In printed circuit board manufacturing, there is always the need to provide power to the circuit being assembled. The simple solution is to plug it into the wall socket, though wires can be cumbersome for the user/operator. In the case of implantable devices (or temporary implantable devices) wires are impractical. So as a result, most companies turn to the battery. For TV Remotes and other household items, the battery can simply slide into the spring loaded compartment and Presto! Your device is up and running. Nonetheless, when dealing with micro-electronic circuits, standard AA or AAA batteries won’t cut it for a few reasons. Mainly because these batteries are often much larger than the Circuit they are powering, but also because small implants do not have the luxury of being attached to a spring loaded battery compartment. So the progression of possible solutions continues, moving from standard AA batteries to coin cell batteries, like the ones used in watches. These batteries come in a variety of small sizes, so they no longer are double the size of the circuitry and attaching the batteries to a PCB can still be difficult.
One customer had just this problem; a need to connect 2 coin cell batteries to a microelectronic circuit.
When the device information was transferred to Valtronic for Prototype building, there was a process for attaching the battery in place. It involved taking “sticky donut rings” and placing them onto the PCB where the coin cell batteries would connect. In the center of the sticky donut, a small amount of conductive epoxy was dispensed to allow for electrical connection between the PCB and battery. After the epoxy was dispensed, the batteries were stuck down onto the donuts and allowed time to cure. While this had worked for the customer in the past for very small runs, using this method for full production build would prove extremely difficult.
A multitude of issues were encountered, including inconsistent epoxy dot dispensing, trouble with proper alignment of the coin cells onto the sticky donuts, difficulty providing sufficient pressure to the battery to allow for solid PCB-epoxy-battery contact, and, most concerning, unreliable powering of PCBs after epoxy cure. Fixtures to align the batteries were created; clamps to provide pressure to the batteries, and EFD dispensing equipment were all used in an attempt to make the epoxy process work, but to no benefit. The yield of successfully powered units was alarmingly low.
By working with the customer, we were able to develop a new method of attachment, one that reversed the post-battery attach yields. The obvious choice was to solder the batteries to the PCBs, yet battery life has an inverse relationship with high heat exposure. That is, if a high temperature is applied to a battery, its life will decrease rapidly. A joint-operation experiment between customer and Valtronic, the contract manufacturer was conducted to determine if it was possible to solder a
battery and still keep the heat exposure below the temperature/life threshold. Using thermocouples and plotting software, we were able to determine the proper temperature and exposure time needed to successfully solder the battery. Additionally, a small amount of epoxy was placed with the battery to help solidify them to the PCBs.
The quick solder method was implemented and has produced yields far superior to the epoxy method. Through engineering teamwork, an appropriate solution was reached. The solution was one that increased product throughput for the manufacturer, as well as, yields and final product functionality for the customer. This is just one example of the many engineering problems that are continuously solved at Valtronic!
