Surface finish for PCB by Azitech ApS

Choosing the correct PCB surface finish for your application requires taking factors into account like cost, the final application requirement like solder joint strength, high-heat, thermal, vibration, stability, RF, component density, single or multiple solder processes, shelf life, shock, drop resistance, production volume, and throughput.

Selecting the right surface finish is therefore extremely important to extend durability of the PCB. This guide will help you understand some of the pros and cons of the most used surface finishes while giving you some insights into new appearing trends and popular selections.

The most common reason, for why a PCB, requires a surface finish, is because copper loses its solderability, when exposed to air. Once copper is exposed to air, an oxidation will start and almost instantly decrease the solderability. Extending the solderability time is therefore achieved by adding a protective layer to the surface copper of the PCB.

Some of the protective layers are widely known as surface finishes like LF HASL, OSP, ENIG, Immersion Silver and Immersion Tin. In this article you will find a full comparison chart across all surface finishes and discover some of the new trending surface finishes like ENEPIG, ENIPIG and EPAG that are outcompeting the old ones by miles.

Other surface finishes EPAG, EPIC and ISIG is even more interesting in their performance, but are yet not as accessible in the PCB factories and may currently NOT be ideal if other criteria’s such as quality, technology and low cost also serve as a priority for factory selection. Some of Azitech factories also delivers these surface finishes, but it may not support all technologies.

Using OSP as surface protection, where you aresoldering directly on pure copper, which providesthe strongest solder joint of all surface finishes.OSP is also the cheapest and is therefore excellentfor low-cost application, and application wherethe soldering work process is simple. Keeping itsimple is ideal, as flux will remove the surfaceprotection, leaving the copper susceptible tocopper oxidation, which will make solderingimpossible. Vacuum packing is therefore a must ifthere are multiple solder processes. Whenvacuum packed, the process window issuccessfully extended.

ELECTROLESS NICKELIMMERSION GOLD (ENIG)

ENIG is the most used surface finish in the PCBindustry. It is a two-layer metallic coating of 2-8μin Au over 120-240 μin Ni. When soldering onENIG, you are soldering on the Nickel layer, asthe Nickel acts as the barrier to the copper.Soldering to Nickel decreases the solder strengthto around 83%, when compared to soldering onpure copper. A rare phenomenon called “Blackpads” can occur. Black-pad defects are formed byhyper-corrosion activity of the immersion goldprocess on the nickel surface. Black pads must becontrolled by the PCB manufacturer, where highlevels of phosphorous must be avoided duringprocessing.
The gold protects the nickel during storage andprovides the low contact resistance required forthe thin gold deposits. ENIG is now arguably themost used finish in the PCB industry due thegrowth and implementation of the RoHsregulation

This finish is most used for aluminum wirebonding, metallic dome contacts, and EMIshielding. Compared to ENIG, Immersion Silveris cheaper, provides stronger solder joint, isequally as good for fine pitch, but is in generalmore sensitive to its environment such as heat,finger acid and sun, which will deteriorate thepreservative, making it susceptible to pollutionlike Sulphur dioxide, which tarnishes the surfaceand forms an AG2S layer (Silver sulfide). It istherefore recommended to keep it away fromdirect sunlight, heat and must be kept in avacuum sealed bag, until it’s ready to be soldered.When used, it is recommended to use gloves.When exposed to heat, finger acid, humidity orpollution, silver can still maintain goodsolderability, if silver sulfide has yet not beenformed on the surface. It is therefore alsorecommended to complete all stages of solderprocesses within 1 working day, depending on theamount of pollution in the air. When ImmersionSilver is used as a contact surface, it will showstrength inferiority to gold, but it will keep itselectrical properties, despite forming AgS2 layer.Prototype tip: An office article like an Eraser canremove the AGS2 layer, making it possible toachieve ok solderability.

ENEPIG, is one of the emerging new surfacefinishes and came on the market in the late 90s.This three-layer metallic coating consists ofnickel, palladium, and gold: it provides optionslike no others. It is wire bondable, has no risks ofBlack pads, is ideal for fine pitch as is a very flatsurface. During its introduction somesolderability issues were reported due to thepalladium layer being too thick.
The need for aseparate manufacturing line was not receptive forthese same reasons. Recently, ENEPIG has made acomeback as the potential to meet reliability andpackaging needs, and RoHS standards are a pluswith this finish. It is perfect for high frequencyapplications where spacing is limited. Whencompared to the other top four finishes, ENIG,Lead Free-HASL, Immersion Silver and OSP,ENEPIG outperforms all on the after-assemblycorrosion level. Today more and more factoriesoffer this as an alternative to ENIG, OSP andImmersion Silver. The cost is still a bit higherthan ENIG, but it also has advantages that mightoutweigh this fact. Today Azitech has many PCBfactories that provide this surface treatment.

LF HASL (LEAD FREE HOTAIR SOLDER LEVELLING)

When the LF HASL process is applied possible delamination weakness of the PCB will be revealed, as the process temperature of HASL is up to 265°C, which will apply unwanted stress to the material. Therefore, choice of material is crucial. It will however also identify any potential delamination issues well before any expensive components are attached to the board. The surface finish is not ideal for fine pitch, but as it is re-workable, it is excellent for prototyping.