Adam T. Neal
University: The Pennsylvania State University
Gradation Date: May 2009
Hometown: Indiana, PA
My Project: Process for In-House Soldering of Printed Circuit Boards
One of the goals of the CMOS Device and Reliability project is to advance measurement capabilities to enable a better understanding of transistor reliability.
With transistors operating at Gigahertz frequencies, the understanding of their transient response after stress is increasingly important in predicting their long-term reliability. To measure the transient response requires us to build high bandwidth, high gain, and low noise circuits not available commercially. Realizing circuits which fit all of these criteria requires that we use surface mount components to minimize parasitic circuit elements. Surface mount components must also be used to achieve a small enough printed circuit board (PCB) so that it can be placed very close to the actual source of signal.
Surface mount technology is well-established in the electronics industry; however, it is viable only to customers who need large quantities of a PCB. In research, we use one-of-a-kind circuits and will have to pay a high premium to get our PCBs custom-built. To make surface mount technology available to our research, my project is to devise and test a process to solder PCBs using readily available equipment - a toaster oven.
The process begins when ordering the PCB. There are many companies which provide quick turnaround manufacturing of small quantities of PCBs. As a standard feature of most companies manufacturing process, the company pre-tins the copper pads with a small amount of solder. In industrial PCB assembling, solder is added to this pre-tinning to solder the components to the PCB. Electronics Interconnect of Elk Grove Village, IL, agreed to modify their manufacturing process to pre-tin the pads with extra solder, allowing the PCB to be soldered without adding additional solder to the pads in-house.
After the PCBs are received a thin layer of tacky flux is applied and the surface mount components are placed on top of their pads. The flux keeps the components in place making it easier to handle the PCB. The PCB is then placed in a toaster oven which is preheated to 238°C. The PCB is baked for a minimum of 90 seconds and removed from the oven to allow it to cool. The toaster oven is controlled by a commercial PID controller.
To optimize the process, I soldered PCBs both with normal pre-tinning and extra solder at various times. The results are shown in the graph. This data shows that increased bake time leads to increased yield over this range. Also, at both 90 seconds and 130 seconds, the heavy solder produced a higher yield than the normal solder, indicating that using heavy solder will, on the average, increase yield.
Baking PCBs with heavy solder for 130 seconds produced 100% yield twice. Of the conditions tested, these would seem to be the optimal.
To verify that I have a successful process, I will use it to build a high-speed (DC to 1.8 GHz), high gain (60 db), and low noise amplifier which I have designed to fit into the small housing of our probe station’s electrical probe. If all works well, I will use this new measurement capability to demonstrate that we can take an existing experiment to new -territory.
Resistor yield vs. bake time for Normal solder (standard pre-tinning) and Heavy solder (extra solder) PCBs
This is the setup used to solder the boards. The metal enclosure houses a relay which is connected between the 110V wall outlet and the toaster heating elements. The PID controller controls the frequency at which the relay turns on and off and adjusts that frequency to control the oven temperature.
