Conformal Coating Dip Systems FAQ’s

Summary

How do you control the coating thickness in the 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?

The viscosity is monitored by utilizing 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 DS100 dip system?

The PCBs are hung on the cross rods by hooks (we can supply or you make / buy your own?) or any other form of jigging ( we can custom build). The immersion and withdrawal of the cross rods is carried out by a custom piston which moves up and down at a controlled rate down to 1-2”/min since we use an air over oil system.

Can the DS100 utilise in line curing?

This is a batch dip system without inline curing. If you wish inline curing you will be adding a zero to the price. Sort of range is £30K-70K depending on size of system, rate of cure, type of material to use.

Can I horizontally dip into the conformal coating using the DS101?

It is perfectly possible to horizontally dip PCBs using the automated batch systems such as the DS101 conformal coating dip system. 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 SCH regularly use this technique to minimise costs for customers.

Is the argon blanket an important option for the DS101?

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.

How often do you recommend a viscosity check on the conformal coating material in the DS101 tank?

Checking viscosity is down to the time the tank lid is off. If it is off all day I would suggest twice a day check to ensure control. If it is a few hours then each time you start. The main issue is building a pattern of use. As you use the dip coating system, record the hours, volume of boards and the conformal coating thinners used, and chart this. Then, you will be able to anticipate when it needs checking and when it doesn’t. Also, contributes to the lean manufacture processing.

SCH have an option of automatic viscosity control and top up system for the DS101 dip coating system. Can you advise how this works?

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 DS101 dip coating system?

On the DS101 dip coating system there are 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!).

Can the capacity of the DS101 tank be reduced?

SCH can make the tank any size you want. However, there is a ratio of surface area of the tank to depth that is important. If you have too little volume of coating in relation to the surface area, the evaporation of the solvent will change the viscosity of the tank too quickly and you will constantly be monitoring it. We have no formula for this but making it too shallow could be a problem.