Laminate Outer 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 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 outerlayer 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 outerlayers..
Aim of Process
To polymerise an image of the inner layer circuitry in the correct image integrity to the correct position onto the dry film etch resist.
The exposure process is the polymerisation of the oligomers/monomers in the resist chemistry. Polymerisation is accomplished by exposing the resist to a
known amount of ultra violet radiation.
The following sections detail eight critical areas that should be taken into consideration to ensure successful exposure.
There are several different types of exposure units used in the industry which range from single-sided exposure manual printers to double-sided exposure fully automated types. Different types of light reflectors are used with varying efficiency of light transfer. The height of these reflectors above or below the panel to be exposed will determine the light energy reaching the panel. This will directly impact the rate of exposure. The quality of the reflectors will determine the ENERGY DISTRIBUTION across the exposure frame.
Light will not be reflected evenly across the frame leading to resist exposure difference across the imaged panel. On some older manual exposure units the difference in light energy across the exposure frame can be as high as 40%. This does mean that we can have a difference of up to one step on a 21 Step tablet across the frame.
PHOTEC photoresists have a peak spectral absorption of about 360 manometers. Therefore, the lamp used must have a peak spectral output at the same wavelength. If there is a mismatch between output of the lamp and the absorption characteristics of the resist there may be incorrect exposure, even if the required number of millijoules is applied. In general, the type of lamp used is a high pressure mercury lamp. To obtain the required wavelength output the mercury is normally “doped” with small quantities of iron.
Lamps should be changed regularly as the efficiency of the lamp changes with age. Regulating the current that is applied to the lamp controls the light output. In addition, spectral output may also change. It is common practice to change the lamps after 1000 hours of use. A meter is normally incorporated in the machine to record the time that the lamp is used.
Some lamps are cooled with a water jacket around the lamp. This cooling does prevent heat build up within the exposure machine but it also reduces the amount of energy transmitted. In severe cases this may increase the length of exposure time to reach the desired number of millijoules.
A short, high illumination intensity is preferred to a longer, low intensity illumination. The optical density of the resist changes during exposure. Since the resists are of the surface polymerisable type, the surface layer of the resist will consume energy, resulting in reduced energy passing to the lower levels within the resist. Therefore, unless there is sufficient energy intensity, most of the energy will be consumed as it passes through the resist and the
resist at the copper interface will not be exposed sufficiently.
During normal exposure we will see an exposure difference between the resist on the surface and that at the base of the resist. The difference in exposure can be two steps on a 21 Step Density tablet. Two steps are a reduction of 50% of the light energy.
Exposure intensity is measured in milliwatts.
The actual time of exposure in seconds is millijoules.
Millijoules = milliwatts x time
Each resist requires a certain amount of energy to reach the optimum exposure state. The amount of exposure will determine the actual chemical properties of the resist. In all cases the technical data sheet of the resist should be followed.
If the exposure energy is too low, then the resist will be attacked by both the developing solution and the subsequent processing chemicals. The adhesion of the resist can be affected leading to open circuits after innerlayer etching and short circuits on electroplated circuits.
If the resist is exposed with too high an exposure energy, the image on the phototool will not be transferred in a 1:1 ratio. This means that the exposed traces will be wider than on the phototool.
Phototool quality is extremely important for high circuit density products.
Image density ( Dmax ) should be in excess of Dmax 3,5. Edge definition of the opaque areas should be sharp and well defined. Diffused image edges can lead to variation in track widths after development or incorrectly exposed edges which may cause problems in the development, electroplating or etching processes.
Background clarity of the clear areas of the phototools is essential to ensure short exposure times. A high background density may increase exposure times by up to 100%. If not corrected to compensate for high background density, poor resolution and resist side wall profile may occur.
The use of phototool emulsion protection systems extends the life of the phototool. When setting exposure times the thickness of the protective coating (normally 3-5 microns) must be considered to ensure accurate line width reproduction. This thickness increases the off-contact distance from the resist and thus makes the beam collimation of the exposure unit more important. If the exposure unit has poor beam collimation, the use of a phototool protective system will be dependent upon the permitted deviation from the normal 1:1
reproduction of the phototool.
Degree of Collimation
To obtain accurate phototool reproduction, the exposing energy (light) which is hitting the panel at right angels to the resist is required. This light should be evenly distributed across the exposure frame.
Collimated light is defined as light that is very close (90 degrees) to the panel surface. Normally the angle of declination is about 0,5 degrees. It must be noted that the higher the degree of collimation, the more dirt and scratches will have on the phototool. These defects will be reproduced on the exposed panel.
There are several factors that can influence the angle at which the light hits the panel being exposed. The light emitted by the exposure lamp passes through several different layers before it actually reaches the photoresist
These include the glass or polyester vacuum frame, phototool, phototool protective layer and the polyester cover sheet on the resist. All of these layers can give diffraction of the light.
Any air trapped between any of the layers will also cause light scatter. This air gap can lead to “off contact“ printing. The actual print out image will be diffused and lead to rejects.
A vacuum delay is necessary to allow time for all the air between the various layers to be removed. We need the closest contact possible between the phototool and the photo-resist, the vacuum delay allows the time for the air to be removed and for the phototool to pull down onto the resist surface. A typical vacuum delay is about 10 seconds but for high circuit density boards or high-resolution boards then the vacuum delay is increased up to 30 seconds in severe cases.
Exposure Unit Temperature
Heat is a key factor during exposure. Polymerisation reactions started by ultra violet light can be continued by heat energy and continue after the light energy has been stopped. This may result in over exposure of the resist even if the number of millijoules applied is correct. The actual degree of over exposure caused by the heat in the exposure unit will depend on the actual temperature and the inhibitors used in the formulation of the resist.
The exposure unit should have sufficient airflow within the unit to remove the heat emitted by the lamp and to maintain the temperature within the unit at or near room temperature.