Proper Furnace Setup
Proper furnace setup is critical for successful high-yield Thick Film processing. The two key parameters that control successful firing are furnace air and the firing profile.
- A sufficient quantity of clean, dry air must be present for thick film to fire correctly.
- Intake air should be drawn from a point 6 - 10 feet above the roof line and as far away as possible from contaminant-containing exhausts.
- Oil-free compressors should be used whenever possible.
- A series of dryers and filters must be used to dry to a dew point of -30°C at 80 - 100 psi. Standard cooling dryers in conjunction with heated desiccant dryers will achieve this value.
- Activated charcoal filters will help to remove halogen and sulfur-type contaminants.
- Micron and sub-micron filters will remove particulate matter.
To determine how much air is required in the furnace, use the following formula:
V = PLAWS where:
- V = The volume of air in liters/minute required in the burnout section of the furance.
- P = Printed area of the most dense circuit expressed as a decimal.
- L= Belt load factor. How much of the burnout section is full when parts are loaded; expressed as a decimal.
- A = Constant of 0.4 liters/cm2
- W = Belt width(cm)
- S = Belt speed (cm/min)
The amount of air required in the firing section is 10 to 20% higher than for the burnout section.
To set up the air flow properly:
- turn off all flowmeters
- turn up the burnout air to the value calculated above
- turn the firing air to its value (1.1 to 1.2 times plaws)
- check to make sure air is flowing out the entrance and exits of the furnace
- turn up the venturi exhaust until room air is just being drawn into the furnace; this will ensure that all air introduced to the furnace is removed.
- turn up the entrance and exit air curtains until flow at the exit and entrance is out of the furnace. This will prevent air from being drawn into the furnace.
There are several critical parameters that must be met in the furnace profile. Furnace profiles may be run with a traveling calibrated ungrounded Type-K or Type-N thermocouple. The Type-N is preferred because it shows less degradation with repeated use. The tables below list the critical parameters and the specifications associated with them.