A guide to:
Surface Mount Technology
(SMT)

- by Luke Enriquez VK3EM        


  Introduction

Many experimenters avoid dealing with SMT (Surface Mount Technology) because of a lack of good information on the subject. Whilst there are several good references for commercial assembly, very little has been written about hand soldering and prototyping with SMT.
In many ways, SMT is the next stage in miniaturisation. A bit like the size-change from value to transistor. This article has been written to introduce you to this interesting technology.

What is Surface Mount Technology?
Put Simply - It is a type of electronic component package. Most electronic components can be divided into two categories - through hole (TH) and surface mount (SM). Through-hole components have been used for many years and are designed to be loaded on one side of a PCB and soldered on the other. Surface Mount components are designed to be loaded and soldered on the same side of the board.

Why is SMT used in industry?
SMT has several important benefits over though hole technology.

They are :
      - Faster for automatic machines to add to a PC board
      - Has a smaller physical size
      - Less parasitic (unwanted) effects
      - Cheaper cost

Why should you be involved in Surface Mount Technology?

"Black Box Operators" aside, SMT is increasingly affecting people involved in the repair, modification and development of electronics. Through hole components are being replaced by SMT equivalents at a rapid rate as manufacturers increase their investment in SMT production equipment to cash in on the benefits.

Whilst there are exceptions, it is rare to see the use of leaded components in modern consumer electronics. Since the demand for leaded parts is decreasing, their cost will rise over the next few years and sourcing them will become difficult. Eventually, supplies will dry up and leaded components will join the domain of valves.

Those who doubt these warnings should spend some time and have a look at a modern mobile phone, computer motherboard or consumer appliance. About the only component to remain leaded are the large electrolytic capacitors.

SMT Myths

Many facets of electronics are hampered by the myths that surround SMT.  Some of these myths are :

      - SMT needs special and expensive equipment
      - Components are hard to find
      - Requires professional PCBs
      - Requires special training and skills
To use SMT and not get stressed, requires the following :
      -  Have a steady hand
      -  Practice your technique
      -  Invest in a good pair of tweezers
      -  Have reasonable eyesight or use magnification
Unfortunately, there is not much you can do about the steadiness of your hand, but all the other obstacles can be easily overcome. There are many other tips that go together to form a good technique and these are covered in this article.


Common SMT Packages


There are three popular package styles used for most passive components. Their names refer to the size (in thousands of an inch or just thou). They are :
      - 0603 (60 thou long, 30 thou wide)
      - 0805 (80 thou long, 50 thou wide)
      - 1206 (120 thou long, 60 thou wide)
In the early days of SMT use, 1206 was the most commonly used package for resistors and capacitors. As SMT technology advanced through the development of faster and smarter "pick and place" machines 0805 became more economical to use. As with many things in life there is a trade-off between size, cost and difficulty of use.
0603 and smaller sizes are often used when "size" is the important criteria. 0805 is now standard for products where "cost" is the important criteria. 1206 is no longer in common use but does find applications where power dissipation and reliability are an issue.
Many other passive components such as inductors, tantalums, trimmers, etc. use other packages that are not mentioned here.


Fig 1 - Common SMT components

Resistors and ceramic capacitors are shown in Fig 1. The MELF package is also used for resistors and diodes. FETS, Diodes, Varicaps, Transistors use the SOT package and the only way to determine the component is via a code marked on the top of the component.

How can SMT help you?

SMT has many benefits over leaded components. These are :  

    - Component values can be "tweaked" (i.e. : changed slightly). SMT capacitors and resistors are easy to parallel together, and quick to solder and de-solder. The chances of "lifting" circuit board tracks are reduced and so is the frustration of trying to work on both sides of a PCB at the same time.
    - Where RF signals are present. There are no wire leads to add parasitic inductance to the component-value. Some modern components are only available in SMT form. Leaded packages do no lend themselves to microwave use.
    - Where space is limited.
     
    - Where drilling holes is a problem. Anyone who has made a PCB understands the frustration of trying to work on two sides at once. SMT simplifies this, because you draw, load and solder on the same side. Components can be used on both sides of the PCB, or a ground plane can be used on one side with holes drilled only for ground connections.
    - Where a pre-existing circuit needs modification. If a capacitor, diode or resistor is needed, a track can be cut and a SMD inserted.

Tips for soldering SMT Parts

Most of the rules applicable to soldering through-hole parts apply to SMT parts.  Good soldering technique will come with practice, but these tips will guide you in the right direction. If you need to practice,  SMT resistors are the toughest of all.

1.) Keep the circuit board clean. Isopropanol or wood alcohol is suitable for removing light oils. PCB's should always be washed under warm water, then oven dried at 60 degrees Celsius for 10 to 15 minutes.

2.) Use the correct soldering iron for the job. You don't need to purchase a temperature controlled iron, special SMT tip or SMT hot gas reflow station (but a temperature-controlled iron will make the job much easier). These tools might be used in industry, but only to save time and increase reliability.  A low-cost temperature controlled iron is really needed as a "normal" soldering iron gets too how for most electronic work. A "normal" iron gets very hot so that it is able to solder most medium-to-heavy connections and these are much larger than electronic connections.
Once you go to a temperature-controlled iron, you will NEVER go back to an ordinary iron.
One type of temperature-controlled iron I don't recommend is the Weller work-station. It has replaceable tips and these are only available in 100 degree increments. These intervals are too large and the temperature of the tips I have worked with are too hot for electronic work.

As with any soldering job, the idea is to have the joint up to temperature and soldered in a few seconds. Think about how much of a "heat sink" the joint will be and choose the tip based on that. Use of larger tips should be limited to areas of large solid copper plane (i.e.: ground plane).

Remember : Components most likely to suffer from over heating are ceramic chip capacitors. Silicon devices are also very sensitive with inductors and resistors being the least sensitive.

3.) Use L.M.P (Low Melting Point Solder) if you are experimenting. LMP solder is very similar to 60/40 solder, except that it contains 2% Silver. This Silver "loading" has two effects. It lowers the melting point and it reduces the rate at which component metalisation leeches into the solder itself.

SMT resistors, capacitors, ferrite beads, etc. make electrical connections via metalised pads. These are the pads on the end of the SMD device. The metal used is often Nickel or a related alloy. One problem with soldering the same joint several times, is that each time the joint is heated, some of the Nickel leaves the component and joins the solder. This is called "leeching". Leeching is only a problem when the solder joint of a metalised component is heated several times. Leeching occurs at a faster rate with standard 60/40 solder than with LMP solder.

The downside of LMP solder is that it is about 3 times the price of 60/40 solder and harder to obtain.

If a kit is being built, where the component values are known,  60/40 solder with extra flux will suffice. If component changes are often and likely, then LMP would be better for a long-term reliable solder connection. Some constructor use "solder cream" sold by  hobby shops. The advantage of solder cream, is that it has more flux than regular solder. The solder cream is made up of very fine balls of solder mixed with a water based flux.

Unfortunately, solder cream was never intended to be used with a soldering iron. In fact, because the solder sits in a water based flux solution, the cream needs to be "dried" out (i.e. : the water has been driven off) before the solder can be melted. This can be done by moving the iron tip close to the joint for a few seconds prior to moving onto the joint.

In experimentation quantities, solder paste or cream is only available as 60/40 mix. In my opinion, the SMT experimenter would be better off to use LMP solder and extra flux (from a tube or a pen) rather than solder paste.

4.) Use solder flux where possible. One of the biggest problems with soldering SMT parts is the amount of flux within the solder core is not sufficient for the joint. Professional SMT manufacturers use "solder cream" and controlled temperature ovens. However, soldering iron temperatures are far less controlled and often the flux has evaporated before the joint has solidified, leading to dry joint which is often dull in complexion. That's why you should use a temperature-controlled iron that does not evaporate the flux too quickly.

Solder flux has other advantages too. It increases the conduction of heat from the iron tip to joint and increases the surface tension of the molten solder. This helps to achieve a nice concave joint and minimises the chance of bridging finely spaced pins.

Flux has the disadvantage that it is generally sticky, and can require special flux removers to remove. Soapy water and ultrasonic baths are one solution, but this requires a second wash in fresh water and a bake in the oven. Flux can also carry contaminants which may effect circuits operating in the microwave region or circuits with very high impedances, especially VCOs. Some fluxes contain lead based chemicals and it is wise to use latex gloves to avoid direct skin contact.

Flux is available from several hobby shops and other outlets in syringe and pen form. In general, the use of extra flux makes SMT soldering much easier and increases solder joint reliability.

5.) Use a good magnifying lamp or other magnification source. SMT parts are generally considered very small. SMT solder joints are at least four times smaller again. Since its the solder joint that should concern you most (especially if you want to build something reliable) it is often worth investing in a decent magnification lamp (Maglamp) or other magnifying source.

Most people with reasonable eyesight should be able to solder without magnification and check the joint under magnification later. For those who have relatively poor eyesight, special "jeweller's eyes" that sit on the head can help.

6.) Buy a good pair of tweezers. You will be amazed how much easier SMT soldering becomes. In fact, out of all the equipment I have suggested, this is the most important. Both soldering and de-soldering will involve tweezers, so they are a worthwhile investment.

How to Solder Small SMT Parts

The following technique should be used for soldering small SMT parts such as resistors, capacitors, inductors, transistors, etc.
 

    1.) Add a small amount of flux to each of the pads for the component. Lightly solder one of the pads.
    2.) Pick up component with tweezers making sure it is horizontal. Place it in position on the pads.
    3.) Hold the component in place by pushing it onto the board with the tip of the tweezers.  Melt the solder you have provided on one of the pads.
    4.) Remove the iron but continue holding the component until the solder has solidified. Check to see that the component is sitting flat on the PCB. If not, re-melt solder whilst pushing gently on top of the component with tweezers.
    5.) Solder the other lead by adding solder and making a good connection. Re-melt the first solder joint and let solidify.
    6.) Check your work under magnification.
    The joint should be shiny and concave. If you added too much solder, wick up the  excess solder with "solder wick."

1206 - Insufficient Solder

1206 - Adequate Solder

1206 - Excessive solder

SOT - Insufficient Solder

SOT - Adequate Solder

SOT - Excessive Solder

Table 1 - 1206 and SOT Solder Joints. Insufficient, Adequate and Excessive Joints
 

How to solder SMT Integrated Circuits

IC's require a similar but slightly different technique.

    1.) Add flux to the pads where the IC is to be soldered.
    2.) Add a small amount of solder to one of the corner pin pads.
    3.) Line up the IC with the pads on the PCB. Double check the IC orientation. Push the IC onto the board with the tip of the tweezers.
    4.) Melt the solder with your iron and let to joint cool.
    5.) Solder the diagonally opposite pin. Check under magnification that all pins line up with their respective pads.
    6.) Solder the rest of the pins and check work under magnification.

De-soldering Small SMT Components

    1.) Add excess solder to one side of the component.
    2.) Whilst the side with excess solder remains molten, move your iron to the other joint and gently push the component off the pads.
    3.) Clean up pads with solder wick.
Note : The trick is make one side of the component a larger thermal mass and heat that side first.


De-Soldering Small Outline Integrated Circuits

Experimenters generally do not like SMT IC's due to the difficulty of making circuit boards to suit them. However, you might need to remove a faulty IC. This technique only works for SO-IC (50 thou spaced devices). Other devices require a hot air gun or a grinding tool (Grind the legs off the IC).

    1.) Apply flux to the IC pins.
    2.) Use solder wick to remove as much solder as possible from each pin.
    3.) Thread fine enamelled wire under one row of pins.
    4.) Secure one end of the wire on a nearby component (i.e. : Large Electrolytic).
    5.) Starting at the loose end, heat each pin and pull wire simultaneously. Pull the wire as close to the PCB as practical. As the solder between the pin and pad melts, the wire will pop out and leave the pin standing free of the pad.
    6.) Repeat steps 3 to 5 for the other side.

What parts can you recycle?

Some SMT parts can be quite expensive when purchased in small quantities. All sorts of SMT parts can be recovered from surplus and junk equipment (providing it uses SMT parts). It will not only save you money, but give you good practice at de-soldering.

If you use recycled components, perform an electrical check on them.  Ceramic capacitors cause the most problems (they crack easily). Inductors, transistors and resistors can all be checked.

Conclusion

This article has described some methods which simplify the use of SMT for the experimenter. It is by no means complete. All methods mentioned in this article are simply a guide and everyone will develop their own technique over time. SMT can be both useful and rewarding to anyone who has an imagination to find an application.

Acknowledgements

The author would like to thank Steve Merrifield VK3ESM and Bryan Ackerly VK3YNG for their help in preparing this article. Pictures for this article were extracted from the Tait T2000 Series II radio manual. Comments regarding this article or any questions may be sent to VK3EM@hotmail.com

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