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PCB Surface Finishes
Surface Finish Options for Solderability, Bonding, Storage, and Contact Areas
SUGA helps you choose the right PCB surface finish for your OEM project by identifying the pad function, package type, bonding requirements, storage exposure, edge contact needs, and verification requirements before finalizing fabrication details.
Finish Review Signals
- Surface finishes are compared to determine compatibility with pad function and package type.
- Bonding, contact, press-fit, and storage requirements are checked against the finish type and fabrication drawing notes.
- Before finalizing fabrication details, the project criteria, finish type, and fabrication drawing notes should align with the selected finish.
What a Surface Finish Can and Cannot Solve
What the Finish Helps Determine
PCB surface finishes protect exposed copper on the final working surfaces of solder pads, bonding pads, connector fingers, and other functional areas of a PCB and create the final surface needed for fabrication and assembly. They affect solderability, oxidation resistance, pad flatness, storage sensitivity, contact performance, and the surface condition needed for bonding or assembly.
Different surface finishes are used to address different problems. For example, ENIG provides a smooth surface for solder pads and fine-pitch packages. OSP can also be used to create flat pads, but its use is limited by shelf life when handling and heat exposure are not controlled. ENEPIG may be required when using wire bonding. Hard Gold is better suited to edge contact surfaces like edge fingers and areas subject to repeated insertion cycles, rather than as a general-purpose finish.
What the Finish Cannot Repair
Although some finish-related issues can be resolved with the proper finish type, most upstream issues cannot be resolved by changing the finish. A finish cannot be used to repair upstream issues such as incorrect pad geometry, an incorrect stackup, a via structure that is not suitable for the plating process, missing connector notes, unclear bonding pads, or an incorrect package-to-pad match. If your drawing simply states that a finish must be used but does not show how the pads are to be used, a finish choice may ultimately be based on undefined expectations.
It should be straightforward to match the finish type to the function of the surface being finished. Solder pads, bond pads, press-fit areas, edge contacts, and storage-sensitive boards do not all require the same surface condition to function as designed. To make the most informed decision when selecting a finish, information regarding fabrication drawings, pad geometries, package lists, storage methods, and bonding and contact areas should be compared.
Compare PCB Surface Finish Options by Function and Limitation
PCB surface finish selection starts with the intended function of each surface rather than simply habit. Two PCBs may both need a surface that can be soldered to; however, if your PCB design includes very fine-pitch surface-mount technology (SMT) packages, wire bonding, edge contacts, press-fit areas, or repeated thermal exposure, you may need to select a different surface finish than for a larger through-hole assembly.
Solderable Finishes
Lead-free hot air solder leveling (HASL), electroless nickel immersion gold (ENIG), organic solderability preservative (OSP), immersion silver, and immersion tin are some of the options you will have for finishing the solderable portions of your PCB. Not all of these can be interchanged. The solderable finish you select will depend on the density of pads, as well as whether there are BGAs, QFNs, or other pads that require coplanar surfaces.
Bonding and Contact Finishes
ENEPIG, Soft Gold, and Hard Gold finishes have more specialized purposes. ENEPIG and Soft Gold should be selected based on bonding needs, with Hard Gold generally associated with edge fingers, connector contacts, and areas that will be subjected to multiple insertions.
Limits to Check Early
The finish for your PCB should match the working surface condition. OSP finishes are well-suited for flat pads and short periods of controlled ambient storage. Immersion Silver may be considered for RF layouts requiring a conductive surface interface, while tarnish prevention requires care in handling, packaging, and storage. Concerns surrounding Immersion Tin with flat pads and some press-fit applications should be considered at the beginning of product development due to tin whisker concerns and the potential for copper-tin intermetallic compound formation. ENIG can provide coatings for flat pads; however, the risk associated with nickel corrosion from either a poorly controlled bath or excessive gold exposure remains an area of concern.
These values should be used as engineering reference points and not as universal finish guarantees. Final thickness, solderability testing, pad bonding, and contact-surface verification should follow the fabrication drawing for each application and the agreed project criteria.
Surface Finish Specifications & Application Scope
| Finish | Layer Composition & Thickness | Primary Function / Use Case | Known Limitation |
|---|---|---|---|
| Lead-Free HASL | Pb-free solder coating with a non-uniform thickness | Through-hole pads, larger SMT pads, general-purpose designs | Not recommended for BGA, dense QFN, or tight coplanarity pads |
| ENIG | Nickel: 3-6 um; Gold: 0.05-0.125 um | Fine-pitch SMT, BGA pads, flat solderable surfaces, storage-sensitive applications | Risk of nickel corrosion from either poorly controlled baths or excessive gold exposure |
| ENEPIG | Nickel: 3-6 um; Palladium: 0.05-0.15 um; Gold: 0.03-0.07 um | Wire bonding, mixed soldering and bonding, contact-capable pads | Higher process complexity; palladium layer control required |
| OSP | Organic copper protection film 0.2-0.5 um | Fine-pitch SMT, short storage cycle, a flat pad before lead-free reflow | Handling-sensitive; solderability loss after multiple heat cycles |
| Immersion Silver | Silver >=0.125 um or specified by the project | RF-sensitive layouts, flat pads, press-fit contact surfaces | Susceptible to tarnishing under an uncontrolled atmosphere; handling protection required |
| Immersion Tin | Tin 1.0-1.2 um | Flat pads, press-fit surfaces, lead-free soldering | Tin whisker concerns; Cu-Sn intermetallic compound formation with prolonged heat exposure |
| Hard Gold | Gold 0.75-1.27 um over Ni 2.5-5.0 um | Edge finger connectors, plug-in connectors, repeated insertion contacts | Limited to contact surfaces; not intended for general solderable finish |
| Soft Gold | Gold thickness specified by the bonding process over Ni | Wire bonding pads, specialized bonding surfaces | Bonding process capabilities must be established before fabrication |
When comparing finishes, the key is to match the finish to the working surface. The solder pad, bonding pad, press-fit hole, and edge connector all have different levels of risk exposure during normal use.
Choose by Pad Function and Package Type
The surface finish of pads or other functional areas will vary depending on their function. An example of this is a circuit board that contains fine-pitch SMT pads, BGA pads, wire-bond pads, edge fingers, and press-fit areas. It will not require just a finish name specified in the documentation, but a description of each area's functional requirements, which may include flatness, solderability, bonding surface, insertion wear, and contact stability.
Fine-Pitch, BGA, and QFN Pads
Fine-pitch SMT, BGA, and QFN areas will generally require a flat solderable surface that provides for the consistent release of paste and secure pad contact. Various finishes such as ENIG, ENEPIG, and OSP may meet the requirements for these areas; however, each area will be dependent upon factors such as package pitch, pad geometry, thermal exposure, storage, and accepted quality criteria.
Wire Bonding and Mixed-Function Pads
The selection of finishes for wire bonding pads may include finishes such as ENEPIG or Soft Gold. The bonding pads provided on the board may also affect the choice of finishes, as the wire type, bond pad layout, and bond parameters should all be determined before fabrication.
Edge Fingers, Contact Surfaces, and Press-Fit Areas
Edge fingers and repeated-insertion surfaces will require different finishes and considerations than those associated with other types of pads. Hard Gold is a finish that is specific to edge fingers or repeated-insertion surfaces, and is not used as a general solderable finish. Press-fit or compliant-pin pads will often be specified to have Immersion Tin, Immersion Silver, or finishes that will be defined within the drawings, based upon the connector specifications and hole tolerances.
Finish Recommendations by Design Scenario
| Design / Application Scenario | Finish Shortlist | Not Recommended When |
|---|---|---|
| Fine-pitch SMT / BGA | ENIG, ENEPIG, OSP | Lead-Free HASL on tight coplanarity pads |
| Wire bonding | ENEPIG, Soft Gold | OSP, Immersion Tin, Immersion Silver |
| Edge connector wear | Hard Gold | ENIG or immersion finishes on insertion-cycle contacts |
| RF / high-frequency layout | Immersion Silver, ENIG, ENEPIG | Tarnish-sensitive storage without packaging controls |
| Short storage before assembly | OSP, Lead-Free HASL | OSP after uncontrolled handling or exposure to atmosphere |
| Long storage before assembly | ENIG, ENEPIG | OSP or Immersion Silver without storage protection |
| Multiple reflow exposure | ENIG, ENEPIG | OSP or Immersion Tin after repeated thermal exposure |
| Press-fit pads | Immersion Tin, Immersion Silver, Hard Gold when defined for contact surfaces | OSP on insertion-contact surfaces |
| Mixed soldering + bonding | ENEPIG | ENIG unless the specified bonding or contact process is qualified for ENIG |
These recommendations are a starting point for project evaluation. A pad will not face the same type of surface requirement when used for soldering, bonding, repeated insertion, or press-fit purposes. Since the functions for each pad type are not determined by the drawing specifications, SUGA will establish the intended use for the pads prior to treating the surface finish as being final.
Match Finish Verification to the Drawing and Function
Determine finish verification from the functional use of the finished area. Not every finish proof is necessary for solder pads, bond pads, edge fingers, OSP-coated copper surfaces, or Immersion Silver pads. Drawings, finish callouts, the storage setting, and the functional use will dictate what information is relevant for each check.
Thickness and Layer Verification
When verifying a metallic finish coating that requires thickness measurement, it is standard to use X-ray fluorescence (XRF) measurement. ENIG, ENEPIG, Immersion Silver, Immersion Tin, Hard Gold, and Soft Gold coatings all typically have thickness records associated with them when defined on the drawings or through the quality agreement with the customer. Where ENIG is concerned, it may be necessary to validate the condition of the nickel layer where specified. For ENEPIG, it may also be necessary to verify the condition of the palladium layer.
Solderability and Surface Condition
Solderability checks fulfill a different requirement than thickness verification. In the case of Lead-Free HASL, ENIG, ENEPIG, OSP, Immersion Silver, or Immersion Tin, solderability records are used to document conditions that could create a risk to solderability due to storage, handling, or assembly exposure. The condition of OSP surfaces, tarnish on Immersion Silver, and the potential for whiskers forming from Immersion Tin should also be assessed based on the finish quality and project condition.
Bonding and Contact Records
Bonding surfaces and edge contacts should also have their own records kept. In cases where ENEPIG or Soft Gold are used for wire bonding, bond pull testing may be required, and records or documentation should be maintained to demonstrate compliance with the bonding specification. Hard Gold applied to edge contacts may involve contact resistance and tape adhesion testing when the contact detail of the product is defined.
Documentation Defined Before Fabrication
Finish records are useful only when the required records are defined before fabrication. Finish thickness records, solderability records, bond pull records, and contact performance records for the finished assembly must match finish requirements, not be added to an assembly after it is made.
When verification and documentation processes are defined during quotation, Request for Quote (RFQ), or project evaluation, SUGA can identify the required records based on the appropriate finish type, functional pad use, process use, quality requirements, and acceptance criteria for the project.
Verification Methods and Documentation per Finish
| Property Verified | Applicable Finish | Verification Approach | Acceptance Criteria | Engineering Documentation |
|---|---|---|---|---|
| Finish thickness | ENIG, ENEPIG, Immersion Silver, Immersion Tin, Hard Gold, Soft Gold | XRF measurement | IPC-4552 / IPC-4553 / IPC-4554 / IPC-4556; tolerance per fabrication drawing | XRF thickness report |
| Nickel layer integrity | ENIG, ENEPIG | Microsection per applicable IPC-TM-650 method | IPC-4552 / IPC-4556; functional callouts in fabrication print | Microsection micrograph report |
| Palladium layer control | ENEPIG | XRF multi-layer measurement | IPC-4556 tolerance range | Multi-layer XRF report |
| Solderability | Lead-Free HASL, ENIG, ENEPIG, OSP, Immersion Silver, Immersion Tin | J-STD-003 wetting balance or dip test | J-STD-003 category requirement; build acceptance level | Wetting balance test report |
| Tarnish condition | Immersion Silver | Visual inspection with magnified optical inspection | Documented storage and handling conditions; visible tarnish grading | Visual inspection report |
| OSP surface condition | OSP | Pre-reflow visual inspection; solderability test after storage | Storage label conditions; J-STD-003 solderability results | Pre-reflow surface inspection report |
| Tin whisker concern | Immersion Tin | Microscopic inspection; JESD201 assessment when applicable | Application exposure; storage parameters; finish definition | Tin whisker assessment record |
| Bondability | ENEPIG, Soft Gold | Destructive wire bond pull test | Bond specifications; wire type; minimum pull target | Bond pull test report |
| Edge contact reliability | Hard Gold | Contact resistance and tape adhesion testing | Mechanical drawing; contact surface specifications | Contact performance report |
These verification methods and documentation records do not apply to every order. However, they help align finish verification with the drawing, functional pad use, and mutually agreed inspection criteria. If thickness reports, solderability reports, bond pull reports, and contact performance reports are to be included in your order, expectations for these documents should be established prior to fabrication.
Missing Drawing Details That Can Change the Finish Decision
Only listing the finish name in the fabrication drawing is not enough. The drawing must also show where the finished surface will be used to support soldering, bonding, repeated contact, press-fit insertion, or storage before assembly.
Missing Bonding and Contact Details
Areas designated for bonding must identify wire type and bond pad locations, along with any pull-test requirements affecting the finish. Edge fingers must be located where Hard Gold starts and where the general board finish ends. Clearly defined limits also apply to selective finish areas, as no matter what the intended application for the contact surface is, it cannot be assumed it will behave as a standard solder pad.
Missing Pad and Package Details
Fine-pitch SMT pads, BGA pads, QFN pads, and press-fit holes must be distinguishable by the drawing and the actual package information. If the drawing does not include geometry, solder mask openings, connector specifications, or drill chart tolerances for these types of pads, there is no way to determine whether one finish is enough or if a local finish designation will have to be treated differently.
Handling, Storage, and Process Notes
Repeated reflow, staged assembly, storage prior to soldering, sea freight, and packaging conditions can influence how OSP, Immersion Silver, Immersion Tin, ENIG, and ENEPIG are evaluated. While these notes do not have to be detailed, they need to clearly indicate how handling conditions may affect finish decision-making.
SUGA may flag missing items from the drawings when it comes to pad function, storage exposure, bonding requirement, or connector usage. This is to ensure that a solder pad, bond pad, press-fit area, or edge contact will not be finished under the wrong interpretation.
Storage and Reflow Risks After Finish Selection
When finishes are selected, there is still more to be done. The behavior of the same surface can vary after it has been stored for lengthy periods, handled in an uncontrolled manner, shipped via sea freight, staged for assembly, or subjected to multiple thermal exposures. These exposures can alter the solderability of the surface, its surface cleanliness, and oxidation risk, and can affect how solderability will need to be documented prior to assembly.
OSP After Storage and Heat Exposure
There are specific risks associated with OSP finishes after they have been stored for extended time or subjected to multiple thermal exposures. The organic OSP coating is a sensitive protective layer and is easily affected by handling, atmosphere, humidity, temperature, and repetitive heating. If parts are going to wait for assembly or go through multiple reflow cycles prior to final assembly, the storage conditions, solderability requirements, and finish expectations must be clearly defined prior to accepting the finish.
Immersion Silver, Storage, and Tarnish
Tarnishing of Immersion Silver surfaces can occur when proper control of environmental exposure and physical protection is not maintained. If the circuit board design is RF-sensitive, then the electrical reason for using the Immersion Silver finish should not be confused with the surface condition related to environmental exposure. Parameters related to shipment and storage must also match the finish selected.
Immersion Tin Over Time and Temperature
When considering Immersion Tin finishes for potential tin whiskers and copper-tin intermetallic growth, the degree of concern will increase with an increase in exposure duration, increased temperatures, and application risk. Therefore, the storage duration, thermal exposure, and connector usage should be evaluated before Immersion Tin is selected for the design.
ENIG and ENEPIG Still Need Defined Records
Regarding ENIG and ENEPIG finishes, both require that the finish be properly defined and the processes used to create the finish be documented. The concern of creating a black pad in the ENIG process is a process and finish verification concern. It should not be assumed that every ENIG finish is a risk. With respect to ENEPIG, when designing a circuit board to rely on palladium layer control, both the palladium layer control and bonding considerations must be clearly defined.
The practical step is to match finish selection with the actual handling path the boards will take. The storage environment, type of packaging used, number of reflows, assembly sequence, and pad use will all impact the documentation required to plan fabrication and assembly.
Why the Lowest Finish Price Is Not Always the Lowest Project Cost
The surface finish cost is more than just the unit cost of the finish; a lower-cost surface finish may be expensive if it introduces solderability risk, storage limitation, additional review requirements, or if it does not meet the bonding, contact, or press-fit needs of the design. At the same time, a higher-cost finish may be unnecessary if the design does not require that finish's functionality.
Single Finish Price vs. Total Project Cost
The OSP finish is frequently used when a flat solderable surface and short storage cycle meet the needs of the project. However, the OSP finish does not necessarily represent the lowest project cost for all projects. If the project lacks a clear understanding of the handling, packaging, reflow exposure, or staged assembly, then the project risk associated with the finish chemistry will shift to solderability assurance and rework exposure.
Higher-Cost Finishes That Serve a Function
ENEPIG, Soft Gold, and Hard Gold have a different cost basis. Specific applications for the finishes include situations in which the bonding performance is included within the design, such as ENEPIG or Soft Gold, or Hard Gold is used for defined edge contact areas, such as edge fingers or repeated insertion surfaces. In these instances, the finish value reflects functionality rather than preference.
Lower-Cost Finishes That Still Need Clear Parameters
Immersion Tin and Immersion Silver finishes also require clarification regarding the conditions under which they will be used. Immersion Tin provides a flat pad and may be used with some press-fit designs when connector specifications support it, but potential tin whisker concerns and growth of copper-tin intermetallics can affect long-term reliability and require assessment prior to implementation. Immersion Silver finishes can provide flat pads and accommodate RF-sensitive designs; however, tarnish protection, storage parameters, and handling are cost-risk factors that require separate evaluation.
The comparison of costs can be made once the hidden cost associated with functional requirements, such as special finish areas, storage requirements, record requirements, and changes made after fabrication, is visible; therefore, these types of costs can be determined prior to fabrication of the item, and none of these costs should be treated as surprises to manage after fabrication.
Who Should Define the Surface Finish Specification?
A surface finish decision can rarely be attributed to only one person. Therefore, several roles can influence the surface finish specification:
Confirmation by Role
| Role | What They Usually Define |
|---|---|
| Design engineer | Pad function, package type, bonding locations, contact surface locations |
| Manufacturing engineer | Reflow exposure, process sequence, solderability risk |
| QA / compliance | Verification records, acceptance requirements, documentation scope |
| Procurement / supply chain | Storage time, shipping condition, availability, cost parameters |
| Project owner / engineering manager | Function, risk, cost, schedule, and required documentation |
A separation of responsibilities among the various roles allows for a greater level of certainty regarding the surface finish of a design and also reduces the need for guesswork. For example, the design engineer determines the working surface, the quality engineer determines what records are needed, and the procurement engineer determines what storage or availability conditions exist for a given finish prior to the finish being quoted.
What SUGA Needs Before Fabrication
To define a finish for a product, a fabrication drawing is used. However, the fabrication drawing by itself is not sufficient; SUGA will require an understanding of the intended use of each finished area, such as soldering, bonding, press-fit insertion, repeated contact, staged assembly, and storage prior to assembly. The evaluation may be placed on hold if SUGA identifies any gaps. SUGA will clarify what assumptions were made regarding the gaps identified prior to continuing the evaluation.
Finish and Fabrication Inputs
The finish designation on any given drawing, purchase order, or fabrication print is the first check for SUGA. If a drawing, purchase order, or print mentions that the finish used on this part will be ENIG, OSP, ENEPIG, Immersion Silver, Immersion Tin, Hard Gold, Soft Gold, or Lead-Free HASL, SUGA is able to compare the callouts with pad geometry and functional areas. If the finish for the pads is unclear or not included on the drawing, purchase order, or print, SUGA will indicate that the quote used an assumption regarding the finish being used, and this gap must be identified prior to continuing with the fabrication process.
Package and Pad Information
The package type used on the board, along with the type of pads used on the board, is very important to completing the finish assessment for a product. Relevant package and pad information may include BGA, QFN, fine-pitch SMT, press-fit connectors, wire-bond pads, and edge fingers. The pad and package lists will indicate whether the finish must support flat solderable pads, bonding surfaces, insertion-contact surfaces, or surfaces subject to repeated insertion or contact.
Thermal, Storage, and Handling Notes
The thermal and storage notes are also relevant for determining what finish is needed for the production of the board. When assessing the requirement for a particular finish, it is critical to consider any unique thermal or storage environment that SUGA may need to work within. Details such as reflow count, assembly side sequence, shipping condition, storage environment, and packaging requirements can influence the selection of finishes for OSP, Immersion Silver, Immersion Tin, ENIG, and ENEPIG. This information does not necessarily need to be compiled into an extensive report; however, sufficient clarity must be provided to avoid unnecessary instances of finish mismatch.
Several items are used to define a finish before fabrication. Not all projects require all items, but inadequate information related to missing finish, pad, thermal, storage, bonding, or contact surface requirements can leave risk associated with the treatment of critical surfaces unresolved.
Pre-Fabrication Data Required for Finish Selection
| Engineering Item | When Applicable | Accepted Format | Role in Finish Selection | Risk If Omitted |
|---|---|---|---|---|
| Finish designation | All PCB fabrication orders | Fabrication print; purchase order note; finish designation | Establishes a baseline for surface treatment before fabrication | Surface treatment is selected by default rather than through design intent |
| PCB stackup | Multilayer or impedance-controlled builds | Stackup PDF; ODB++; IPC-2581 | Provides the context for copper weight, dielectric material, and copper plating | Finish may conflict with the layer structure or stackup plan |
| Package list | SMT, BGA, QFN, fine-pitch, or press-fit builds | Bill of materials (BOM); assembly drawing | Establishes the relationship of component density to coplanarity and solderability requirements | BGA, QFN, or fine-pitch packages may experience finish mismatch |
| Pad geometry | Fine-pitch, BGA, contact, or press-fit areas | Gerber RS-274X; ODB++; IPC-2581; pad drawing | Establishes flatness limitations, contact surfaces, and solderable areas | Pad-level plating issues may remain undetected until production |
| Reflow count | Multi-side SMT or multiple thermal exposures | Assembly process note; thermal profile | Validates thermal exposure for OSP, Immersion Tin, ENIG, or ENEPIG | Without thermal data, solderability after the second reflow cannot be confirmed |
| Storage environment | Poor storage conditions, long-term storage, or sea freight | Procurement specification; storage instructions | Establishes vacuum seal requirements, desiccant requirements, and storage atmosphere | Risk of tarnish, oxidation, or loss of solderability due to storage |
| Wire bonding limits | Wire-bond builds only | Bonding specification; wire type | Establishes the target for ENEPIG or Soft Gold deposition | Bond pull failure or unstable bond interface |
| Edge connector profile | Edge connector fingers or repeated insertion zones | Mechanical CAD / DXF; contact layout | Establishes the Hard Gold plating zone, thickness, and bevel angle | Risk of contact wear-through or mating-cycle failure |
| Press-fit mechanics | Press-fit pins or compliant-pin connectors | Connector specification; drill chart tolerance | Matches Immersion Tin, Immersion Silver, or Hard Gold to the insertion forces when contact surfaces are defined | Risk of pin insertion damage or long-term contact instability |
Request a Quote
To obtain a quote from SUGA, customers should provide a fabrication drawing, Gerber files, and finish specification along with any instructions related to bonding, edge contacts, press-fit areas, storage, or repeated reflow. If an assembly evaluation is required, a BOM and package information should also be submitted so that the surface finish can be evaluated against the entire project context.
SUGA will review the finish specification against the files submitted for the project and note any details that must be resolved prior to finalizing the quote or fabrication plan.
Request a Quote
FAQ
Some of the most commonly used finishes for PCBs include Lead-Free HASL, ENIG, ENEPIG, OSP, Immersion Silver, Immersion Tin, Hard Gold, and Soft Gold. Each of these finishes has a specific purpose and use within the PCB manufacturing process; while some finishes primarily provide support for solderability, other finishes provide support for surface flatness, and still other finishes provide support for bonding or contact areas. The finish selected will depend on the function of the pads, type of package being used, the storage parameters of the PCBs, and how bonding and contact areas are defined. A commonly used finish is not necessarily the correct finish for every design.
To select the right surface finish for your PCB, you must first consider the working function of the surface. For example, fine-pitch SMT pads, BGA pads, wire-bond pads, edge fingers, and press-fit pads all require different specifications for their respective applications. Each finish can be matched against several criteria including solderability, flatness, bonding surface, contact wear, storage exposure, and the requirements for confirming the use of the finish. Detailed drawings, pad geometries, package specifications, storage notes, and bonding or contact areas will allow you to narrow down your choices for finishes prior to submitting for fabrication.
There is no single recommended surface finish for all PCBs. While ENIG may work well for flat pad mounting and fine-pitch packaging, OSP may be effective for PCBs with short-term storage cycles where handling is controlled. There are instances where ENEPIG or Soft Gold will be required for wire bonding, and Hard Gold is typically specified for edge-contact surfaces, such as edge fingers. The best decision regarding PCB surface finish is determined by the individual PCB function, project context, and quality records necessary for acceptance into production.
OSP is an organic surface finish used as a protective coating for exposed copper surfaces prior to soldering. OSP will provide a relatively flat solderable surface; therefore, OSP is well suited for applications involving fine-pitch SMT pads or projects with short-term storage cycles. The primary drawback of OSP is susceptibility to handling, storage atmosphere, and repeated thermal exposure. OSP is not inherently inferior to other finish materials, but for OSP to remain acceptable, OSP needs an appropriate storage plan, assembly sequence, and solderability expectations that match OSP finishes.
The primary difference between OSP and ENIG is that OSP is an organic coating used for protection of copper surfaces, while ENIG consists of two metal layers: a nickel layer and a thin gold layer. While both finishes provide a flat solderable surface, OSP is limited to applications where there is short-term storage and controlled handling exposure, whereas ENIG is chosen for applications requiring flatter pad surfaces, fine-pitch packaging, or extended storage. The processing of ENIG is often more complex than the processing of OSP, whereas OSP requires more rigorous handling and reflow discipline. The best choice of these finishes is determined by the application and project conditions.
Wire-bond finishes are typically selected based on bondability rather than only solderability, so common finishes used with circuit boards that contain wire-bond pads include ENEPIG and Soft Gold. The finish choice will vary depending upon the type of wire being used, how the bond pads are configured on the circuit board, bonding parameters used during bonding, and pull-test record requirements. OSP, Immersion Tin, and Immersion Silver are generally not considered to be wire-bond finishes; therefore, all bonding details should be known prior to fabrication.
Through storage and reflow, solderability and surface condition can change. The storage environment, packaging, number of reflows, and assembly order should be evaluated as early in the process as possible.
Information SUGA needs includes the finish type, fabrication drawings, Gerber files, pad configuration information, package information, and all other applicable notes related to storage, reflow, bonding, edge contacts, and press-fit areas. If SUGA will be evaluating assembly information, SUGA will also need the BOM and package information in order to relate the finish to all aspects of the project.
The scope of work depends upon the design itself. Inconsistencies in notes about the surface finish and the omission of needed functionality within the design’s functional areas can create working assumptions during quoting and fabrication evaluation.
Related Manufacturing Capabilities
Surface finish selection also depends on material choice, tolerance limits, finish records, and specialty-board constraints when the finish decision involves more than the finish name.