What is Surface Mount Components on a PCB? (SMD Components Identification Guide in 2021)

Surface mount components 0

Why SMD Component Sourcing is in Great Demand?

In the previous lesson, we looked at push-through wiring of electronic components and other elements to a printed circuit board. I am sure that everything worked out for you and there were no difficulties, since this assembly method was originally designed for manual soldering. It is good enough for making relatively simple electronics designs.

Technologies for the production of electronic devices began with this. Once upon a time everything was done by hand. However, technologies are improving, and automated machines gradually began to replace manual labor. Special machines have appeared which in automatic mode are able to arrange electronic components on the board.

Wiring Principle of Automatic Assembly of Boards

It is obvious that the through-wiring principle is not suitable for automatic assembly of boards for objective reasons. The need to bend the pins of the components and orient them into the holes makes automation very difficult, expensive and impractical. Therefore, a surface mount components method has been developed to simplify the mounting of components on boards. When mounting in this way, special pads are provided on the board, to the surface of which electronic components are soldered. And the method itself boils down to several stages:

  • Application of a layer of solder paste through a stencil directly to the places of the board to which the components will be soldered. The solder paste contains solder and flux at the same time.
  • The machine automatically places the components on this paste in their intended places (it is sticky enough to hold the components)
  • The board is placed in a special oven, in which it is heated to the melting point of the solder in the paste, and as a result all components are soldered.
  • Cleaning the board from the residues of the flux contained in the paste is the same as with through-hole assembly. Only for washing are solvents used that are able to dissolve the flux that is part of the paste.
  • Drying the board.

New electronic components in suitable housings have been developed for this installation method. Such components can be with legs-contacts, having a special shape for installation on the contact pads of the board (Photo 1). Or they can be without legs – contacts in this case are the side parts of the component body (Photo 2). They are directly soldered to the board pads. But all these components, regardless of type and standard, belong to the class of SMD components, that is, surface-mounted.

Surface mount components 02Surface mount components 02

                                  Photo 1                                                             Photo2

In catalogs with electronic components SMD, you can find microcircuits with different cases. It can be:

  • SO (Small Outline) – enclosures with small outputs (contacts);
  • SOIC (Small Outline Integrated Circuit) – integrated circuit (microcircuit) with small outputs;
  • TSOP (Thin Small-Outline Package) – a case with especially small outputs;
  • QFP (Quad Flat Package) – a family of IC packages with flat contacts for surface mount components ing, located on 4 sides of the microcircuit;
  • and other body types, as well as variations of the ones listed above.

They all differ in size and shape, and can also have a different number of contacts, which is often indicated immediately after the designation. So from the designation of the SO28 case it follows that the microcircuit in this case has 28 contacts for soldering.

Also, for each type of chip package or component, standards are set in terms of dimensions. For example, the distance between the centers of the contacts of SO and SOIC packages is 1.27 mm, and the distance between TSOP or QFP chips can be from 0.8 mm to 0.5 mm, depending on the chip model. Which, you see, is much less than 2.54 mm in DIP chips intended for through-mounting. 

And if you solder them by hand, then the significantly smaller dimensions of the SMD components make the soldering process much more difficult. The smaller the distance between the pins, the more difficult it is to align the component on the board and solder it. And you will definitely have to solder them.

The Impact of Size on the Production of SMD Components

The fact is that due to the small size, the production cost of SMD components is noticeably lower than that of bulky components in a DIP-type package. As a result, manufacturers of electronic devices use mainly ICs in such packages for sealing onto their printed circuit boards. This makes the end devices cheaper and more compact. 

Therefore, SMD components are in great demand and the demand for them is growing, and the demand for microcircuits in a DIP package has dropped dramatically. This led to the fact that the production of many popular microcircuits in a DIP package is completely curtailed, and only their SMD version for surface mount components ing is produced. One example of such microcircuits is the FT232 microcircuit, which is used for communication with a computer on some models of the Arduino programmable controller.

So sometimes, to implement the plan and install the desired microcircuit on the board, you will inevitably have to solder some components in the SMD case. Therefore, skills in manual soldering of components that are intended for surface mount components ing are also required.

Why Soldering SMD Components are Complicated?

As mentioned above, soldering such components is more complicated, but not particularly difficult. It is enough to take tweezers in your hand, arm yourself, if necessary, with a magnifying glass, master a new soldering technique, and you can safely solder such a “trifle” as well.

When soldering components with SMD housings, almost the same steps are followed as when soldering components using through wiring (lesson # 2). The only difference is that you do not need to bite off the legs sticking out of the board with pliers. For the rest, everything is done exactly the same:

  • The place for installing each component is determined (do not forget about the keys and markings);
  • The flux is applied to the site to which the component is soldered;
  • A component or microcircuit is placed on the board and held with tweezers so that all contacts strictly coincide with the pads intended for them;
  • After preliminary fixing, all contacts of a component or microcircuit are soldered;
  • The board is washed from slag and flux residues;
  • The board is dried after washing.

Since all these stages are well described in the previous lesson, in this lesson we will dwell in more detail only on the direct process of soldering SMD components.

So, from the point of view of the principles of soldering components, they are divided into 2 groups: components without legs (these are resistors, capacitors), and components with legs-contacts (microcircuits, transistors, stabilizers, etc.).

Soldering Components without Feet

Everything is very simple with them. Lubricate the board surface with flux. Put quite a bit of solder on the tip of the soldering iron and hold the soldering iron in one hand. With your other hand, hold the tweezers and use the tweezers to grip the middle of the body so that the edges of the body are free. Next, use tweezers to bring the component to the designated place on the board so that the ends of the component are directly above the soldering pads. Hold the component, and at the same time touch the edge of the case closest to you with the sting (photo 3).

Surface mount components 03

                                                                                    Photo 3

Due to the presence of flux, the solder instantly spreads over the board area and over the part of the component body that is intended for soldering. This creates a small droplet that is connected to both the component and the board surface at the same time.

Once the solder solidifies, the component will be fixed. Now you can remove the tweezers and repeat the same procedure with the other end of the component. Take some more solder with the tip and touch it to the second end. After the solder has solidified, the component will be soldered. In this case, the solder will not get on the surface of the board covered with varnish (solder mask) or on the PCB without copper foil, since it does not chemically interact with it. This way the component will be soldered exactly where you need it  (photo 4).

Surface mount components 04

                                                                                    Photo 4

Soldering Components with Pins

But with the soldering of components with legs, everything is more complicated. There are several important subtleties here.

First . After applying the flux to the pads to which the microcircuit is soldered, it is necessary to install it very accurately with tweezers so as not to accidentally solder the legs to other people’s pads. When installing, do not forget about the coincidence of the keys on the board and the microcircuit.

 And it is desirable that the contacts of the microcircuit not only clearly coincide with the pads in the longitudinal direction, but also be symmetrically located on the pads of the board in the transverse direction (in the direction of the legs). This means that there must be the same length of the part of the pads protruding from under the legs of the microcircuit on one side and on the other.

Second . For convenience, first, one leg is soldered on each side of the microcircuit (or 1 leg, if a component) so that it is fixed (photo 5). While soldering the legs, hold the microcircuit with tweezers so that it does not move when soldering. After that, it is necessary to lubricate the legs of the microcircuit with flux again (photo 6).

Surface mount components 05

                                                                                      Photo 5

Surface mount components 06

                                                                                      Photo 6

Third. The legs of the SMD component are very close together, so you cannot touch the tip of the soldering iron to each leg individually. When you bring the tip to the contacts, you will inevitably touch several legs at once. Therefore, a soldering technique called “drop-wave” is used. It is a sequential conduction of a very small drop of solder on the soldering iron tip through a row of microcircuit legs (in the direction across the legs). It turns out that the drop rolls from one leg to the other. 

Thanks to the flux, the solder spreads only along the legs of the microcircuit and the pads on which the legs are located (the solder does not get into the space between the pads on the board due to the presence of a solder mask). This process is similar to running a wave. Hence the name “drop-wave”. At the same time, it is necessary to carry out the sting slowly, so that each leg warms up, the flux has time to activate, and the solder spreads along the legs (photo 7). When the solder spreads over the legs, you will see it well, because they immediately become shiny. As a result, the microcircuit or component will be soldered, and their legs are still tinned (covered with a thin layer of solder), which improves their resistance to corrosion.

Surface mount components 07

                                                                                         Photo 7

In order to perform such a soldering technique without stress and to obtain a stable result, it is desirable to have a special tip of the “drop-wave” type, or it can also be called “microwave”, at your disposal. We will talk about him and other types of stings in the next lesson.

Fourth .  When soldering SMD microcircuits, there should not be a large force of pressing the tip to the legs, so as not to damage the tip of the soldering iron.

Fifth . It is very important that there is very little solder on the soldering iron during drop-wave soldering. If there is more solder than needed, then solder jumpers will appear between some contacts of the microcircuit as in photo 8.

 That is, excess solder will connect adjacent legs with a single drop. In this case, the excess solder will have to be removed with a soldering iron tip. To do this, you need to lubricate the contacts with a flux and run the tip of a soldering iron over them from top to bottom so that the solder remains on the tip. Then this solder is removed from the tip with a sponge. The procedure is methodically repeated until the adhesion of the legs is eliminated.

Surface mount components 08

                                                                                       Photo 8

Sixth . Extreme care must be taken when soldering microcircuits. The permissible temperature for them is not higher than 260 ºС. Therefore, they are not allowed to warm up for a long time to higher temperatures. It is highly desirable to use a soldering iron with power or temperature control (soldering station) so that the temperature of the tip does not exceed the permissible one and the contacts of the microcircuit do not overheat.

Conclusion Surface Mount Components  

Perhaps these are all the subtleties that you need to know for the successful soldering of SMD components and microcircuits. Try to practice and gently solder the microcircuits to the board when assembling devices containing such components (included in the extended set “Evolvector” for studying soldering, or can be purchased additionally). Make sure that there is nothing difficult in such a soldering. You will get better and better with experience.

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