How Are PTFE Immersion Heaters Used in the Hot Rinsing of Ceramic Substrates for Electronics?

A ceramic substrate, the hard, insulative basis for a high-power LED module or a sophisticated automotive sensor circuit, is screen-printed with precious metal pastes and then burnt. When finished shooting, the gun must be cleaned with hot ultrapure deionised water to eliminate any loose particles and ionic residues. Any metal contamination from the rinse heater, in particular mobile ions such as sodium or iron, would be deposited onto the ceramic surface, would move under the electric field and would form a conductive channel that shorts out the fragile circuit. The rinse heater must be as pure as the water itself. This important cleaning stage needs a metal free, stable thermal input, which is supplied by a PTFE heater ceramic substrate rinsing system.

Hot Rinse Process for Ceramic Substrates
Uses of Hot DI Water
After burning of screen printed conductor pastes (e.g. silver, gold or palladium-silver), ceramic substrates are generally moved to a hot deionised water rinse station. The water is heated to 60–80 °C (140–176 °F) then sprayed or cascaded on the substrates. A hot rinse eliminates sintering residues, loose ceramic dust, and any ionic impurities that can cause electromigration or dendritic development later. The water itself must be of the utmost purity, normally with a resistivity of greater than 10 MΩ·cm, so as not to redeposit ions back onto the substrate.

The PTFE Immersion Heater's Role
A PTFE immersion heater is installed directly in the deionised water tank or a recirculation loop. The PTFE sheath is fully inert to the ultrapure water and does not give off any of the metal ions. Unlike metal-sheathed heaters (e.g. stainless steel, Incoloy) a PTFE heater does not leach iron, chromium, nickel, sodium or any other species into the rinse bath. Its non-stick, smooth surface also prevents any scale or particles from forming and sloughing off into the substrate. The heat is pure, dependable and free of metal, leaving the ceramic surface spotlessly clean and ionically sterile for the next step in circuit building.

The PTFE heater is an ionic void. A quiet, pleasant presence. Adding nothing but pure, cleaning heat to the final bath that washes the foundation of a powerful electrical circuit.

Process Note: Closed Loop Rinse System with Continuous DI
The hot rinse system is usually always a closed loop recirculating system to maintain the ultrahigh purity of the rinse water. Water is continually pushed from the rinse tank through a mixed-bed deionisation (DI) cartridge or an electrodeionization (EDI) module and returned to the tank. The PTFE immersion heater is put either directly in the tank or in an in a side-stream heating loop. This permanent polishing preserves the water resistivity higher than 10 MΩ.cm, and removes any trace of organic compounds or particles that may come from the ambient air or from the substrates themselves. The heater needs to be placed behind the DI polisher, so that no potential leachates can be deposited directly onto the surfaces.

Design Problems of PTFE Heaters in This Service Sanitary, Crevice Free Design
The heater should be designed in a sanitary, crevice free manner so as not to allow bacterial growth in the warm, nutrient lean water. Bacteria in itself are not a big issue for electronics cleaning but biofilms can trap particles and cause local ionic gradients. The PTFE sheath is smooth and non-porous and all mounting interfaces (the tank wall flange or the thermowell for example) are built with radiused corners and compression seals, with no stagnant pockets. There are no removable pieces, however a PTFE over-jacket welded or integrally moulded is employed.

Temperature Regulation and Stability
Rinse temperature is generally adjusted to ±2 °C. Thermocouple or RTD with PTFE sheath installed directly adjacent to the heater or in a separate thermowell. The PID controller maintains the setpoint, usually between 70 and 80 °C, for the best removal of organic wastes without excessive evaporation. The higher temperature would increase evaporation losses and concentrate ionic species, undermining the objective of the ultrapure rinse.

Resistance to Thermal Cycling
The rinsing system is a batch system and the heater may be cycled off and on. PTFE is very resistant to thermal fatigue, and an immersion heater properly constructed (with a tight fit between the heating element and the PTFE sheath) will survive tens of thousands of cycles without breaking or delamination.

Comparison to other heating methods
Heating Method Metal ion leaching Corrosion resistance Scale formationCompatibility with Ultrapure rinse
Ptfe Dip HeaterNone GoodNone (non-stick)Excellent
Stainless steel immersion heater Fe, Cr, Ni, MnGood (but can pit in chlorides)Moderate Not desirable for high purity
Immersion heater (Incoloy, Ni-Cr-Fe) Ni, Cr, FeVery good Moderate Not appropriate
External heat exchanger (steam or PFA-lined)0 (assuming all wetted parts are PFA/PTFE)ExcellentLow Suitable but less efficient
Infrared or microwave heating NoNot applicable Not applicableNot feasible for liquid bulk cargoes
The PTFE immersion heater is a direct, energy efficient, compact solution, much simpler than an exterior lined heat exchanger and far cleaner than any metal heater.

Quality Assurance & Validation
In a typical electronics production facility, ionic contamination is routinely monitored in the rinse system. A sample of the hot DI water is analysed by ion chromatography (IC) upstream and downstream the heater. There should be no increase over the detection limit (usually <0.1 ppb) in any metal ion (sodium, potassium, calcium, iron, copper, etc.) in the PTFE heater. The heater also has periodic megohm testing to ensure that the sheath insulation resistance stays high (>100 MΩ) to prevent any electrical current from leaking into the rinse water and causing electrolytic contamination.

Conclusion Pure heat for pure surfaces
A PTFE immersion heater is the pure, non-contaminating and reliable heat source for cleaning ceramic electronic substrates, assuring the ionic cleanliness that is the basis for long term circuit reliability. It offers a metal free, scale free and biologically inert warm water to prepare the ceramic surface for subsequent metallisation, die attach or wire bonding without risk of leakage currents or device failure. The cleanest and purest surfaces are where the most powerful electronics are produced. Quietly, the PTFE heater provides the heat that enables that cleanliness.