Capacitors for pcba SMT assembly

Capacitors are marked in a variety of ways. The actual format of the capacitor code or marking depends upon the type of capacitor.

There are different codes used dependent upon whether it is a traditional leaded component, or surface mount and the capacitor dielectric or technology. Size also plays a major part as it determines the space available for the marking.

Some of the marking systems have been standardised by the EIA - the Electronic Industry Alliance, and these provide commonality across the industry.

Capacitor marking basics

Capacitors are marked in many different ways. There are a number of basic marking systems that are used and different capacitor types and different manufacturers use these as needed and best fits the particular product.

Note that on some occasions the abbreviation MFD is used to denote μF and not a MegaFarad.

Normally a knowledge of these basic capacitor codes and capacitor marking systems enables codes on most capacitors to be easily decoded.

• Non-coded markings:   The most obvious way of marking a capacitor parameters are to directly mark them onto the case or encapsulation in some way. This method works best on larger capacitors where there is sufficient space for the markings.
• Abbreviated capacitor markings:   Smaller capacitors may only have room for a few figures printed as a code for the value. This capacitor marking code uses three characters. It bears many similarities to the colour code system adopted for resistors, but without the colour part of the coding scheme. The first two figures refer to the significant figures, whereas the third one acts as a multiplier. The value of the capacitor is denoted in picofarads for ceramic, film, and tantalum capacitors, but for aluminium electrolytics the value is denoted in microfarads.

   

  
  For small values the letter R is used to denote a decimal point, e.g. 0R5 is 0.5, 1R0 is 1.0 and 2R2 is 2.2, etc..
  
  This scheme is widely used with surface mount capacitors where space is very limited.
  
  
  Abbreviated capacitor code
• Colour code:   Some older capacitors use a form of colour code. This type of capacitor marking is used less these days but may be seen on some components.
• Tolerance codes:   Some capacitors have a tolerance code. The code used is actually the same as that used with resistors, but for completeness this tolerance capacitor code is included here:
  
  
• Capacitor working voltage codes:   One key parameter of any capacitor is its working voltage. This is widely marked on capacitors and particularly in situations where there is space for alphanumeric coding. In many instances where the capacitor is small no voltage coding is provided and care must be taken if there is no marking on the reel or other storage container.

   

  
  On some SMD electrolytic and tantalum capacitors a one character code is used. This occupies much less space and bears many similarities to the EIA system.
  
  
  

Temperature coefficient codes

It is often necessary to mark a capacitor with a marking or code that indicates the temperature coefficient of the capacitor. These capacitor codes are standardised by EIA, but also some other generally used industry codes may also be seen in common use. These codes are typically used for ceramic and other film type capacitors.

The temperature coefficient is quoted in terms of parts per million per degree C; PPM/°C.

Capacitor polarity markings

One important marking for polarised capacitors is the polarity. Great care must be taken to ensure the polarity markings are observed when inserting these capacitors into circuits otherwise damage to the component, and more importantly to the remainder of the circuit board can result. Polarised capacitors effectively mean aluminium electrolytic and tantalum types.

Many recent capacitors are marked with the actual + and - signs and this makes it easy to determine the polarity of the capacitor.

Another format for electrolytic capacitor polarity markings is to use a stripe on the component. On an electrolytic capacitor the stripe indicates the negative lead.

If the capacitor is an axial version having leads at both ends of the package, the polarity marking stripe may be accompanied by an arrow that points to the negative lead.

For leaded tantalum capacitors the polarity markings indicate the positive lead. A "+" sign is placed close to the positive lead. When new, a further polarity making may be used because it may be seen that the positive lead is longer than the negative one.

Markings for different types of capacitor

Many larger capacitors like electrolytic capacitors, disc ceramics, and many film capacitors are large enough to have their markings printed on the case.

On a larger capacitors there is sufficient space to mark the value, the tolerance, working voltage, and often other data such as the ripple voltage.

There are a number of subtle differences in the capacitor codes and markings used for different types of leaded capacitors:

• Electrolytic capacitor markings:   Many leaded capacitors are quite large, although some are smaller. As such it is often possible to provide the complete value and details in a non-abbreviated format. However many smaller electrolytic capacitors need to have coded markings on them as there is insufficient space.
  
  A typical marking may fall into the format 22μF 50V. The value and working voltage is obvious. The polarity is marked by a bar to indicate the negative terminal.
• Leaded tantalum capacitor markings:   Leaded tantalum capacitors generally have their values marked in microfarads, μF.
  
  
• Typically the markings on a capacitor may give the figures like 22 and 6V. This indicates a 22μF capacitor with a maximum voltage of 6V.
• Ceramic capacitor markings:   Ceramic capacitors are generally smaller than types like electrolytics and therefore the markings need to be more concise. A variety of schemes may be used. Often the value may be given in picofarads. Sometimes figures such as 10n will be seen and this indicates a 10nF capacitor. Similarly n51 indicates a 0.51nF, or 510 pF capacitor, etc . .
• SMD ceramic capacitor codes:   Surface mount capacitors are often very small and do not have the space for markings. During manufacture the capacitors are loaded into a pick and place machine and there is no need for any markings.
  
  
• SMD tantalum capacitor markings:   Some tantalum capacitors like many of their ceramic counterparts do not have a value marking ont hem. Possibly they only have the polarity marking to ensure that the capacitors are inserted the correct way onto the circuit board.
  
  
  
  
  Bar across one end of this tantalum provides the capacitor polarity marking
  
  On the occasions that there is space for a marking or code, the simple three figure format like that shown below is often used, especially for capacitors such as ceramic formats. For the example of the capacitor code shown in the diagram, the two figures 47 indicate the significant figures and the 5 indicates the multiplier of 5, i.e. 100 000, i.e. 4.7μF.
  
  
  Tantalum SMD capacitor code
  
  
  
  In some cases the only marking shown on the capacitor may be a bar across one end indicating the polarity. This is particularly important because it is necessary to be able to check the polarity and to have a marking to identify the polarity of the capacitor. It is particularly important to have a capacitor polarity marking because reverse biasing tantalum capacitors leads to their destruction.

Being able to read the value of a capacitor is particularly important. With so many different capacitor codes, it is often necessary to have a basic understanding of the codes and apply them as appropriate for a particular capacitor.

Although examples for capacitor markings have been given for different capacitor types, these can only be examples of the more likely variants, and experience and a little practice will enable the values of most capacitors to be determines.

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