OSP Film Thickness Control on Copper Surfaces

An organic solderability preservative is a thin organic film applied to bare copper so that the copper stays solderable through storage and through the assembly process. The film is only a few tenths of a micrometre thick, it protects the copper from oxidation, and it is displaced by flux during soldering. Film thickness is the parameter that decides whether the coating does its job without becoming a barrier of its own.

What the Coating Has to Do

The film has three requirements. It has to cover the copper completely, including the walls of a plated hole, it has to survive the storage and handling between fabrication and assembly, and it has to break down or dissolve quickly when the flux and the heat arrive so that the solder can wet the copper underneath.

All three are thickness dependent. A film that is too thin oxidises through in a few weeks, and one that is too thick survives the flux in places and leaves a joint that wets slowly or not at all, which is the more damaging failure because it appears during a build rather than in the warehouse.

How the Film Forms

The coating forms by a chemical reaction between the copper surface and the active components of the bath, which are usually an imidazole or a benzimidazole chemistry. The copper is not etched away as it is in plating, so the reaction is self limiting: once the surface is covered, the film stops growing.

That self limiting behaviour is what makes control possible. Thickness is set by concentration, pH, temperature and immersion time, and it plateaus at a level that is characteristic of the chemistry, so a bath that is far out of specification produces a film that is either patchy or at the plateau and no better. Extending the immersion time beyond the plateau does not thicken the film either, it only gives the bath more opportunity to attack the resist edges and the laminate.

Target Film Thickness

Typical production films are between 0.2 and 0.5 micrometres, with the heavier end used where the board will be stored for months or will pass through multiple reflow cycles. Values below about 0.15 micrometres are usually taken as a coating defect rather than as an in specification result.

Thickness also has to be uniform. Copper that is covered by 0.4 micrometres at the pad centre and 0.1 micrometres at its edge will fail first at the edge, where the film is thinnest, and the failure presents as a patchy wetting pattern that follows the geometry of the pad rather than the position on the panel.

Measuring the Film

Laboratory measurement is normally done by weight loss, by stripping the film in acid and analysing the solution, or by measuring copper oxidation with a coulometric method that reports thickness in micrometres directly. All three are destructive, so they are run on coupons.

A quick line check uses a colour standard or a wetting balance test on a coupon, and a solderability test after a simulated storage period is the evidence that matters to a customer. The solderability test is the end point that thickness control exists to deliver.

Copper Preparation Before Coating

The film can only be as uniform as the copper beneath it. Oxide, fingerprints, resist residue and a rough etch all change the local reaction rate, so the coating step is normally preceded by a micro-etch and a rinse sequence that presents clean, slightly roughened copper, which is the same condition that anti-tarnish control maintains on uncoated copper.

Where the surface is not uniform, the film follows the contamination. A panel handled without gloves therefore carries a pattern of thin coating that matches the fingerprints rather than the copper distribution, and the plating appears correct until the wetting test is run. A shadow of a fingerprint appears in the coating as a region of different thickness, and it is often invisible until the wetting test is run at the end of the line. Boards are therefore handled by their edges for this reason, and gloves are changed before the panels are packed rather than after.

Storage and Shelf Life

The film protects the copper for a defined period rather than indefinitely. Oxidation proceeds through the coating at a rate that depends on thickness, humidity and temperature, and a board stored in a humid warehouse without packaging loses solderability long before the calendar limit is reached.

Storage control is therefore part of coating control. Boards are normally kept in a dry environment or in moisture barrier packaging with a desiccant, and the shelf life quoted by the process assumes that environment. A board that has been unpacked and handled has a much shorter usable window than the sealed figure suggests.

Defects and Their Symptoms

A film that is too thin shows as non wetting at the pad edges and as a wetting balance result that degrades with every week of storage. A film that is too thick shows as slow wetting, incomplete spread and, in the worst case, a joint that only wets after the flux has been extended by extra time or heat.

Patchy coating shows as a wetting pattern that follows the surface preparation rather than the design. Where the defect is limited to one area of the panel, the rinse or the micro etch before coating is the place to look; where it is spread across the panel, the bath parameters are.

Reflow Behaviour of the Film

During reflow the film decomposes and is displaced by the flux, and it must do so quickly enough that the molten alloy meets clean copper. A heavy film that survives the first pass can still be present at the start of a second reflow, which is a common reason a board passes the first side and fails the second.

Double sided assemblies therefore need a thinner film, or a process that tolerates the second thermal excursion, and the decision should be made at the coating step rather than at assembly. Where the film thickness is at the top of the range, the second side profile has to be reviewed as well.

Process Window and Records

The window is described by bath concentration, pH, temperature, immersion time and the rinse that follows, and it produces a specified film thickness on a coupon. Each parameter is recorded at the start of a shift and the thickness on a coupon is measured against the specification.

Records should also carry the copper preparation parameters, the storage conditions and the age of the board at assembly. A finish selection decision that chooses this coating on cost alone ignores the storage discipline that it requires, which is the most common reason the choice disappoints.

FAQ

How thick should an OSP film be? Most processes target 0.2 to 0.5 micrometres, with thicker films used where the board will be stored for a long time or will pass through more than one reflow cycle.

Does the coating survive multiple reflows? A thin film is largely consumed by the first reflow. Heavier films can leave material in place for a second pass, which is why double sided assemblies usually specify the thinner end of the range.

What shortens the shelf life most? Humidity and handling. Unpacked boards in a humid room lose solderability far sooner than the sealed shelf life figure implies, even when the film thickness is correct.