Surface Mount Technology in the Educational Environment
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Electronics Technical Sameul Ginsberg Surface mount technology in the educational environment by Samuel Ginsberg, University of Cape Town The consumer-driven demand for electronic devices to become smaller, lighter and more functional has been driving electronic components into ever smaller packages. The early development of integrated circuits mount technology (SMT) has brought great This article proposes a shift in thinking and focused on increasing the number of transistors variation in the physical size and shape of suggests methods of implementing that shift. per package, leading to very large scale devices and is commonly believed to require The shift in thinking is that SMT must be embraced integration (VLSI) devices but in the last few great skill in handling because of the smallness wholeheartedly by educators, an investment in decades the limitations of traditional through- of the connections that it requires. equipment and skills must be made and the full hole integrated circuit packaging have SMT has been seen as unfriendly in the benefits of this technology reaped. become clear and industry has moved steadily educational environment since it became The differences between educational and away from this cumbersome but easily-handled available. Educators actively avoid the use of production environments technology. Educational institutions have been surface mount devices (SMDs) in their teaching slow to follow this trend and now face increasing work and some educational institutions actively SMT is well accepted in commercial products, limitations in the variety of usable devices. avoid stocking SMDs in their component stores. and many designers prefer to use SMDs even when more traditional packaging is Some of the advantages of through-hole The problem with this approach is that in available. In a product of fixed overall size packaging are that it is highly standardised, with recent years it has become untenable. The (such as a replacement board to fit into an few package options and that it is large and variety of through- hole components is steadily existing enclosure) the reduction in size of the robust, allowing quick and cheap prototypes decreasing as older parts reach obsolescence components often leads to easier routing of to be constructed using products such as and the adoption of a lead-free policy might the printed circuit board (PCB). On double stripboard and solderless breadboards. Surface well have contributed to this trend [1]. sided and multilayer boards this advantage is particularly pronounced because SMD's only occupy space on one layer, whereas through- hole components occupy space through the whole PCB stack. On a product which is to be made small SMDs have obvious advantages in component density. Once the board is in production SMDs are cheaper to populate than through-hole components [2]. Pick and place machinery works fast and efficiently and soldering techniques such as and reflow soldering are suitable for fast and cheap bulk soldering of SMDs. These placement and soldering processes incur setup costs. When producing a medium to large production run the setup costs Fig. 1: A 128 pin surface mount package (left) and a 40 pin through-hole package (right) are amortised over a large number of boards occupying roughly the same surface area. and the efficiency of automated assembly results in lower overall costs. In a small run it is often difficult to pay back the initial investment required to automate production. Commercial products are always designed onto a professionally made PCB. Any modern PCB manufacturer will be able to produce boards that can accommodate the majority of SMDs. This is not true of “backyard” PCB facilities, which are often limited to crude coarse pitch boards. Fig. 2: A quad flat pack soldered to a Commercial products are usually designed mass produced carrier board. by experienced engineers. This means that EngineerIT - September 2009 37 Reasons for the adoption of SMT in board. This board will have one or several SMT educational environments footprints and these are connected to header pins with a standard 0,1 inch pitch. The SMD is The primary reason for adopting SMT is the soldered on to the carrier board and the carrier broader range of components that become board is then used in a standard solderless available when SMT is adopted. Many new breadboard or stripboard. The advantage of components are only available in SMT. this method is that the populated carriers can This includes digital components such as be manufactured in moderate volumes by a microcontrollers, microprocessors and their commercial supplier. Coarse pitch devices peripherals as well as analogue components. can be hand soldered to commercially made In addition the frequency limitations of through- carrier boards. Commercial carrier boards can hole components often preclude their use be custom made by the institution which needs in wireless communications systems. This them or bought off the shelf. In the case of prevents educators from making full use of the custom carrier boards it is easy to amortise the 433 MHz, 868 MHz and 2,4 GHz bands for set up costs of the PCB over a large number of student projects. The adoption of SMT opens up Fig. 3: A basic SMT soldering setup. boards for a low cost solution. all of these exciting possibilities. The disadvantage of this method is twofold. Levels of surface mount sophistication Carrier boards do not often offer sufficiently comparatively few attempts are made before The adoption of SMT is not an all-or-nothing good high frequency performance for use the product actually works. Each attempt incurs decision. The wide variety of surface mount with RF and microwave devices. In addition it is full setup costs, as it is fairly rare for a redesign packages range from large pitch devices with inevitable that there will be devices in packages to have enough in common with an earlier exposed solder points to ultra high density ball that cannot be accommodated on the attempt to avoid re-tooling. grid arrays which require sophisticated handling available carrier boards. The educational environment is rather different. and inspection facilities [4]. Even adopting Outsourced PCB manufacture, Generally students only ever wish to make small the most basic surface mount packages will quantities of any product. The lessons that the provide a useful upgrade. As the devices get outsourced assembly more sophisticated the cost of adopting them design contains can generally be learned It is possible to have device-specific boards goes up. At the same time the number of users through one or two boards. Large volumes made up commercially and to have devices who will benefit from the extra devices enabled are rare and can be treated as a production hand-soldered to those boards by companies by the facility will decrease incrementally. This job. On the other hand however, education is with the requisite equipment and skills. Generally indicates that there is a sophistication point often incremental. Students start designing with PCB manufacturers charge an “origination fee” beyond which it is no longer economical to little experience of the practicalities of circuit to cover the processing phase of the circuit service. design and so problems such as marginal board's manufacture. Generally this fee is design, mechanical incompatibilities and other The fineness of pitch of devices with exposed constant per board regardless of the size of the impracticalities abound. This is perfectly normal. solder points has a small effect on the price board, within reasonable bounds. Some PCB As a student gains more design experience they of adopting the technology. Slightly more manufacturers are generous in allowing multiple will begin to evolve a feeling for the realities of equipment is needed to conveniently solder boards to be merged into one large board, electronic design. Often a supervisor will allow fine packages with exposed solder points. provided that future use of the boards will always a student to build a flawed device, in the The level of skill required to solder the devices be in sets, as laid out on the developed artwork. knowledge that sometimes “the hard way” is increases, but it is understood that this develops This requires that the person wishing to use SMT the only way of learning. Because designs are with practice. Devices with hidden pads require need only have access to a computer and disposable in an educational environment it is much more complex equipment, and the cost/ PCB layout software in order to use the devices. benefit ratio often makes this impractical. important that they can be tested in a cheap There are several highly suitable free PCB layout way with short turn-around cycles. A short list of packages in each of these packages available [5], [6]. In education it is common to ignore some categories is shown in Table 1. This list is far from aspects of a problem in order to focus on exhaustive as there are very many surface other aspects. Inexperienced students must mount packages.The decision that must be be protected from “cognitive overload” and made is therefore one of which column of this require that design problems be simplified to table to adopt. Packages Fine pitch Packages with exposed packages with with hidden the minimal working solution [3]. For this reason Possible solutions it is common practice for student projects to pads exposed pads pads have far fewer requirements than commercial Once the decision to adopt SMT has been taken SOIC MSOP QFN projects. Physical size and shape are rarely and the types of devices that are to be used SOT SSOP LGA have been selected then the next decision is specified and students are seldom under SOJ TSSOP BGA pressure to cram their designs into tight space how to implement those decisions.