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Working With OSP Treated PCBs

Published Date
Author Cadence PCB Solutions

OSP PCB

Once a PCB passes through the fabrication process, one of the final steps is to apply a surface plating to the exposed copper on the surface layers. The exposed copper will oxidize over time if it is not treated with a surface finish, and the various surface finishes used in PCBs are used to both prevent copper degradation and provide a solderable surface. Tin-lead or immersion tin are the typical entry-level surface plating materials, but there are more specialized options for certain builds.

One of these more specialized options is organic solderability preservative (OSP), which is the only organic compound-based copper surface treatment. Leveraging this surface treatment for PCBs requires correct storage and handling practices that are not implemented for metallic surface treatments. In addition to storage and handling, rework of OSP treated PCBs can be difficult without compromising the reliability of treated conductors.

How to Work Boards With OSP

OSP is not a new coating technology, but it is less-often used than metallic finishes due to the special storage and handling procedures required before assembly. Like immersion silver coatings, which have low losses comparable to solder mask, OSP coatings offer low loss and have a shelf life that must be considered. Due to the limited shelf life of OSP coatings, there are several challenges associated with the storage, handling, and processing of OSP-coated boards.

Challenge

Description

Solution

Oxidation of copper pads

Copper pads on OSP-finished PCBs can oxidize if exposed to the atmosphere for extended periods.

Store OSP-finished PCBs in vacuum-sealed bags immediately after fabrication. Reapply OSP finish if storage period exceeds 30 days.

Deterioration of OSP finish

OSP finish can deteriorate if exposed to high temperatures or improper storage conditions.

Avoid baking OSP-finished PCBs before assembly. Store them in a controlled environment with a temperature between 15 °C to 35 °C and relative humidity within 60%.

Damage during rework

OSP finish can be damaged during the rework process if high temperatures are used during desoldering and soldering.

Use low temperatures during the rework process and handle the PCBs with care to avoid damaging the OSP finish.

Limited shelf life

OSP-finished PCBs have a limited shelf life compared to other surface finishes.

Plan production schedule to ensure OSP-finished PCBs are used within their shelf life. Reapply OSP finish if necessary.

After the coating is applied in fabrication, the boards are often shipped to an assembler, or they may be put into storage for a short time before in-house assembly. The storage conditions will greatly influence the solderability of the OSP coating, so a storage plan should be in place if OSP is to be used in a PCB.

Storage and Handling

Ideally, OSP-coated boards should be assembled immediately after fabrication. If this is not possible, they will need to be stored properly in order to prevent degradation of the OSP coating and oxidation of copper. OSP PCBs have specific storage requirements that must be met to ensure the coating does not degrade prior to assembly. Boards coated with OSP should be stored just like moisture-sensitive ICs; packing and storage procedures include:

  • Seal the unassembled boards in vacuum bags
  • Include a desiccant included and humidity indicator card in the bag
  • Maintain storage temperature from 15 °C to 35 °C
  • Maintain relative humidity below 60%
  • Only inert gasses (e.g., nitrogen gas) should be in the packaging

Adhering to these storage procedures will significantly extend the useful shelf life of OSP-finished PCBs. As a rule of thumb, the storage period of OSP PCBs should not exceed 30 days. Should this occur it is recommended to return the boards to the PCB manufacturer to reapply the OSP finish, or to reapply an off-the-shelf OSP finish yourself following recommended procedures from the vendor.

The handling of OSP-coated PCBs follows similar principles as storage; avoid exposure to gasses and liquids that could damage the coating and reduce the solderability of the exposed conductors. Sometimes boards will be-prebaked to remove moisture before assembly; OSP-coated boards should not be baked before PCB assembly as baking can cause the OSP finish to deteriorate.

Rework

Rework procedures for OSP-coated boards can be difficult due to the possibility of damaging the coating through both excessive heat and through cleaning. There are three possible sources of damage to the OSP coating during rework:

  • High temperatures
  • Exposure to aggressive flux
  • Exposure to cleaning chemicals

Fluxes can degrade and strip the OSP coating when heating the board to remove components and when resoldering components. Flux normally needs to be cleaned after rework, and the cleaning agent will also damage the coating. Isopropanol (rubbing alcohol) is normally used for cleaning flux residues, but this can remove a significant amount of the OSP coating. The best approach is to use a specialized cleaning solution; this may be provided by the OSP coating vendor.

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