Surface mount technology in the educational environment by gyvwpsjkko

VIEWS: 16 PAGES: 4

									                                                                                                                                   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
away from this cumbersome but easily-handled                                                                The differences between educational and
                                                        available. Educators actively avoid the use of
technology. Educational institutions have been                                                              production environments
                                                        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
                                                        variety of through- hole components is steadily     (such as a replacement board to fit into an
few package options and that it is large and
                                                        decreasing as older parts reach obsolescence        existing enclosure) the reduction in size of the
robust, allowing quick and cheap prototypes
                                                        and the adoption of a lead-free policy might        components often leads to easier routing of
to be constructed using products such as
                                                        well have contributed to this trend [1].            the printed circuit board (PCB). On double
stripboard and solderless breadboards. Surface
                                                                                                            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
                                                        the most basic surface mount packages will              Outsourced PCB manufacture,
Generally students only ever wish to make small
quantities of any product. The lessons that the         provide a useful upgrade. As the devices get            outsourced assembly
design contains can generally be learned                more sophisticated the cost of adopting them
                                                                                                                It is possible to have device-specific boards
through one or two boards. Large volumes                goes up. At the same time the number of users
                                                                                                                made up commercially and to have devices
are rare and can be treated as a production             who will benefit from the extra devices enabled
                                                                                                                hand-soldered to those boards by companies
job. On the other hand however, education is            by the facility will decrease incrementally. This
                                                                                                                with the requisite equipment and skills. Generally
often incremental. Students start designing with        indicates that there is a sophistication point
                                                                                                                PCB manufacturers charge an “origination fee”
little experience of the practicalities of circuit      beyond which it is no longer economical to
                                                                                                                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.
important that they can be tested in a cheap            benefit ratio often makes this impractical.
                                                                                                                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
the minimal working solution [3]. For this reason                                                                with exposed      packages with           with hidden
                                                        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
specified and students are seldom under                 have been selected then the next decision is
                                                                                                                 SOJ               TSSOP                   BGA
pressure to cram their designs into tight space         how to implement those decisions. We discuss
                                                                                                                                   QFP                     TFBGA
envelopes. In addition the number of potential          a range of options here.
                                                                                                                                   TQFP                    CSP
failure points must be reduced to facilitate
                                                        Carrier boards                                                             MQFP                    CASON
easy debugging. This reduces the pressure on
educational facilities to adopt surface mount           The cheapest and simplest option is to have                     Table 1: Three levels of sophistication in
technology.                                             commonly used SMTs soldered on to an adapter                          surface mount packaging.



38                                                                                                                                        September 2009 - EngineerIT
                                                     useful as it allows more precise usage on fine         of devices may become connected together.
                                                     joints. Generally 0,5 mm diameter is the finest        Solder wick may be used to remove the excess
                                                     solder available. Liquid flux is critical to making    solder. Plunger type desoldering pumps are not
                                                     solder “take” well to components, bearing in           suitable for surface mount work. Small tools such
                                                     mind that most soldering techniques require            as tweezers and a scalpel are often supplied
                                                     the solder to be applied to one surface only           with the hot air station. According to preference
                                                     and then reflowed (re-melted). In order for the        solder paste may be used instead of wire solder.
                                                     reflowed solder to form a proper connection            There are advantages and disadvantages to
                                                                                                            both forms of solder. Experimentation is key.
                                                     to the previously dry metal flux is required. It is
                                                     advisable to clean all soldering residues from         Techniques for soldering surface mount
                                                     the PCB once soldering is complete because             components vary. There is a large and highly
                                                     the fine pitch of the components makes them            informative body of information on the Internet,
                                                     more susceptible to bridging by contaminants.          complete with videos demonstrating a variety
                                                     Contaminants may stick in residual flux if it is not   of techniques applied to a variety of different
                                                     removed. If too much solder is applied then pins       package styles [7]. As with any unreviewed



          Fig. 4: Protomat S42 rapid PCB
               prototyping machine.




  Fig. 5: An example of what can be achieved. The
     spacing between the centres of pins on the
                central SMD is 0,5 mm.




Outsourced PCB manufacture, in-house
assembly

For a moderate investment it is possible to
purchase the equipment needed to solder all of
the devices mentioned in the first two columns
of Table 1. A typical setup is shown in Fig. 3.

The equipment consists of a fine-tipped soldering
iron, a hot air station and a variety of simple
tools and a few consumables. The soldering iron
should have a 0,25 mm conical tip. 0,25 mm
is the finest integrated circuit pin encountered
in this level of surface mount device. The tip of
the soldering iron should have good thermal
transfer properties, not all are equally good.
The soldering iron must have interchangeable
tips as the finer tips do wear down with use.
Variable temperature control is essential,
particularly when looking ahead towards lead-
free soldering. The hot air pencil needs to have
variable temperature and variable air flow
control. The locally made station shown below
is popular, low cost and effective. Fine solder is


EngineerIT - September 2009                                                                                                                               39
                                                                                                              very helpful for this. Industry has a range of highly
                                                                                                              sophisticated options for circuit board inspection,
                                                                                                              but generally they are not appropriate for
                                                                                                              a small volume educational environment.
                                                                                                              The most basic tool for inspecting boards is
                                                                                                              an illuminated magnifying lamp as shown in
                                                                                                              Fig. 3. If more magnification is required there
                                                                                                              are some very affordable and effective USB
                                                                                                              connected PC based microscopes. The Esquire
                                                                                                              USB digital microscope is shown in Fig. 6. More
                                                                                                              comprehensive microscope kits, such as the
                  Fig. 6: Esquire USB microscope. The transparent end is held against the PCB
                                    and the grey knob is used to adjust focus.
                                                                                                              ProScope HR, have interchangeable lenses and
                                                                                                              more advanced features.

                                                                                                              Conclusion
source information found on the Internet must            process the copper layer is milled off a copper-
be evaluated carefully for reliability.                  clad base material. This may be done with a          This article has outlined the benefits of adopting
                                                         general purpose milling machine, although            surface mount technology in the educational
In-house PCB manufacture, in-house assembly                                                                   environment. It has reviewed the reasons for
                                                         the process is vastly tedious. Small computer
More affluent institutions might well consider           numerically controlled desktop mills are used        resisting the change and it has argued that these
producing their own PCBs. This allows low                by some educational institutions [8]. Custom         problems may be overcome if a determined
cost low turnaround-time prototypes to be                                                                     effort is made.
                                                         made “PCB plotters” are the most capable and
fabricated. Depending on the technology used             convenient milling machines for this application.    Different levels of sophistication in surface
to produce the PCBs an in-house plant might              There are low cost kit-form machines available       mount technology have been mentioned and
well pay itself off with the reduction in origination    [9], [10]. There are high-end dedicated PCB          their characteristics have been discussed.
fees. It must be noted that the requirements             prototyping machines on the market and
for a useful PCB have increased substantially                                                                 The tools required to make the move to SMT have
                                                         these yield excellent results even with the most
over the last decade, so that a PCB facility that                                                             been discussed and a variety of options have
                                                         demanding packages. One of these machines            been weighed up, ranging from outsourcing
was useful in the 1990s might not serve much
                                                         is shown in Fig. 4. These machines are simple to     to simple tools to complex and expensive
purpose today. When through-hole technology
                                                         operate and there are few parameters to control      machinery. None of these options are mutually
was standard a circuit board facility could be
                                                         in order to produce highly usable PCBs. The          exclusive, and so an exploratory path may be
useful if it could produce track widths and track-
                                                         better machines can drill holes through the PCB      followed.
to-track clearances of 1 mm. Current exposed-
                                                         as well and can produce double sided boards
pad SMDs require track widths and clearances
                                                         with ease. Quick and effective (albeit expensive)    References
as fine as 0,25 mm.
                                                         through hole plating options are also available.     [1] RoHS to Obsolete Entire Catalogues Overnight
If the relatively costly option is taken to have                                                                  Dataweek 24 March 2004
                                                         Fig. 5 shows a circuit board with a fine pitch SMD
an in-house PCB facility then it makes sense                                                                  [2] RS Khandpur Printed Circuit Boards: Design, Fabri-
                                                         soldered to it. The circuit board was made on
to invest the small additional amount for in-                                                                     cation and Assembly McGraw Hill First Edition pp
                                                         the machine shown in Fig. 4 and the SMD was
house assembly so that the entire prototype is                                                                    488 - 489
                                                         soldered using the equipment in Fig. 3.
produced without the use of external resources.                                                               [3] This idea is touched on in S. Papert Mindstorms:
                                                                                                                  Children, Computers and Powerful Ideas Harvester
In-house PCB manufacturing facility options              Laser circuit structuring
                                                                                                                  Press Ltd. 1980 Chapter 6

A number of very practical options exist for             The drawback to milling PCBs is that there           [4] Bob Willis, SMART Group BGA – Ball Grid Array In-
                                                         are fundamental limitations on the speed of              spection Workshop Available at www.smartgroup.
institutions who wish to have an in house PCB
                                                         machine operation, as for any machining                  org/pdf/bgainspection/bobwillis.pdf last refer-
prototyping facility. The more popular methods
                                                                                                                  enced 6 July 2009
are discussed here.                                      process. This limits the throughput of such a
                                                         machine. It would be ideal if each student in        [5] EAGLE CAD Lite, www.cadsoft.de last referenced 3
                                                                                                                  July 2009
Chemical etching                                         a course could produce their own circuit board
                                                                                                              [6] Kicad Open Source (GPL) Software for the Creation
                                                         during the course of a laboratory session. If such
Commercial PCB's are etched using a chemical                                                                      of Electronic Schematic Diagrams and Printed Cir-
                                                         a capability is required, and sufficient funding
process. This is also a very common method                                                                        cuit Board Artwork. www.lis.inpg.fr/realise_au_lis/
                                                         available, then a laser based PCB production             kicad/ last referenced 3 July 2009
of prototyping. A simple chemical process is
                                                         facility may be ideal. These machines use a          [7] See, for example, www.youtube.com, search for
cheap to set up but difficult to operate. There
are a large number of parameters that must               laser to ablate the unwanted copper from                 “soldering smd”. Last referenced 6 July 2009.
be controlled in order to produce boards of              the substrate. These machines can produce            [8] N A Gershenfeld Fab: The Coming Revolution on
sufficient quality for surface mount work. Should        much smaller track widths and clearances than            Your desktop – From Personal Computers to Per-
                                                         the milling machines generally can. Naturally            sonal Fabrication New York Books 2005
a chemical facility of insufficient capability
already exist then an upgrade in equipment or            machines this complex and capable are                [9] P rofiler: Build a Versatile Milling Machine Elektor Elec-
                                                         costly.                                                    tronics Vol. 33 No. 361 Jan 2007
staff training might suffice to make it usable for
                                                                                                              [10] Profile Pro Elektor Electronics Vol. 35 No. 390 June
surface mount components.
                                                         Circuit inspection                                        2009
Milling
                                                         Fine pitch SMDs and their PCBs require visual        Contact Samuel Ginsberg,
Another method of rapid prototyping that may             inspection to ensure that they are correctly         University of Cape Town, Tel 021 650-5297,
be used for PCBs is milling. In this subtractive         made, placed and soldered. Magnification is          samuel.ginsberg@uct.ac.za


40                                                                                                                                       September 2009 - EngineerIT

								
To top