By Al Wright, PCB Field Applications Engineer
Epec Engineered Technologies
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Anyone involved within the printed circuit board (PCB) industry understand that PCBs have copper finishes on their surface. If they are left unprotected then the copper will oxidize and deteriorate, making the circuit board unusable. The surface finish forms a critical interface between the component and the PCB. The finish has two essential functions, to protect the exposed copper circuitry and to provide a solderable surface when assembling (soldering) the components to the printed circuit board.
Hot Air Solder Leveling (HASL) was once the tried and true method of deliver consistent assembly results. However, the ever-increasing circuit complexity and component density has stretched the capabilities of even horizontal solder levelling systems to their limits.
As component pitches became finer and a need for a thin coating became greater, HASL represented a process limitation for PCB manufacturers. As an alternative to HASL, alternative coatings have been around for several years now, both electrolytic and immersion processes.
Listed below are some more common surface finishes used in PCB manufacturing.
HASL is the predominant surface finish used in the industry. The process consists of immersing circuit boards in a molten pot of a tin/lead alloy and then removing the excess solder by using 'air knives', which blow hot air across the surface of the board.
One of the unintended benefits of the HASL process is that it will expose the PCB to temperatures up to 265°C which will identify any potential delamination issues well before any expensive components are attached to the board.
Printed Circuit Board with HASL / Lead Free HASL Surface Finish
According to IPC, the Association Connecting Electronics Industry, Immersion Tin (ISn) is a metallic &#;nish deposited by a chemical displacement reaction that is applied directly over the basis metal of the circuit board, that is, copper. The ISn protects the underlying copper from oxidation over its intended shelf life.
Copper and tin however have a strong affinity for one another. The diffusion of one metal into the other will occur inevitably, directly impacting the shelf life of the deposit and the performance of the &#;nish. The negative effects of tin whiskers growth are well described in industry related literature and topics of several published papers.
Printed Circuit Board with Immersion Tin Surface Finish
Immersion silver is a non-electrolytic chemical finish applied by immersing the copper PCB into a tank of silver ions. It is a good choice finish for circuit boards with EMI shieldingand is also used for dome contacts and wire bonding. The average surface thickness of the silver is 5-18 microinches.
With modern environmental concerns such as RoHS and WEE, immersion silver is environmentally better than both HASL and ENIG. It is popular also due to its lesser cost than ENIG.
Printed Circuit Board with Immersion Silver Surface Finish
OSP (Organic Solderability Preservative) or anti-tarnish preserves the copper surface from oxidation by applying a very thin protective layer of material over the exposed copper usually using a conveyorized process.
It uses a water-based organic compound that selectively bonds to copper and provides an organometallic layer that protects the copper prior to soldering. It's also extremely green environmentally in comparison with the other common lead-free finishes, which suffer from either being more toxic or substantially higher energy consumption.
Printed Circuit Board with OSP / Entek Surface Finish
ENIG is a two layer metallic coating of 2-8 μin Au over 120-240 μin Ni. The Nickel is the barrier to the copper and is the surface to which the components are actually soldered to. The gold protects the nickel during storage and also provides the low contact resistance required for the thin gold deposits. ENIG is now arguably the most used finish in the PCB industry due the growth and implementation of the RoHs regulation.
Printed Circuit Board with Electroless Nickel Immersion Gold (ENIG) Surface Finish
ENEPIG, a relative newcomer to the circuit board world of finishes, first came on the market in the late 90s. This three-layer metallic coating of nickel, palladium, and gold provides an option like no others: it is bondable. ENEPIG&#;s first crack at a printed circuit board surface treatment fizzled with manufacturing due to its extreme high cost layer of palladium and low demand of use.
The need for a separate manufacturing line was not receptive for these same reasons. Recently, ENEPIG has made a comeback as the potential to meet reliability, packaging needs, and RoHS standards are a plus with this finish. It is perfect for high frequency applications where spacing is limited.
When compared to the other top four finishes, ENIG, Lead Free-HASL, immersion silver and OSP, ENEPIG outperforms all on the after-assembly corrosion level.
Printed Circuit Board with Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) Surface Finish
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Hard Electrolytic Gold consists of a layer of gold plated over a barrier coat of nickel. Hard gold is extremely durable, and is most commonly applied to high-wear areas such as edge connector fingers and keypads.
Unlike ENIG, its thickness can vary by controlling the duration of the plating cycle, although the typical minimum values for fingers are 30 μin gold over 100 μin nickel for Class 1 and Class 2, 50 μin gold over 100 μin nickel for Class 3.
Hard gold is not generally applied to solderable areas, because of its high cost and its relatively poor solderability. The maximum thickness that IPC considers to be solderable is 17.8 μin, so if this type of gold must be used on surfaces to be soldered, the recommended nominal thickness should be about 5-10 μin.
Printed Circuit Board with Gold &#; Hard Gold Surface Finish
It is important to select the appropriate surface finish for your project by considering the various options while factoring in performance requirements and material costs.
For an example, if you are looking for the lowest cost then Tin-Lead HASL might seem like a good choice, but it is not suitable for RoHS-compliant products. If your product does require RoHS, you might consider lead-free HASL. That is only if there are no fine pitch components, since LFHASL cannot be applied perfectly flat. If your design needs to be RoHS compliant and does use fine pitch components, then you'll need to select a flat, lead-free finish, such as Immersion Silver or ENIG. Bear in mind that doing so will necessitate the use of more costly high temperature laminate.
Unsure of what you will need? Consult with a PCB fabricator prior to you making a selection. This will ensure that the combination of the surface finish and material will result in a high-yielding, cost-effective design that will perform as expected.
For more information on circuit board surface finishes, check out our blog posts on why you should bake OSP circuit boards before use and proper circuit board handling with immersion finishes.
Our team of experienced engineers is here to help you design a custom PCB solution, utilizing the various surface finished from HASL, Immersion Tin, OSP/Entek, ENIG, ENEPIG, or Hard Gold.
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First let&#;s discuss the basics when it comes to a typical rigid PCB. During the production process with a rigid PCB, we would image and etch our inner layer cores, laminate them together, drill and plate with Electro Deposited copper (ED copper), and then move on to pattern plating. So you have copper foil with essentially three additional copper deposits on top of it.
With flex PCBs it&#;s different. We don&#;t want to stack a ton of copper over our pattern because it reduces flexibility, which is the objective when creating a flex PCB. Also, the type of copper material is different. Flex PCBs uses a base copper of Roll Anneal (RA copper). This has a horizontal, linear grained structure and when it&#;s bent, it doesn&#;t have the tendency to fracture. ED copper has a more granular grain structure. If we plated it like we did a traditional multilayer PCB, we&#;d be stacking ED copper on top of that and the result is a dissimilar copper grain structure. Where RA copper is intended to bend and where ED copper is not, you could in theory have reduced flexibility or potentially a defect with the copper partially fracturing or failing to bond to itself.
That brings us to the need for button plating. Whether it&#;s to support a dynamic bending application by allowing for a single deposit of RA copper or facilitating higher speed impedance control by supporting a homogenous base copper. In the process of button plating we only plate the areas of the PTH (plated through hole) and supporting pads. The term button plating refers to the visual aspect of the final product. If you were to look at the cross-section, you would see your flat surface and then you would have what looks like buttons as the PTH is slightly raised from the other copper features. The process is done by selectively masking the areas which do not receive the plating, allowing the plating chemistries to only access the areas of the PTH. We&#;re putting copper in the PTHs, we just sometimes do it differently between flex PCBs and rigid PCBs.
This process can also be utilized in some cases for other types of technology where a heavier deposit is required in the PTH but not across the entire surface of your outer layers. An example of this could be .002&#; minimum through hole plating with a design not intended to support heavy copper. I would be cautious with this technology as such a process can be demanding on the PCB manufacturer and in turn a significant cost driver.
A potential added benefit to PCB button plating is the ability to have smaller line width and spacing. Since the increase in miniaturization, many times a flex PCB is built to be in a smaller space. Although this is not always the case. I&#;ve built flex boards that are almost two feet long. If you visualize the copper plating process on a multilayer PCB, we&#;re including the plating on all features, and all the circuitry is going to get that additional plating on it. So we&#;re plating in a 3-dimensional trapezoidal shape from the top as well as the sides. All the features that are close together get even closer as we plate them.
On a flex PCB, because we use button plating, once you image and etch, the circuitry is done. The plating process does not further reduce any spacing. This allows us to achieve finer features. For reference, on a multilayer PCB, we&#;re limited at about 2.99 mils line, width and spacing. With a flex PCB we can get around 1.99 mils. Although this is just 1 mil difference, depending on the design, this could really mean all the difference. The design can be smaller which supports miniaturization and the ability to have smaller features closer together.
So there you have it. Rigid PCBs and flex PCBs often utilize different materials and employ a different production process. This is important to keep in mind during the design and procurement stages of producing a PCB. If you&#;re new to flex PCBs, more than likely you&#;ll want a different fabricator than a fabricator producing traditional multilayer PCBs. Another benefit to working with us at NCAB Group, as we produce a high mix of various technologies, we are always able to provide design support and source the right factory for the right design, getting it right from the start.
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