SMT & Inspection | January 12, 2010

Care-free storage of OSP coated PCBs

To find a reference value for the proper stocking and moisture protection of electronic components, you can refer to IPC standards (IPC/JEDEC J-Std-033B.1). However, this is unfortunately not the case if you are looking for guidance to handle printed circuit boards.
Because there are no published standards yet for storage and moisture protection for printed boards, they’re generally overlooked. But with the correct stocking control and the help of professional drying methods, considerable advantages can be gained: Coated printed boards will stay useable for a much longer time and damage during reflow due to moisture can be completely eliminated.

A common high Tg FR-4 afterlead free assembly

Organic solderability preservative (OSP) coatings are among the leading surface finish options in lead free soldering because they provide an attractive combination of solderability, ease of processing and low cost. Compared to alternatives, however, they tend to be the most prone to oxidation.

The cause for this lies in the pure copper surface protected only by the OSP coating layer. Under normal climatic conditions in a manufacturing process, after only a few minutes there will be a separation of a water film at the surface (3-5 atom layers). This then starts a diffusion process which leads to a vapour pressure balance through the OSP coat.

The electrolytic effect of the water molecules leads to rapid oxidation of the copper surface which in turn allows the printed boards to fall under the minimum value of wetability. This effect can also be found with non gas-proof coatings (e.g. nickel) but is not as pronounced.

The packaging quality of PCBs as received from the manufacturers can also have a significant effect. Often a simple foil bag or an ESD bag is used instead of a Moisture Barrier Bag (MBB). With such packing, coated boards are likely to arrive having already absorbed moisture, and stored like this they will be useless after a short time. Desorption process desiccant drying cabinets, such as those from Totech can be used for the careful drying of these and other moisture sensitive devices.

In addition to extending the safe and stable storage time, defects and damage such as popcorning and delamination during the reflow process caused by moisture can be avoided with the company's desorption technology. The previously absorbed moisture is removed from the printed board without temperature stress and with a controlled, revertive drying process in an atmosphere of less than 0,5 g/m³ vapour content (in effect, a moisture vacuum). Further oxidation is stopped by the removal of the electrolytic water molecules.

Dry storage is also necessary for a less critical coating; e.g. in the beginning the wettability of the coating of a hot air tin-plated printed board is better than of OSP coated printed boards. Here an oxide film forms on the surface under the influence of moisture which can also limit good wetting and solderability. This problem can also be averted with the help of a dry cabinet, enabling the retention of a constant wettability for a much longer period of time.

Source: Totech Europe


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