Issue link: https://iconnect007.uberflip.com/i/288471
26 SMT Magazine • April 2014 CliMaTiC rEliaBiliTY OF ElECTrONiC DEviCES aND COMpONENTS continues ated chemicals mainly from the fire retardant chemistries. Typical examples are the release of bromine resulting from the use of fire retardant tetrabromobisphenol (TBBA) (See section on fire retardants and degassing). Permeation of water through laminate: Wa- ter permeability of polymer laminate is an im- portant aspect in determining the corrosion of buried conduction lines in the case of multilay- ered PCBs. For FR-4 types of laminate, easy path- way for water is along the glass fibre interface, which will be contaminated with various species along its way through the polymer. In this case, the fiber-epoxy interface is also the preferential solution connecting path for two conduction lines situated inside the PCB to form corrosion cell. Water permeated to the polymer can act as a conductive media causing corrosion between the conducting lines especially when the PCB is working, which introduce potential bias be - tween the lines. A typical corrosion issue related to water entry into the polymer is the corrosion of copper conduction lines due to conductive an- odic filament formation as described earlier. Ma- jor PCB factors controlling this corrosion are the amount of water, fibre matrix interface acting as a connecting path, and the presence of ionic resi- dues. Typically, CAF is found between two oppo- sitely charged vias due to the possibility of a gap between vias and epoxy to collect water and the possibility of several fibre-matrix interfaces act- ing as a pathway for the solution layer between the vias overall length of the hole. Corrosion of multimetallic surface finish: Depending on the surface finish of the PCB, there is a possibility of two or more different metals/alloys comes in contact, which can to- gether interact with the environment leading to galvanic corrosion (See section on galvanic corrosion in Part 2). Galvanic coupling be- tween the various metallic layers is a clear pos- sibility especially for the features that are open on the PCB surface namely electronic contacts such as the pin contacts or edge contacts. Oth- er features usually get buried below the solder joints after the component mounting, there- fore expected to have comparably less effect on corrosion. fEATurE surface and interior of the PCB is approximately 100 µm. Thickness of the conduction lines are also of similar magnitude. Sources of contamination: Although at tiny levels, leftover chemical species from the PCB manufacturing process can influence the corrosion process by increasing the conductiv- ity of the water layer formed on the PCB dur- ing service. Overall cleanliness of the PCB after manufacture is ensured by the ROSE test (sol- vent extract test, IPC-TM-650 2.3.25 ROSE test standard) which defines an overall allowed con- tamination level of 1.56 µg/cm 2 NaCl equiva- lent. However, contamination at localized sites can be significantly higher if there is local con- densation especially at sites where hygroscopic residues are present. Important trapping sites for process chemicals on a PCB surfaces are via holes (plated through holes), edges of conduct- ing lines or any other features where entry of the cleaning solution is difficult. Figures 7a and 7b show examples of trapped chemicals in vias mainly constituting ionic residues such as chloride, sulphates, hypophosphite etc. Vias can also act as water trapping sites due to the capillary effect during service, while also main - taining water for longer times compared to the flat regions during thermal cycling. Therefore, the chances of forming a thicker layer of water at high humidity surrounding a set of vias are higher than typically seen in flat regions. The aspect ratio of the vias is an important factor in achieving cleanliness. High aspect-ra- tio vias have higher chances of trapping con- tamination than low aspect-ratio vias due to the difficulty of entry for the cleaning solutions. Similarly, it is quite common to find defective plating inside the vias (example wicking) for higher aspect ratio vias. This can expose the un- derlayers and glass fibers of the polymer to the opening of the vias acting as a site for easy water transport. This can cause formation of conduc- tive anodic filament (CAF) between buried con- duction lines. The release of chemicals from the PCB lami- nate is another issue, especially during reflow or wave soldering due to the increase in tempera- ture as high as 250–270 o C. Most important in this case is the possibility of release of fraction-