Surface Finishes on Printed Circuit Boards
Then finally, you have finished your design and the moment has come to order the PCB. You have considered all the important aspects. The DRC check is done and you are relieved: no errors remaining.
The online calculator shows you the default pooling options and offers you the possibility to adjust some of them to your specific need. En there you bump onto the Surface Finish. What is Surface Finish and how to make the right choice?
Since all finishes have advantages and disadvantages, it is important to stand still by their application and to check how your boards will be treated during assembly. The different finishes we offer are Lead-free HAL, Electroless Nickel/Immersion Gold (known as ENIG) and Immersion Silver (ImAg). All of these are Lead-free and can be used for a RoHS design but also in a SnPb assembly. Edge connectors can be covered with hard Gold (electroplated Ni/Au).
The use of HAL as a finish results in PCBs with the highest level of solderability and solderability robustness with regards to multi-step assembly and storage and all this for a reasonable price. On the other hand, the HAL process requires the submerging of the complete PCB in liquid solder and is responsible for extra thermal load on the PCB. For that reason, it isn’t the best choice if your board requires small via holes or the board exceeds a normal thickness (when the aspect ratio is high). Another aspect is the less flat surface it may create. Although we try to achieve a flat surface, the variety of the amount of solder present on the solder-pads can make this finish less suitable for small sized components with small pad sizes.
ENIG (chem. Ni/Au)
Many customers choose this finish because of the flat surface, the good solderability and the acceptable shelf live, but also because they are already familiar with this finish from before the introduction of the Lead-free assembly. At least, this parameter could remain unchanged and gave at that time some confidence. However, the ENIG process is a complicated one with a higher risk of defects (skip plating, black pad or interface embrittlement). When using ENIG, the solder-joint is formed between the solder and the Ni layer of the NiAu surface, not with the underlying Cu. The Au is completely dissolved in the solder-joint. This interface is considerably more brittle than a SnCu interface and therefore not recommended for applications where shock, bending or strong vibrations are part of the picture. And last, but not least: it is the most expensive surface finish of the list. For some applications, like key pads or wire bonding, it is the better choice.
Immersion Silver (chem. Ag)
This surface finish is often a bone of contention, some love this finish, and some hate it. It offers a flat surface, a very good solderability and a long shelf life. The solder-joint is created with the underlying Cu, since the Ag is dissolved during soldering. Sounds good, but the ImAg is susceptible to sulferdioxide (SO²) which tarnishes the surface and creates the AgS² layer. This layer affects adversely the surface solderability. To avoid this tarnish, we pack the PCBs in silver saving paper and require hermetic sealing to avoid moisture and atmosphere SO² coming in. In case of multi step assembly, the partially assembled boards should better be stored in a sulfur-free atmosphere. If all this handling is not a problem and taken into account, it is the best choice and… the cheapest.
Any Lead Free
If you don’t choose a finish, your board will be produced on a production panel where the finish is lead-free HAL or chem. Ag or chem. NiAu. The choice is defined by other PCBs appearing on the same panel.
Gold for Edge Connectors
Due to the need of abrasive resistance of edge connectors we can finish these connectors with a layer of electroplated NiAu (hard gold). This finish is processed in a special sized bath construction and only used for these connectors. It is not possible to have this hard gold processed on other locations of the PCB.
Carbon combines a high mechanical strength with a good electrical conductivity and can often be used as a substitute for gold on contacts. It is applied directly on the Cu and used for switch contacts, foil keyboards and can enable the creation of cross-over conductors. It is resistant to HAL and soldering processes without showing practically any change in resistance. Carbon is printed (conductive ink) and the accuracy in position and image is therefore limited by the printing process.