Introduction

Organic Solderability Preservative (OSP) is a thin organic coating that is applied to the surfaces of printed circuit boards (PCBs) to protect the copper from oxidation and preserve solderability. OSP coatings provide excellent shelf life for PCBs, maintaining solderability for up to 1 year of storage under typical conditions.

OSP coatings work by forming a very thin polymer film on top of the copper that acts as a barrier to prevent oxidation. This allows the copper to remain solderable for a prolonged period of time. OSPs are designed to be removed during the soldering process, exposing clean copper for excellent wetting when solder is applied.

Some key benefits of OSP coatings:

• Long shelf life – preserves solderability for 6-12 months typically
• Excellent solderability – promotes good wetting when solder is applied
• Halogen-free formulations available
• Environmentally friendly low-solids aqueous process
• Compatible with other PCB finishes like ENIG and immersion silver
• RoHS compliant formulations available
• Can be applied to large variety of PCBs and components

OSP chemistry is continuing to evolve, with newer generations providing longer protection against oxidation while remaining easy to remove during soldering. This article provides an in-depth look at OSP technology, processing, performance, testing, and industry standards compliance.

OSP Formulations

There are a variety of OSP chemistries available, with the most common based on organic acids like benzimidazole, benzothiazole, and imidazole. These OSP solutions contain an organic acid, a reducible salt, and sometimes proprietary accelerators/activators in an aqueous vehicle.

During processing, the OSP chemistry reacts with the copper surfaces in an acidic reducing environment. The organic acid component polymerizes on the board surface, forming a thin polymer film that encapsulates the copper traces. Popular OSP chemistries include:

• Benzimidazole
• Benzothiazole
• Imidazole
• Mercaptobenzothiazole
• Bis-benzothiazolythio-methane

The thickness of OSP coatings typically ranges from 0.05-0.3 microns. The coating needs to be thin enough to completely volatize and decompose during soldering, while still being thick enough to provide adequate oxidation protection.

The newest generation of OSP chemistries use proprietary blends and additives to enhance performance. This includes improved thermal stability to prevent coplanarity issues, along with optimizing viscosity and coating quality.

OSP Processing

OSP coatings are applied to PCBs using a vertical spray process. The OSP chemistry is sprayed onto the top and bottom board surfaces as the PCBs are conveyed through the processing equipment.

The typical OSP process sequence includes:

1. Cleaner – An alkaline or acid cleaner removes oils, residues and other contaminants from the PCB surface. Thorough cleaning is essential for OSP adhesion.
2. Microetch – A mild microetch roughens the copper surface removing a thin layer of copper, which improves OSP adhesion.
3. Water Rinse – Rinses remove any etchant solution from the PCB surface.
4. OSP Application – OSP solution is sprayed onto the top and bottom board surfaces as it passes through the equipment. The OSP application line includes multiple spray stages with drying in between.
5. DI Water Rinse – A final rinse stage removes any residual OSP chemistry.
6. Dry – Hot air knives thoroughly dry the PCBs exiting the line.

Process control needs to be tightly maintained throughout the line. Parameters like temperature, conveyor speed, spray pressures/exposure times, and chemistry concentration need to be dialed in based on the OSP chemistry and board designs being run.

OSP Equipment

OSP is applied using custom spray equipment designed specifically for PCB finishing. These spray application lines utilize a conveyorized equipment configuration with a series of spray stages and drying zones.

• Loading zone – Boards are loaded into carrier racks or racks with removable panels. Loading can be automated or manual.
• Pre-treatment – Incorporates cleaner, microetch, and rinse stages.
• OSP application – Comprised of multiple spray stages with drying in-between. Often 3-5 stages.
• Final rinse – DI water rinse stage after OSP application.
• Drying – Hot air knives thoroughly dry boards exiting the line.
• Unloading – Boards are manually or automatically removed from the line.

Programmable logic controllers (PLCs) are used to control equipment sequences, conveyor speeds, spray pressures, and chemistry flow rates. Programming recipes makes it easy to quickly switch chemistries and process different board designs.

OSP Chemistry

OSP chemistries are engineered formulations containing organic acids, reducible salts, accelerators, and sometimes additional proprietary ingredients:

• Organic acids – Form the protective polymer film on copper surfaces. Common acids used include benzimidazole, benzothiazole, imidazole, and mercaptobenzothiazole.
• Reducing agents – Help initiate polymerization reaction and maintain plating bath stability. Some common reducing agents are formaldehyde and tetrahydroquinone.
• Accelerators – Proprietary ingredients to accelerate polymerization reactions. Improves solderability life and thermal stability.
• pH buffers – Help maintain bath pH and chemistry stability.
• Surfactants – Used to optimize solution drainage from boards and improve coating quality.
• DI water – Used as the main solvent. Chemistry concentrations are typically 5-15%.

OSP chemistries have an optimized pH range they need to be maintained at through replenishment. Running too low in pH can hinder OSP polymerization while too high can cause excessive build-up and precipitation.

OSP Process Control

Maintaining consistent OSP process control is key to achieving optimal coating quality and performance. Some of the main process control parameters include:

• Conveyor speed – Typically range from 1-10 feet per minute depending on the line. Faster speeds may require higher spray pressures to get adequate chemical exposure time.
• Solution temperature – Typically 65-80°F. Higher temperatures accelerate OSP reaction rates.
• Spray pressures – Need to be optimized to get complete chemical coverage while not over-exposing. Pressures range from 15-40 psi.
• Chemistry concentration – Mix ratios need to be maintained through titrations and specific gravity measurements. Low concentration can result in uneven OSP deposition.
• Solution filtration – Removes particulates that can clog spray nozzles. 5 micron pre-filters are commonly used.
• PCB preheating – Boards are often preheated to 80-100°F prior to OSP to enhance coating performance.

Monitoring OSP thickness on finished boards through periodic measurements helps validate that the line is dialed in properly.

OSP Performance Properties

The main functions of OSP coatings are to protect copper surfaces from oxidation while maintaining excellent solderability. Some key performance characteristics include:

Solderability Protection

The primary purpose of OSP is to preserve solderability on printed circuit boards over extended storage times. Solderability refers to how well molten solder will wet to and spread over copper surfaces during assembly processes like wave soldering.

OSP coatings keep copper traces in a clean, solderable state preventing the formation of surface oxides over time. Well-coated OSP boards can maintain good solderability for 6-12 months when stored properly.

Proper dry storage conditions are important for maximizing OSP shelf life. Storage at low humidity prevents moisture absorption into the coating which can degrade oxidation protection. Storing boards in vacuum sealed bags along with desiccants helps prolong shelf life.

Solderability Testing

There are several test methods used to evaluate OSP solderability performance:

• Wetting balance testing – Measures the wetting force as molten solder is applied to OSP coated copper coupons. The wetting force provides a quantitative measure of solderability.
• Spread testing – Visual test method where the spread/coverage of an applied solder droplet is evaluated and rated. Provides a qualitative analysis.
• Surface insulation resistance (SIR) – Used to analyze the cleanliness and non-conductivity of OSP coatings.
• Copper mirror corrosion testing – Copper coupons are coated with OSP, aged in humidity chambers, and then visually examined for oxidation and corrosion.

Continual solderability testing of OSP boards under storage provides valuable data on the real-world coating lifespan and performance over time.

Thermal Stability

An important OSP selection criteria is its thermal stability, or its ability to withstand elevated process temperatures without charring, cracking, or causing other defects.

Many newer generation OSP chemistries are engineered for improved thermal stability to withstand processes like reflow soldering without impacting quality or reliability. However, there are some process limitations:

• Multiple high temperature exposures can gradually degrade OSP coatings.
• Lead-free reflow profiles above 245°C may require adjustments to limt OSP exposure time above 200°C.
• The use of nitrogen reflow environments is preferred over air when using OSP.

Careful process control is required when using OSP on boards that will undergo multiple high temperature exposures. Extended reflow times or excessive temperatures can sometimes carbonize OSP coatings.

Coplanarity

For multilayer and fine pitch boards, it is important that OSP coatings do not build up excessively and cause coplanarity issues with pad or component heights.

Proper spray application along with optimized chemistries prevents excessive OSP deposits. Frequently cleaning spray nozzles, solution baths, and drain traps helps minimize build-up of coatings over time during processing.

Newer generation OSP formulations also contain leveling agents to help minimize coating thickness variances across circuit board surfaces.

Removability

A defining characteristic of OSP finishes is that they are designed for easy removal during soldering processes. The thin organic coating volatizes completely when exposed to soldering heat, leaving behind fresh copper for excellent wetting.

However, if OSP coatings are not properly removed, it can negatively impact solder joint quality and long-term reliability. Insufficient heat or short exposure times can result in solder beading off pad surfaces rather than wetting and sticking properly.

When process conditions are optimized, OSP coatings provide very clean copper surfaces after reflow for high wetting forces and reliable solder joints. The temporary nature of OSP makes rework easy.

Comparing OSP to Other Final Finishes

OSP provides an appealing combination of low cost, environmental friendliness, and excellent solderability. It offers some advantages over other common final finishes:

OSP vs. Immersion Tin

• Lower material cost than immersion tin plating
• Easy rework compared to possible tin whisker issues
• No dendrite formation like electroless tin

OSP vs. Immersion Silver

• Significantly lower cost finish than immersion silver
• No discoloration or tarnishing like silver does

OSP vs. ENIG

• Much lower cost finish than electroless nickel/immersion gold (ENIG)
• OSP has smaller minimum feature size capabilities

OSP vs. HASL

• Lower processing temperatures than hot air solder leveling (HASL)
• Eliminates concerns over tombstoning defects with HASL

OSP provides the optimal combination of cost, shelf life, and solderability for many PCB applications. It is a versatile finish option suitable for consumer, automotive, telecom, medical, and industrial electronics needing 6+ months of shelf life.

OSP Process Troubleshooting

Like any surface finish, OSP requires careful process control and maintenance to achieve optimal results. Here are some common OSP issues along with potential corrective actions:

Problem: Short or failing solderability life

• Solutions:
  • Increase OSP thickness by lowering line speed
  • Check spray pressures, nozzles, chemistry concentration
  • Replace aged OSP solution
  • Improve dry storage conditions
  • Confirm reflow profile stays under 200°C for no more than 90 sec

Problem: OSP cracking, charring, outgassing

• Solutions:
  • Lower reflow temperature or shorten time above 200°C
  • Switch to a more thermally stable OSP chemistry
  • Use airless nitrogen reflow when possible

Problem: Excessive thickness, copper staining, carbon residue

• Solutions:
  • Increase line speed
  • Reduce number of spray stages
  • Lower chemistry concentration
  • Increase rinse exposure time
  • Clean spray nozzles and line equipment

Problem: Dewetting, solder beading, non-wet opens

• Solutions:
  • Check reflow profile and increase time above 183°C
  • Increase OSP thickness
  • Switch to more stable OSP chemistry
  • Improve dry storage and handling
  • Eliminate multiple high temp excursions

Problem: Poor adhesion, OSP peeling

• Solutions:
  • Increase microetching time/strength
  • Use cyanide or plasma etch processes
  • Lower conveyor speed
  • Increase number of spray stages
  • Check cleaner concentration

Careful attention to process details, chemistry management, storage conditions, and reflow profiling is key to maximizing OSP performance and lifespan.

OSP Quality Control Testing

To validate OSP coatings are being deposited correctly, there are several analytical tests than can be performed:

OSP Thickness

The thickness of OSP coatings is typically measured using a coulometric test method. This involves taking copper test coupons through the OSP line, dissolving the OSP coating in an acid solution, and then measuring the copper content to determine coating thickness.

Target thickness ranges from 50-300 microinches (1.27-7.62 microns) depending on the OSP chemistry and application. Thickness measurements on production boards should be conducted periodically to ensure optimal coatings.

Surface Insulation Resistance (SIR)

Also referred to as surface resistivity, SIR testing is used to measure the electrical resistance across an OSP coated surface. It provides a way to assess coating quality and contamination levels.

High SIR values greater than 10^9 ohms indicates an non-conductive, defect free OSP coating. Lower SIR can mean poor adhesion, gaps in coverage, or surface contamination.

Solderability Testing

As explained previously, multiple test methods can be used to validate the wetting and spread performance of OSP coatings after deposition and over time. This includes wetting balance testing, spread tests, and copper mirror corrosion exams.

Routine solderability testing demonstrates the real-world shelf life of OSP protected boards under typical storage conditions. This data helps determine maximum storage times to maintain soldering process yields.

Visual Inspection

Visual inspection under a microscope checks for OSP coating quality. Evaluating for complete copper coverage, thickness uniformity, no cracking or pitting, and a consistent surface appearance.

Coupon cross-sections should show a smooth, tightly adherent OSP coating free of voids, inclusions, contamination or other defects.

OSP Industry Standards

OSP performance needs to meet established industry specifications when used on circuit boards targeted for military, aerospace, automotive, telecommunications, and other specialized applications.

IPC-CC-830 Qualification and Performance

This standard sets test methods and acceptance criteria for OSP coatings. Key parameters covered include:

• Coating thickness
• Solderability
• Adhesion
• Corrosion resistance
• Coplanarity
• Thermal stress resistance
• Solution chemistry

Both reduced and elevated temperature solderability testing is included in IPC-CC-830 along with environmental exposures like mixed flowing gas and pressure cooker testing.

IPC J-STD-003 Solderability Tests

J-STD-003 has an entire section focused on evaluating the solderability of OSP finished boards through various test protocols:

• Wetting balance tests
• Spread tests
• Copper mirror tests
• Surface insulation resistance
• Test durations and conditions

The standard establishes minimum values for wetting force, spread coverage, and SIR that OSP must meet at initial production and after aging.

IPC-9252 OSP Performance Guide

This guide provides a wealth of detail on all aspects of OSP technology including chemistries, corrosion protection, processing equipment, reliability testing, troubleshooting, and process optimization.

It contains a substantial amount of data from industry studies on OSP performance under various conditions along with recommendations for maximizing results.

Comparing OSP Chemistries

With the variety of OSP formulations available, how do you determine the optimal chemistry for you application?

Important factors to consider when selecting an OSP chemistry:

• Solderability protection – How many months of storage protection does it provide? How does it perform in solderability testing?
• Thermal stability – What reflow temperature can it withstand without charring, outgassing, or causing other issues?
• Coating quality – Does it coat evenly without excess buildup? How consistent is thickness across panels and different board geometries?
• Compatibility – Is the OSP compatible with your board designs, components, and assembly processes?
• Reliability – Will the OSP impact long-term reliability or create any quality risks?
• Cost – What is the material cost per liter? How does it impact total processing costs?
• Conformance testing – Has the OSP been qualified to key industry standards for your end applications?
• Service – What level of technical support does the chemistry supplier provide? How quickly can they address issues and questions?

Careful chemistry evaluation and selection helps ensure optimal OSP performance tailored for your specific PCB requirements and operating conditions.

OSP Design Guidelines

To maximize OSP protection and minimize any potential issues, following