Pan Pacific Symposium Conference Proceedings


Author: Karthik Vijayamadhavan
Company: State U of NY - Binghamton
Date Published: 2/13/2001   Conference: Pan Pacific Symposium

Abstract: Flux spatter is the violent expulsion of flux from the solder paste onto a Printed Circuit Board (PCB). This phenomenon occurs during the reflow soldering process of the PCB assembly when the flux and volatiles are activated and later evaporated. Flux splatter becomes a concern especially when it lands onto critical areas of the PCB assembly such as gold finger contacts. There are two concerns. First, the insulative properties of flux that resides on gold fingers due to flux spatter may indicate failures during functional testing, when there are none. Second, flux spatter impacts the cosmetic appearance of the gold fingers. In addition, it is often hypothesized in industry that the presence of post-reflow flux residues on the surface of the board could result in a decrease in surface insulation resistance (or SIR) values. This is inspite of the residues exhibiting no chemical activity.

The occurrence of flux spatter was systematically investigated to understand the root cause and develop a mechanism that would eliminate or minimize flux spatter on high-density PCB assemblies. All the processes involved in the assembly of a PCB were studied for their impact on flux spatter. The reflow ovens used consisted of the five and seven zone ovens and were of the forced convection type. Reflow was in an 'air environment' (no nitrogen). Different reflow profiles were experimented with to develop a profile that would eliminate or minimize flux spatter. Two similar rosin based no-clean solder pastes were used to verify if the problem of flux spatter was restricted to one solder paste manufacturer. The flux in the solder paste had a UV (Ultraviolet) die thereby enabling the identification of flux spatter with a UV lamp. Storage and handling of solder paste was checked. Two types of exhaust venting systems that included a fumes extraction system and routing of exhaust fumes straight upwards were evaluated.

The goldfingers of experimental panels were covered with Kapton tape before printing, to prevent them from being smeared with paste during printing which was removed before reflow. Flux spatter due to the presence of solder paste on the goldfingers was thus avoided. An oven characterization metal plate was used to characterize all the reflow ovens involved. The ovens, which did not exhibit significant differences in their profiles, were selected for experimentation. The reflow profiles used differed mainly in their preheat (between 90-1300C) ramp rates as the majority of flux evaporated in this range according to the paste manufacturers. Varying percentages of metal content (88%, 89% and the standard 90%) in the solder paste were used to observe the effect of a lower metal content and a corresponding higher flux content, on flux spatter. One set of reflow ovens had fumes extraction systems attached to remove the oven exhaust fumes. The other set of ovens relied on venting the fumes upwards into the atmosphere. Experimentation was carried out on multiple lines at different locations.

For the 5-zone and 7-zone reflow ovens with the fumes extraction system attached, there was no flux spatter on the goldfingers for all the cases involving reflow profiles with minimum and maximum preheat ramp rates; both the rosin based solder pastes; and varying percentages of metal content in the paste. On the other hand, flux spatter was observed in all the above cases for the ovens, which vented the exhaust fumes upwards. With this system, a reflow profile with the minimum preheat ramp rate could only minimize and not eliminate flux spatter. Flux spatter increased with the profile having the maximum preheat ramp rate. Having the minimum allowable preheat ramp rate is therefore critical in minimizing flux spatter on goldfinger contacts. The fumes extraction system helped maintain a constant exhaust flow rate and thus prevented the flux in the paste from being deposited on the goldfinger contacts during the reflow process. For the exhaust venting upwards to the atmosphere, the exhaust fan involved could not maintain a constant exhaust flow rate and therefore could not prevent flux spatter. Maintaining a constant exhaust flow rate is therefore critical in preventing flux from getting deposited on the goldfinger contacts.

Key Words: Fumes extraction system, preheat ramp rate, exhaust flow rates, venting.

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