally high density Þ relatively complex board Þ narrow tracks/gaps
|high density Þ greater complexity, so more layers of finer traces|
1 Wires should be clamped next to the body, then carefully bent, in order to avoid internal shock damaging the component.
2 The ‘via-in-pad’ exception to this will be considered in the Technology Awareness module
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As indicated in Table 3, the choice of component reflects on the choice of assembly process, and consequently on the board specification. Higher specifications are needed for reflow soldering than for hand soldering and wave soldering. As a result, most designs for reflow soldering use epoxy-glass laminates (such as FR-4), whereas commercial applications with through-hole components will use the cheaper phenolic-paper or composite types wherever possible.
|board spec|| laminate
|assembly process||component type|
| hand soldering
||through-hole||laminate only has to withstand quite low temperatures: can use paper- phenolic types||only has to be marginally solderable, as hand process can ‘scrub’ the joint surfaces|
| wave soldering
||through-hole and mixed technology||laminate has to be flat, but only contacts the wave for a short period, so can use most paper-phenolic and composite boards, provided that they are flat||preferably should be easily solderable, but process can use relatively active fluxes, provided that residues are removed|
| reflow soldering
||surface mount||whole volume of laminate has to withstand reflow temperatures for an extended period: requires epoxy-glass laminate or better||surface has to be both solderable (limited time available to reflow) and flat (for paste printing)|
In the same way, surface finish is dictated by the use: a Hot Air Solder Levelled (HASL) finish is adequate for processes involving exposure to quantities of liquid solder, whereas reflow soldering benefits from a flatter surface, particularly for fine pitch components. There is more discussion on this in Board Finishes.
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Components need to be able to withstand the assembly experience! Depending on the type of component and choice of process, this will involve some exposure to liquid solder and flux, and may involve immersion in quantities of liquid solder and a cleaning medium (water or solvent based).
Although the reflow process is carried out at a lower temperature than wave soldering, the time of exposure to liquid solder is much longer, and the whole of the component body reaches the soldering temperature, both of which impact on the temperature rating of the part.
Components are usually rated in terms of their ‘resistance to soldering heat’, tests of time against temperature which are carried out to international standards. Typical ratings are 10s at 260ºC and 60s at 235ºC, for wave soldering and reflow soldering respectively (Table 4).
Other soldering constraints affect both the choice of materials and the physical construction of the part. For example, the wipers on potentiometers and trimming capacitors must protect the internal wiping parts from molten solder, and cleaning solvents must either be excluded entirely or allowed such free access that complete cleaning is ensured. DIP switches are an example of components which may be supplied with the top face of the part covered by a removable tape, which provides a contamination barrier during assembly.
|component rating||temperature rating||metallisation||packaging|
hand soldering without cleaning is suitable even for sensitive ‘non- wet’ components
wave soldering needs 10s at 120°C rating
solder immersion needs 10s at 260°C rating
no special requirements for hand soldering
machine soldering needs better solderability than hand
automatic insertion needs reel or bandolier for taped components; tubes for DIPs
otherwise can come in any form convenient for manual assembly
|surface mount||needs 235°C/60s rating||
needs good wettability
exposure to molten solder is far longer than with through-hole
|wide range of standard tapes and trays suit all components; sticks also for smaller volumes|
|mixed technology||depends on soldering process (see above)|
The metallisation of the component termination is not usually an issue, provided that components are solderable, and resistant to exposure to molten solder. However, some metallised ceramic components, where contact is made to a surface which is not wholly metal, have a tendency to lose their solderability very quickly. As will be explained later in the module, special care has to be taken in processing, and use of these components may impact on the choice of solder materials.
Table 4 also includes some comments about ‘packaging’, referring in this case not to the format or construction of the component body, but to the way in which the finished components are presented to the assembly equipment.
Where it is decided to clean the assembly after soldering, there are two main implications for the components:
Variable components such as switches present the most obvious challenge from the point of view of cleaning agent ingress because contamination carried into the device by the cleaning agent can result in intermittent switch action, particularly if circuit voltages are low. For this reason, a number of products are supplied with a cover tape which has to be peeled off after all assembly stages have been carried out. The alternative, for sensitive components which are not sealed, is to treat them as what the assembly industry refers to as ‘non-wets’, that is adding them at the very last stage, and hand-soldering the leads, and at most cleaning just the pad areas by local application of cleaning agent.
Your company is redesigning an early 1990s computer-based product that used only through-hole parts on a six-layer PCB. The microprocessor was a 132-lead Pin Grid Array, with thirty support ICs in dual-in-line format; there were many connectors, several crystals and some board-mounted switches; the integral power supply contained a transformer and aluminium electrolytic capacitors.
The size of the new board is at your discretion, the product dimensions being determined by other factors. All the components are now available also in surface-mounted format except the transformer and large capacitors in the power supply, and the special-purpose crystals.
Describe to your colleagues some of the options available for the components, and explain the implications of this choice for board design and for component and board specification. If you are able, feel free to suggest areas for circuit redesign which would cut cost and/or complexity.
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