Laminate Inner Layers
Aim of Process
To laminate an ultra violet sensitive resist onto the copper surface.
The film has the inner polyester sheet automatically unrolled from the film roll, the dry film etch resist is then laminated onto the core under heat ( 110 degrees c) and under pressure ( 3 bar ) while the core is passed through the hot rollers at 1.5 m/min
The freshly chemically cleaned inner layer cores are now laminated with a dry film etch resist.
The configuration looks like the following
What is important is laminating temperature and lamination speed, and the rollers must be in a good condition or they can produce defects such as shown below
The panel when laminated should be free from any debris, wrinkles or visible defects.
The innerlayer core is now laminated
Now some theory on Dry Film Lamination
Lamination is the application of the dry film resist to a properly prepared substrate.
The lamination process must be carefully controlled to ensure that the required mechanical adhesion of the resist to the substrate is obtained by flowing the resist into the surface irregularities. The resist should not flow too much into any drilled holes or slots. Over flowing of the resist will cause thinning along the periphery of holes and result in tent breakage. Therefore, a correct balance of all the lamination parameters is crucial to ensure optimum
performance of the resist.
There are different types of proprietary laminators on the market ranging from manual to fully automatic and from hot rollers being heated by resistance heaters within the roller to indirect heating of the rollers by infrared. In all cases the recommendations of the manufacturer must be followed. The basic principle of the operation is to preheat the resist to a temperature of 110 +/- 100C to lower the viscosity of the resist just prior to application, under
pressure, to the substrate.
A normal sequence of operation is: –
· Preheat substrate (40-500C)
· Heat photoresist (110 +/- 100C)
· Apply resist by roller pressure to the substrate. Typically a pressure of 2-4 Kgf/cm2 is applied.
· Lamination speed 1,0-3,0 meters per minute
Although preheating the substrate is not essential, it does ensure that the cold substrate does not act as a heat sink and thus reduce the actual temperature of resist at the lamination stage. If it were to do so it would affect the adhesion and conformance of the resist to the substrate.
There must be a correct balance of lamination temperature, pressure speed and resist tension to ensure that maximum resist adhesion and tenting ability from the photoresist is obtained.
During lamination the resist is heated on one side and is laminated onto the cooler substrate surface, thus a temperature gradient exists through the resist. The lower molecular weight and lower boiling point fraction of the chemicals within the resist will migrate to the cooler surfaces. A hold time after lamination prior to further processing is to ensure that mobile chemicals equilibrate with the higher molecular weight fraction.
The actual preheat temperature required is dependent upon both the thickness of the dielectric and the copper on the surface. As the thickness of either or both increase, the greater the thermal heat sink. In order to obtain a surface temperature of 40–500C, the preheat temperature must be adjusted accordingly.
If the preheat temperature is too over 550C, wrinkles may occur during lamination. When using an automatic cut sheet laminator, wrinkling is particularly predominant at the edges of the panel. Thinning of the resist at the periphery of holes or slots can occur if lamination speed is too low or the lamination pressure too high.
If the preheat temperature is too low poor resist adhesion immediately after lamination will result. Resist conformity to the substrate, especially in deep and narrow areas, will be imperfect at best.
The condition of the rubber on the rollers is important to ensure a constant pressure over the entire panel. Any imperfections in or on the surface will appear as defects on the laminated resist. A cut or a piece of rubber removed from the roller will give a lower pressure at that spot during lamination and will in severe cases show up as a blister on the surface of the laminated resist.
The hardness of the rubber should be about 65 Shore hardness. If it is harder than this then the resist will be pushed into the holes or slots on the panel. If the hardness is too soft poor conformance will result. The thickness of rubber on the rollers should be as recommended by the equipment manufacturer.
Care should be taken when re-coating any rollers since the removal of the rubber is normally mechanical and at this stage the steel shaft is reduced in diameter. To obtain the required outside diameter of the roller, a thicker rubber will be applied. Rubber is a poor conductor of heat and therefore, the actual transfer of heat from the heating elements inside the roller will be less than normal.
During lamination of a batch of boards the roller may not be able to maintain the correct lamination temperature. On most laminators the heating is accomplished by a resistance heater located within the steel core of the roller.
To ensure that the heat is transferred from this element to the actual roller a heat transfer gel is used. Unless this gel is evenly coated around the heating element, irregular heat transfer will occur along the length of the core and around the diameter. All rollers should be checked whenever they are changed and also on a regular basis. If there are major temperature differences on the roller defects will occur. The two heated rollers should be checked periodically to ensure that they are parallel with each other.
If pressure is applied by air at both ends of the rollers and narrow width panels are laminated frequently, a “bow“ may form on the rollers. If this condition exists, pressure applied to the centre of the panel will be less than that applied to the centre. In severe cases there is a possibility that poor adhesion of the resist to the substrate will result.
During lamination the resist must be heated above its glass transition temperature to make it semi-liquid and hence be in a state to be pressed into substrate defects by roller pressure. Although the glass transition temperature is about 350C, the resist must be heated to a higher temperature to account for cool air and the heat sink effect of the substrate. Heat must pass through the polyester support film prior to heating the resist. It has been shown that the roller temperature, measured by a contact temperature-measuring probe, should be 105-110 0C to provide optimum flow characteristics of the resist. On many laminators the hot rollers are heated by resistance elements in the core of the roller. If the contact gel used to transmit the heat from these elements (firstly to the steel core and the rubber coating) is not operative, the heating will be much slower and the rollers may not return to the set point between
lamination of subsequent panels.
The temperature indicated on the read out on the laminator is via a contact probe situated at one end of the roller. If this contact point is dirty or loose it will lead to an incorrect read out temperature.
The dry film resist is heated to about 1100 C so that it becomes semi-liquid and will flow under the influence of pressure. The pressure that is applied to the rollers is to ensure that the dry film resist is forced into the micro roughness and surface defects that are present on the copper surface.
Unless the pressure is sufficient to enable this action to take place, the resist will not have the necessary physical properties to withstand subsequent processing.
If insufficient pressure is applied, poor conformance of the resist to the substrate irregularities will result. Development, etching or electroplating chemicals will penetrate under the resist resulting in rejects being produced. If the applied pressure is too high, the resist will be forced into any holes that require tenting and the tent strength will not be sufficient to withstand
Pressure must be sufficiently high to enable the resist to flow into the macro roughness of the panel formed by the different thickness of glass fibres used in the construction of the dielectric substrate. A normal pressure range of 3-5 bars is used depending on the type of laminator.
The resist requires a finite time to flow into the irregularities of the substrate under the influence of temperature and pressure. The actual speed of lamination determines the time of applied pressure.
The lamination speed is adjusted to give optimum conformance and at the same time the productivity that is required. Lamination speed is 1,0 to 3,0 metres per minute.
The overall lamination parameters are a balance of temperature, pressure, speed and substrate thickness. Substrate thickness is important since this will impact the amount of heat required to obtain the correct temperature on the panel.
If the speed is too high then the resist will not have sufficient time to flow into surface irregularities and hence poor conformance. If the speed is too low and all other parameters are at optimum, the resist will flow into tented holes causing tenting failures.
Hold Time After Lamination
After lamination the temperature of the laminated substrate must cool to room
temperature as quickly as possible.
· The resist is made semi-liquid at the laminating stage and as such will flow into surface irregularities. Once laminated the resist must be cooled quickly to prevent continual flow into holes that require tenting. If the resist continues to flow into the hole the resist thickness around the periphery of the hole will thin and may not have sufficient mechanical strength to withstand subsequent processing. A tent failure will result.
· The resist must stabilise prior to subsequent processing. The photo sensitisers and photo initiators move during the time that the resist is exposed to ultraviolet light and continue to move for a period after the light is switched off. The polymerisation of the resist will stop once the molecules are at rest. The higher the temperature of the resist, the further the molecules will travel. This is why the resist should be cooled as quickly as possible.
· The minimum hold time after lamination is the time taken for the resist to cool to room temperature.
· The maximum hold time after lamination is normally four days. However, to hold the boards for an extended time the panels should be covered in black opaque plastic. Yellow light does contain an element of visual light that dry film resists are sensitive. In the worse case, polymerisation will occur and prevent development of the resist.
You now know all there is to know about dry film lamination of innerlayers and outerlayers