The heat sink paste will come in a small syringe type applicator and a small amount will need to be applied to the back of each triac. The triac's are the small black squares to the left of the PCB board labeled by the letter G in the picture below. The Bosch AE-115/WH17 will have 4 small black triac's to the left of the PCB board and the AE-125/WH27 will have 6 small black triacs to the left of the PCB board. There are service bulletins that show the PCB board replacement and heat sink application. Bosch Service bulletin EP-22 will show the PCB board replacement for all the newer high KW units with polymer modules. Bosch Service bulletin TWH-E2-22 will show the PCB board replacement for all the older high KW units with copper canisters.
For further questions or to purchase a new PCB control board visit this link:
http://www.globaltowne.com/shop-by-category/bosch-tankless-water-heaters-spare-parts.html?spare_part_type=162or please call GlobalTowne at 888-726-9752 or visit our website.