A lot of people avoid dealing with SMT (Surface Mount Technology) because of a lack of good information about it. Whilst there are several good references for commercial assembly, very little is written about hand soldering and prototyping with SMT. In many ways, SMT is what "Silicon" was to those who were used to valve technology. This article has been written to introduce the amateur radio operator and experimenter to this interesting technology.
What is Surface Mount Technology or SMT? 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 PCB.
Why is SMT used in industry? SMT has several important benefits over though hole technology.
"Black Box Operators" aside, SMT is increasingly effecting people involved in the repair, modification or development of electronics. Through hole components are being replaced by their SMT equivalents at a rapid rate as manufacturers increase their investment in SMT production equipment to cash in on the benefits of SMT.
Whilst there are exceptions, it is rare to see the use of leaded resistors, capacitors, transistors or integrated circuits in modern consumer electronics. Since the demand for these types of leaded parts is low and 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 of you who doubt these warnings should spend some time and have a look at a modern mobile phone, computer motherboard or amateur radio. An alert observer will note that leaded connectors and electrolytic capacitors are usually among the only leaded parts used. This is mainly because connectors often rely on their leads for mechanical strength and electrolytic capacitors (radial type) have a shape that does not lend itself towards implementation as a SM component. Eventually the solutions to these problems will become cheaper and they too will disappear from electronic equipment in their leaded form.
Many new facets of amateur radio and experimentation with electronics in general are hampered by the myths that surround them. SMT is no exception and this is no doubt one of reasons why amateurs have been slow to adapt to SMT. Some of these myths are :
Common SMT Packages
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 and IC's all use the SOT package and
often measurement with a multi-meter and the marking on the top of the
package is the only way to guess what the component. Several good web sites
exist for determining SMT parts from their marking and these can be found
on the VK3EM web site.
How can SMT help you?
SMT has many benefits over
leaded components. These are :
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, practice with SMT resistors as they 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 right soldering iron for the job. You don't need to purchase a temperature controlled iron, special SMT tip or SMT hot gas reflow station. These tools might be used in industry, but only to save time and increase reliability.
All sorts of SMT soldering jobs can be done with the common Weller workstation. The important point is select the right tip (i.e.: have several tips on hand). As with any soldering job, the general 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 stress are ceramic chip capacitors. Silicon devices are less 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. all make there electrical connections via metalised pads. 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. The 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 what it does 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, although sources of supply have been quoted at the back of this article.
If a kit was being built, where the component values are known, then 60/40 solder with extra flux would suffice. If component changes are often and likely, then LMP would be more advantages for a long term reliable solder connection. Some people use "solder cream" sold by various 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 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 that 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.
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 impedance's, especially in 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 application 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.
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 your tweezers,
so they are a worthwhile investment (and they double for removing splinters).
How to Solder Small SMT Parts
The following technique should
be used for soldering small SMT parts such as resistors, capacitors, inductors,
IC's require a similar but slightly different technique.
7.) Special techniques may be needed for some packages.
De-Soldering Small Outline Integrated Circuits
Amateurs generally do not prefer SMT IC's due to the difficulty of making circuit boards for them. However, you might need to repair a faulty IC on your favourite RIG. 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).
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 of course!). It will not only save you money, but give you good practice at de-soldering. The VK3EM web site contains colour pictures of many SMT components so you can identify them.
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 verified for correct operation.
This article has described some methods which simplify the use of SMT for the amateur experimenter. It is by no means complete, and further information can be obtained from the VK3EM web site. 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.
The author would like to thank Steve Merrifield VK3ESM and Bryan Ackerly VK3YNG for there 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 or see http://www.oocities.com/vk3em
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