Bob Willis - The Industry Specialist for Surface Mount Technology (SMT) Screen Printing Process Defects

What are the main problems with reflow soldering copper finish boards ?

Despite the literature from the protective finish manufacturers, heat does affect the copper circuit. It increases the time to obtain full wetting hence increasing the possibility of poor yield. Double sided reflow again increases the surface oxides and poor yield. You need to confirm the quality of the basic coating with your supplier and consider the other process stages which affect solderability. Cleaning the board, washing off poor paste prints, poor board handlin and, the original surface preparation of the copper before treatment can all affect reflow yields.

TOP

What are the benefit of using 62% tin/2 % silver alloy paste during reflow ?

Today the benefit is a slightly lower temperature for reflow. The eutectic allooy, 63% tin/37% lead has a reflow temperature of 183-184oC. The silver alloy reflows at 179C. The lower temperature is worthwhile for any process. The silver alloy in the past has been beneficial for soldering silver palladium parts to reduce the leaching of the termination. As people purchase chip parts with a nickel and tin/lead plating this should not be a problem today.

TOP

I want to profile my reflow oven to try and reduce reflow temperatures, where should I place my thermocouples ?

Positioning of thermocouples comes down to experience. They should always be soldered in place using high temperature solder. First position a probe at the centre of the board and one at the corner edge of the board. This should theoretically give yiou the lowest and highest temperatures. Next, consider where the largest mass of components are located on the board and place a probe at one termination point. In a convection oven you should see a maximum temperature differential of 15-20oC. Experiment again with the same board placing the probes in different positions noting the temperature changes.

TOP

If I change my extraction system or my process settings do I need to perfom further profiles ?

Yes, depending on the type and position of the extraction points it can have an effect. If the ducting is poorly designed other equipment in the factory can affect the extraction rates and your board temperatuure during reflow.

TOP