Surface Mount International Conference Proceedings


SOLDERABILITY OF PRINTED WIRING BOARDS WITH ORGANIC SOLDER PRESERVATION UNDER NITROGEN ATMOSPHERE

Author: Torsten Zachert
Company: Messer Griesheim GmbH
Date Published: 8/23/1998   Conference: Surface Mount International


Abstract: During the last decades a lot of studies describing the influences of nitrogen on solderability of electronic assemblies have been published. Generally, all these studies are divided into two groups, On the one hand you can find pro-nitrogen examinations which demand oxygen levels less than 50ppm. On the other hand there are contrary statements which can not find any advantages within nitrogen because of controlling their soldering processes accurately. Therefore, in most cases the outcomes of these papers are not very helpful for electronic manufacturers. Additionally, numerous of such papers are not comparable, because they do not include all parameters and data necessary for result evaluation. The developments in the electronics industry are going in the direction of applications with higher complexity. Further miniaturization and stronger environmental demands. Therefore it becomes increasingly difficult for the manufacturer to choose the best technology. To overcome these difficulties several hundred electronic assemblies with organic solder preservations (OSP) were soldered in a nitrogen reflow oven to define the necessary technology steps for this critical copper passivation. During the soldering processes the oxygen levels were controlled manually with a gas mixer. The residual oxygen levels (ROL) were measured online in the different oven zones in operation stand-by and during processing the assemblies. The result is a bandwidth of ROLs showing the dependence of the oxygen level on loading the oven. It allows to describe the oven behavior, to control the assembly quality and to decide whether to install a continuous nitrogen control unit. All assemblies were inspected visually, the brightness of the solder joints and all other kinds of soldering failures were documented and analyzed. In order to obtain a more detailed data base a test board was designed, Defined solder volumes were soldered under different ROLS and the wetting angels as well as the solder joint diameters were measured afterwards. Additionally, all these results of soldering under nitrogen atmosphere were compared with the outcomes of soldering under normal atmosphere. This paper describes the influences of selected process parameters on solderability of the coming up low cost OSP board finish. For the manufacturers the results represent an important step on the way to integrate this organic preservation finish in their process lines, KEYWORDS Assembly, Soldering, Nitrogen, Organic Preservation



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