Choosing the Right PCB Surface Protection: A Guide by RUSH PCB UK
Anyone in the PCB (Printed Circuit Board) industry understands that every PCB has copper finishes on its surface which when left exposed in air copper oxidizes and deteriorates thus making the Printed Circuit ineffective. The surface finish, therefore, is a critical interface between the PCB and the components placed on the board. There are two essential functions of a PCB finish; protection of the exposed copper circuits and provision of a solderable surface when assembling or soldering the components on the PCB. Equally significant, Printed Circuit Boards finishes vary depending on the availability, price, reliability, shelf life and the assembling process. There are different PCB finish techniques which in turn offer surface protection to the boards. While each PCB finish has its own merits, in most cases the product, process or environment where the PCB will operate dictates the type of PCB surface protection to be used. It is, therefore, recommended that the user, assembler and designer of a PCB work close together so that the best finish is used for a PCB. Please join us as we discuss some of the PCB surface protection techniques that are in use in the PCB industry as well as those that were used before them.
Surface protection using the Organic Solderability Preservatives (OSP) finishes
Organic Solderability Preservatives have been used by PCB manufacturers since the 1970’s after introduction into the market by IBM. OSP preserves the copper on the surface of a PCB from oxidation by the application of a very thin protective layer over the exposed parts of copper using a conveyorized process. Moreover, OSP uses water-based organic compounds which selectively bond to copper thus providing an organometallic later that offer protection to copper before and after soldering.
The thickness of an OSP finish is almost immeasurable (in angstoms). On the other hand, the initial formulas had a shorter life-time of around 3 to 6 months and they could withstand 1 or 2 heat cycles. Looking at today’s standards of the OSP, such conditions can only be considered applicable for much lower PCB technologies. The most recent OSP formulas are designed for lead-free PCB assembly and they are more robust thus they can withstand multiple heat cycles. The advantages of this technique are that it has a flat surface, it is a simple process, no lead is used, its cost-effective and highly re-workable. Different from that, the demerits are that there is no sure way to measure thickness, it has a shorter shelf-life, there are high chances of having exposed copper on final assembly and this method is not recommended for Plated Through Holes (PTH)
Electroless Nickel/Immersion Gold (ENIG)
ENIG PCB surface protection method is the most commonly used method in the PCB industry. This can be attributed to the growth and the implementation of RoHS regulation.On the other hand, Electroless Nickel/Immersion Gold comprises of a 2-layer bimetallic coating of 120-240µin Ni under 2-8 µin AU. Nickel is used as the barrier to the copper and its where the components are soldered to. On the other hand, gold offers protection to Nickel especially during storage and provides a lower contact resistance needed for the thin gold deposits.
The typical thickness of gold is 3 to 5 micro inches while that of Nickel is 75 micro-inches. The comparative merits and drawbacks of this method are: In this method, no lead is used thus the method is RoHS compliant, it has a longer shelf-life compared to some methods like OSP finishes, the method can be used for almost all types of PCBs and for Plated Through Holes (PTH) and ENIG can be used for flat PCB surfaces. On the other hand, the demerits are that this method is expensive hence not used especially in small-scale PCB manufacturing, it is not re-workable, higher chances of damage from ET, the process to having an ENIG finish is very complicated and more technical than other methods like HASIL and OSP and there might be signal losses (Radio Frequency)
HASL (Hot Air Solder Leveling)/ Lead Free HASL
Hot Air Solder Leveling is the most predominant surface finish protection used in the PCB industry. This process involves immersing the PCBs in a molten pot of a lead or tin alloy then the excess solder is removed using ‘air knives’ which blows hot air across the surface of the Printed Circuit Board. The HASL procedure can lead to additional stress to high layer PCBs thus causing long-term reliability issues. This method is cheap since it uses air unlike some methods like immersion silver or hard gold finishing. Comparatively, the method can be used anytime by anyone since air is readily available and its free to all. PCBs whose finishing is by HASL have an excellent shelf-life and they are re-workable.
However, the increase in stress and uneven solder height on dense BGA or SMT pads are a sufficient reason why HASL should be replaced by another PCB surface protection method. Another drawback of this method is that it results into uneven surfaces and solder bridging. Different from that, HASL contains lead which makes the PCBs with such surface protection RoHS non-compliant. The method also leads to reduced Plated Through Holes; PTH’s unlike the other methods discussed above.
According to the association connection the electronics industry, IPC, Immersion Tin (ISn) is a metallic finish that is deposited by chemical displacement reactions applied directly over the basis metals of Printed Circuit Boards which is basically copper. The Immersion Tin finish offers protection to copper from getting oxidized thus reducing its shelf-life. Different from that, copper and tin have a very strong affinity for each other with the diffusion of one metal to the other occurring inevitably thus affecting the performance as well as the shelf-life of the PCB surface protection or finish.
This method is sometimes recommended due to the fact that it can be used in a flat surface and it does not use or produce lead which might lead to environmental depletion. Equally significant, this PCB surface protection method is the best and most recommended choice for press fit pin insertion. However, tin has adverse effects especially due to the growth of tin whiskers which demands careful handling of the PCB. It is also very difficult to measure the thickness of PCBs with tin surface PCB due to the nature of the finish.