How do you control the coating thickness in the inline dipping process and what is the tolerance of the coating thickness?
The dip speed is controlled by air over oil hydraulics which can control the speed down to 2″ per minute which is very slow and more than adequate for conformal coating.
Repeatability is about getting accurate viscosity control since the withdrawal rates and viscosity control the coating thickness.
Typical speeds for conformal coatings at viscosity of 200 cps are 6″ withdrawal speed giving a 25-50um coating for an acrylic coating. This does vary from coating material to material so control measures should be put in place if accurate thickness is required.
The typical coating tolerance is depending on what you measuring on? If it is a flat coupon, then with a known viscosity of product you should be able to achieve +/- 5um.
How do I monitor the viscosity of my conformal coating dip system?
The simplest method for viscosity monitoring is by using a zahn type measuring cup.
This is essentially an egg cup with a hole in the base on a wire. You dip the cup into the liquid and lift it out of the coating.
Since the volume of the cup is known and fixed, if you time the flow of the coating out of the cup you get a relatively accurate and simple method of measuring viscosity which is certainly suitable in 99% of cases.
What is the loading mechanism for the inline dip system?
There are many options for holding the PCBs and they can be customised to the customer project.
For example, a PCB can be held vertically or horizontally by a connector or through holes in the PCB.
In fact, the system is a conveyor system and the engineering is part of the project to present the printed circuit board correctly to the coating tank.
Can the dip coating system utilise in line curing?
The inline system is designed to incorporate custom modules of infrared (IR)/ convection heating or ultraviolet (UV) cure to dry and cure the printed circuit boards.
It is also possible to integrate the dip module with standard IR / Convection ovens depending on the design of the PCB.
Can I horizontally dip into the conformal coating using the inline dip coating process?
It is perfectly possible to horizontally dip PCBs using the automated inline system and this has been done several times very successfully.
There are factors such as the size of the board, the depth of dipping allowed, the type of conformal coating to be used and the masking & components on the board that must be considered but we regularly use this technique to minimise costs for customers.
Is the argon blanket an important option for preserving the life of the conformal coating?
The argon blanket is a useful accessory for overlaying conformal coatings that are sensitive to moisture such as moisture cure silicones like Dow Corning 2577.
The principle is the argon gas is heavier than air and using a series of valves a blanket of argon gas is bled over the conformal coating tank, effectively trapping the solvents under the argon.
This is also effective in regions of high humidity and can help prevent moisture ingress.
Do you recommend a viscosity check system on the conformal coating material in the inline dip tank?
For inline processing this tends to be volume process where regular coating of PCBs takes place. Therefore, the tank lid is open for long periods of time and the material is being used up regularly.
This will mean there is a change in viscosity of the conformal coating on a regular basis too.
Therefore, measuring the viscosity change is important.
This can be done manually but will need downtime of the system. It is better to use an automatic system which measures and doses without operator interference or error and controls the process fully.
How does the automatic viscosity control and top up system for the dip coating system work?
Automated viscosity control is a sophisticated process reserved for companies who are going to process a lot of PCBs and change the viscosity of the tank a lot of times in a shift or for companies who want a totally closed loop process without manual adjustment.
The viscosity system is an inline viscometer with feedback monitoring the tank constantly.
There is a dosing system that would adjust the coating as required, feeding back into the conformal coating tank either conformal coating material or conformal coating thinners.
How is the height of the conformal coating material in the tank controlled on the dip coating system?
On the inline dip coating system there are generally two parts to the tank. These are the dipping area and the sump, which are separated by the weir edge.
The material is pumped into the tank, over the weir into the sump. Therefore, the weir edge holds the height constant and the weir sump drops with material use.
Monitoring the weir sump depth is crucial again to learn how fast it is drained.
We recommend a 25mm (1”) edge difference between the top of the weir and the sump material and keeping the material close to that avoids a wide evaporation area over the weir which means more solvent evaporation.
This can also be critical with materials that do not re-dissolve into the material when dry like water based coatings. If they do dry /cure then these bits float around in the sump and then eventually could clog the pump.
We would recommend using a stainless steel basket which we have designed to catch these bits like a sieve and any other bits floating (or even PCBs dropped!).
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