PCB surface finishing

PCB surface finishing

To preserve the solderability of printed circuit boards for a considerable period of time, it is important to protect the copper surface of the soldering pads with a suitable coating or in other words, to provide a so-called surface finishing.

Surface finishing is made on soldering pads and other copper elements opened from solder mask. In modern productions, as a rule the several types of surface finishes with different properties are used.

For the appropriate selection of surface finishes and for their parameters specification there are a number of regulations, of which the most common are the following IPC standards:

  • J-STD-003 Solderability Tests for Printed Boards - defines the solderability test methods;

  • IPC 2221 Generic Standard on Printed Board Design - defines the basic requirements for the design of printed circuit boards;

  • IPC-7095A Design and Assembly Process Implementation for BGAs - focuses on the BGA components;

We offer a wide range of surface finishes to make an appropriate selection regarding the PCB project requirements.

  • HAL or HASL (Hot Air Leveling or Hot Air Solder Leveling) uses the tin-lead (Sn-Pb) alloys and the alignment by the hot air knife. This finishing is currently most commonly used due to its properties. It provides excellent solderability with substantial shelf life. HAL finishing is easy to manufacture and inexpensive. It is compatible with all methods of soldering or assembling - the manual soldering, wave soldering, reflow etc. The negative feature of this type of finishing is the presence of the lead - one of the most toxic metals that are prohibited for use on the territory of the European Union by the RoHS directive (Restriction of Hazardous Substances). Another limitation - it is surface non-uniformity that is not acceptable in case of fine pitch components. Besides it is not compatible with the COB technology (Chip on Board).

  • Lead free HASL finishing is similar to the normal HASL, except that it could comprise of different alloys like Sn100, Sn96,5/Ag3,5, SnCuNi, SnAgNi and does not contain lead. The finishing is fully RoHS compliant and meets  all the requirements of safety and solderability. However, due to the fact that finishing is applied at the considerably higher temperatures, more rigorous requirements are imposed on PCB base materials. Lead free HASL is compatible with all methods of soldering or assembling both in lead and lead free technologies, but it requires the appropriate temperature profiles during the soldering. As compared to SnPb HAL, it is more expensive due to the higher prices of alloys as well as higher energy consumption.

  • Immersion Gold or ENIG (Electroless Nickel/ Immersion Gold ) - the finishing from a Ni/Au family. The thickness of the finishing: Ni 3 – 5.0 µm, Au 0,06 – 0,1 µm. The finishing is made by chemical method. The main function of thin gold layer is to protect the nickel layer from oxidation, and the nickel layer prevents a mutual diffusion of gold and copper. The excellent flatness of the finishing makes it suitable to use in case of fine pitch components. The finishing is fully RoHS compliant. Compatible with all methods of soldering. The main limitation is a higher price. Besides there could be a risk of immersion gold defect due to the oxidation, so-called "black pad" that is critical for BGA assembly.

  • Gold Fingers - the finishing from a Ni/Au family. Plating thickness: Ni 5-9 µm, Au 0,2 - 1,0 µm. It is applied by electrochemical deposition (i.e. electroplating). Most often the finishing is used for PCB edge connectors. It has high mechanical strength, resistance to abrasion and adverse environmental effects. Indispensable where it is important to ensure excellent electrical contact with long service life.

  • Immersyjna tin - the finishing made chemically. It is compatible with all methods of soldering or assembling. The finishing has an acceptable shelf life period - up to one year. This is achieved by using organic compounds to make the barrier to intermetallic bonds that affect the oxidation of the surface. Such kind of insulating also prevents tin from crystallization. The finishing with the thickness of 1 µm has a good flatness and suitable for fine pitch components.

  • OSP (Organic Solderability Preservatives) - a family of organic coatings that applied directly onto the bare copper to protect it from oxidation during storage and soldering. This inexpensive finishing, has a  flat surface and is suitable for SMD assembly. It complies with the RoHS directive. As a result it is a cheap alternative to the HASL finishings.  Unfortunately, it has a limited shelf life (months) and quickly degrade during the soldering process.

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