Vacuum Web Coating

Telescoping after metallizing.

Our company experiences telescoping inside the vacuum metallizer but on the rewinding side when the film is already metallized. We observe that telescoping occurs in the inner layers not on the outermost layer.

What could have caused this and how can it be resolved?

Answer

Below are a couple of typical telescoping problems where the telescoping is close to the core.  The top one of the two is caused by the rewind roll not being aligned properly with the unwind roll.  If the core is mounted and the web treaded through and taped onto the core, it is most likely that the web has been taped on to one side of where the true alignment is and so when winding the rest of the web through it re-aligns to its true position. Winding slowly this may not be immediately noticeable but as the web speeds up this will become more noticeable but by then it would be under vacuum.  Sometimes this is easily remedied by making the operators take a little more time to pull a little material through and more carefully checking the alignment before taping to web to the core. 

The lower picture can be produced by either not having the tail of the web aligned properly or by having a problem with the acceleration of the web. If the web is not properly aligned or if the profile of the web is not good the tension on one edge of the web may be different to the other side and so when the web is accelerated the tension difference pulls the web over slightly and gives the telescoping effect. The larger the tension mismatch the greater the telescoping offset will be.

I hope this gives you some ideas as to what you might be seeing in your films.

Lower than expected barrier performance

I represent a converter who is trying to achieve good barrier properties (wvtr<0.2,otr<0.5) using metallized PET as part of the laminate. We have our own metallizer and the highest OD we can achieve on a single pass is 3.0.However, at this OD, we are unable to achieve the above noted barrier properties. Are there other methodologies that are available to enable to hit these properties. We have heard of double side metallized films utilizing plasma and chemical treating? Are these possible solutions and have you had experience with these types of films? Secondly, we are also concerned with loss of properties with downstream processing like rewinding and slitting. Are there special processing requirements for finished metallized film in order not to degrade the barriers achieved during metallizing?

Answer

The biggest limitation to producing good barrier films is generally pinholes.

Pinholes are produced primarily by debris that remains on the film surface that is larger than the very thin metal coating you deposit so that after metallization if the debris is moved it leaves behind a pinhole.

A second source of pinholes is spitting from the boat. This can be caused by a combination of reasons such as a low purity aluminium wire or the wire having a thick oxide coating on as well as an unstable pool of molten metal that as the pool size changes encourages spitting from the ends of the collection of crud built up from the wire oxide and impurities.

Another source of pinholes is any pickoff. This is where any high spots on the reverse surface presses hard against the freshly deposited metal and in some cases it overcomes the adhesion and picks off the metal from one surface and it transfers to the second surface. Again leaving a pinhole.  Typically this is associated with large fillers and a hard wound roll.

If the incoming roll is cleaned to a high standard it is possible to reduce the number of pinholes and so improve the barrier performance.

This can be done by using techniques such as the tacky roll type method of removing debris.  All polymer film will be covered with debris. This is partly because the polymer film as it is wound generates an electrostatic charge, which attracts airborne debris to the surface, this can include slitter dust. Thus if the roll is cleaned just before the final rewind before the roll then goes into the vacuum system this can help. If it is done earlier it can become recontaminated very easily. Ideally after cleaning the film should be in under a clean filtered air hood to limit recontamination. Also the film should be cleaned on both sides otherwise the debris from the back surface can become transferred onto the front surface as the film is rewound.

Similarly it may be possible to improve the wire purity and make sure the wire surface has a minimum of oxide present and may also be possible to improve the wire feed control to reduce some of the pool variations.  This will help reduce any spitting problems

Other possible things that can be looked at is to see if it is possible to wind the material between metallization and lamination with fewer or no front surface rolls in order to minimise the change of moving the debris and so limiting the number of pinholes that appear. Although it is better to clean the material and not have the debris present but this may not be easy to accomplish.

The use of adhesion promoting measures such as plasma pre-treatment if done well, with the process carefully optimised, can improve the adhesion as well as the metal wetting. This has two advantages. The improved wetting means that the metal will spread out on the surface and so will produce a continuous coating at a reduced thickness and this can be seen as either the same OD at a thinner coating thickness or for the same thickness a higher OD. The second advantage is that the coating is less likely to have problems with pick off as a method of producing pinholes.

In addition there may be other changes that can show benefits. If the metallized film is laminated very soon after metallization it may lose more barrier performance than if any further processing is delayed by a day or so.  The aluminium coating is very soft and is more prone to damage if it is handled very soon after metallization. If the roll is stored for a short period of time the native aluminium oxide is allowed to build up on the surface of the aluminium and this is much stranger and any damage is likely to be less.

Where double side metallization wins is that there statistically it is unlikely that any of the pinholes on each of the two aluminium coatings lines up with each other and so the increased tortuous path reduces the gas or moisture transmission.

I hope this gives you some explanation about where the lack of barrier comes from and some possible routes forward.

Problem with Rootes Blower

What could be the cause of a roots blower pressure to plunge intermittently?

Answer

There are some questions and checks that you can do.

Is the same pressure change seen by more than one pressure gauge?

If the pressure variation is seen only on one gauge then does the noise the Rootes pump makes change with the change in pressure indicating that the extra gas load is real or if you measure the current to the pump does this change with the change in pressure also indicating a real change in gas load.  If the sound or current does not change then it might mean that the pressure gauge is faulty rather than the pump.

Assuming it is not a faulty gauge the ask what has changed between the pump not having a problem and the problem appearing?

This could range from nothing at all to things such as the system being overhauled, or maintained where water or air joints have been undone and re-made, or the system has been greased/oiled as part of a routine maintenance.  If something has been done to the system then this could be the source of the problem. Whatever has been done may have disturbed something such as a seal is not longer sitting properly and so is leaking, etc

Check the simple things such as the oil level in the pump, has the correct oil been used, and check the pressure gauges are reading correctly. 

Having done all this then there are two basic possible problems.

The first is that there is a problem with the vacuum. This could be a water leak where the water is leaking at a low rate into the vacuum such that as it evaporates into the vacuum the heat required to evaporate is taken from the water and pipe and this is cooled to the point where the water freezes. The ice then has a lower vapour pressure and so the vacuum pressure falls. As the evaporation rate has fallen this takes less heat and so the thermal mass begins to heat the ice and it returns to water and so the higher rate evaporation returns and the pressure rises. The cycle then repeats and the water freezes and the pressure falls again.  This generally only occurs for relatively small leaks and the two pressures are characteristic of the vapour pressure of water and ice.

There could also be a small air leak and a pipe that is deforming so that sometimes the leak is closed and other times open.

The second possible type of problem is that there is a problem with the Rootes pump.  This could relate to gas loading. If the gas load is too high the rotors could be bending and touching the housing causing an increase in friction and so slowing down the rotation speed. Other problems could be a problem with the seals that is causing an intermittent leak.

I hope this gives you a few things to think about and try.

Pinholes, cracks and barrier performance.

Are there typical industrial standards and referenced test methods for pinholes and gaps in metallized coatings on heat sealable multilayer barrier laminates? 

What is likely to cause gapped streaks (~ 1/4 inch wide) in the aluminium coating in such films?  (A supplier recently switched to a new bag which exhibited MVTR problems traced to gaps and pinholes).

Answer

As far as I am aware there is no standard for the number and size of pinholes per unit area.  I have searched for standards and found the following comments relating to pinholes in metal foils for blister packs.

Blood Compatible Materials and Devices: Perspectives Towards the ... –

by Chandra P. Sharma, Michael Szycher - 1991 - Medical

Therefore, although no standards exist which control the frequency or size of pinholes that can be tolerated in packaging materials, it is important to ...

books.google.com/books?isbn=0877627339...

Industrial Sterilization - by G. Briggs Phillips, William S. Miller - 1973 - Medical - 440 pages

Therefore, although no standards exist which control the frequency or size of pinholes that can be tolerated in packaging materials, it is important to ...

books.google.com/books?isbn=0822302993...

Admittedly these are no old references but I have not found anything to suggest there are now standards to work to.

There are some ASTM Test methods but these relate more to the testing to evaluate the level of leaks and they do not set any standard.

Eg. ACTIVE STANDARD: ASTM F2227-02(2007) Standard Test Method for Non-Destructive Detection of Leaks in Non-sealed and Empty Medical Packaging Trays by CO2 Tracer Gas Method

ACTIVE STANDARD: ASTM F2228-02(2007) Standard Test Method for Non-Destructive Detection of Leaks in Medical Packaging Which Incorporates Porous Barrier Material by CO2 Tracer Gas Method

Again these two are directed at medical packaging and are more aligned to metal foils and foil laminates for blister packs than for metallized laminates.

In metal foils pinholes would appear to mean larger diameter holes than pinholes seen in metallized films.  There are a couple of papers ‘Examining Defects of Various Sizes in Device Packages’ a report by Assistant Professor Laura Bix , Ondrea Kassarjian, Ronald A Iwaszkiewicz and Jane E Severin    Michigan State University (MSU)  & The Impact of Foil Pinholes and Flex Cracks on the Moisture and Oxygen Barrier of Flexible Packaging  By Lee Murray  Alcan Packaging, Neenah Technical Center  (as located by ‘Jimmy’ in his comment on pinholes) whereby in each of these papers they use lasers to mimic holes in foils.  This is done to compare what barrier performance should be achievable and the barrier performance that is being achieved in practice and then to suggest why there is a difference.  What is surprising is that the size of the holes they make are around 50 microns in size although the images they show of real pinholes in foils are closer to 10 microns or less. In this second paper they list the number of pinholes achievable in foil products they sell.  I have looked at other foil manufacturers and they have similar levels of pinhole numbers although none appear to define the diameter of the pinholes.  Thus from one manufacturer the following was in their published literature.

Aluminium foils sales literature for use in 'blister' packaging applications

Pin Holes : 0.025 and 0.03 mm foils are free from pin holes.

In 0.02 mm foil, 40% of foil is free from pin holes, rest less than 10 PSM (pinholes per sq m)

So relatively low numbers at 10 pinholes per sq m but these could be big pinholes or little ones and the difference in barrier performance could be huge.

Enough of foil if we turn to metallized film there does not appear to be any standards for this either and again there are only the standard test methods published for establishing the barrier performance of polymers, metallized polymers or laminates.

As far as testing is concerned a popular method is to use a photographic light box (a number of lights in a box with a translucent top plate to give a bright diffused back light) to put the metallized film on and to then simply count the bright spots.  This can sometimes be modified to using a cardboard sheet with a square of specific size to overlay on the film to then give a number of pinholes for a particular area.

This method only gives a value for the number of pinholes but no information about the size distribution.

Pinholes are also not always perfect in that they are not a cylindrical tube cut through the metal coating.  If you imaging the debris on the surface to be a sphere it will shadow the depositing metal flux differently as the web approaches, moves through and departs the deposition zone. Thus the pinhole with have a graded edge and if the debris falls off during the deposition it will have some coating across the surface but just have a different optical density to the full coating.  Thus there will always be some error in counting pinholes because of the brightness of the light passing through. The fully bright spots are easy to count but there will always be some that are questioned as to whether to include them or not.

It is possible to automate this process by using Image Analysis where a digital image of the area is taken and the contrast difference is used to distinguish the pinholes.  This does require setting thresholds for the contrast difference and so again will have an error.

By converting the captured image into a high contrast black and white image it is also possible to take the size of each pinhole. This too will have errors as the light tends to flare and so most pinholes will be sized too large but this is something that can be compensated for.

It is also possible to use microscopy to look for pinholes. This can use transmission microscopy (as per the light box) and the option here is to use different magnifications to look for the smaller pinholes.  Again the Image analysis can be used to count and size the pinholes.

It will depend on your applications as to what level of barrier you are requiring. This will determine how much of a problem you have with pinholes and how much effort you are having to spend on cleaning the web and how detailed the hygiene requirements are for the cleaning and management of the winding and deposition systems.  This will also determine how detail you need to be in evaluation the level of pinholes. It is obviously cheaper and quicker to use a light box with a cardboard template than it is use a microscope and image analysis system.

Pinholes and Gaps.

Pinholes are generated by several possible means. The most common being dust/debris on the surface that is metallized and then the pinhole appears when the debris is subsequently removed. But this is not the only method of producing pinholes. Spitting during the metallization process where crud from the molten pool may be ejected and damage the coating or web and result in a pinhole.  The pressure from fillers from the back surface of the web pressed hard onto the metallized coating can pick off the metal, particularly if the metal is not well adhered.

Looking at the shape and the edges of the pinholes can sometimes provide information about the source of the pinholes. Pick off is likely to have sharp edges whereas the shadowing of the debris is likely to have a graded edge and the spitting source is more likely to have evidence of the hot material landing on the polymer surface, such as a crater and maybe even the crud still in place.

Where there is debris on the surface it can be removed in two ways. Firstly it can simply drop off or rolled away leaving behind the pinhole. Secondly it can be pushed along the surface where if it slips on the metallized surface may leave a track from the pinhole in the direction of sliding. If the debris is sharp, or hard and the pressure high enough, the debris may scratch the metal and remove some of all of the metal producing a surface scratch. Again the direction of this damage can be useful as it can be an indicator of the quality of the winding.  If the scratching is in the machine direction then at least the winding is aligned. If the scratches are angled to the machine direction then it is an indication that the web is slipping sideways at some point in the winding process, which is adding further damage to the film besides the simple pinholes.

These scratches can be one possible start to the 'gaps' referred to.

Generally I do not hear any reference to gaps. What is common is that metallized (or transparent barrier coatings too) decrease in barrier performance when they are made into finished packages. Often there are specifications to the amount of loss that is permissible (i.e. no more than a 2x increase).

In evaluating coatings one of the tests used is the tensile test. If this is done under a microscope it is possible to watch the onset of cracking of the coating and the increase in cracking with increasing strain.  The onset and amount of cracking is dependent on the adhesion between the coating and the substrate. If the adhesion is poor less stress can be transmitted through the interface between the coating and substrate and so the point where the system has to find a way of relieving the stress is lower and cracking starts earlier.  The aluminium metal is less elastic than the polymer and it is only the fact that the coating is so thin and the adhesion reasonably good that the coating can deform without too much damage during packaging.  If the material is laminated the metal coating can also be near the neutral axis that may also help.

However where the package includes dead folds or creases there are likely to be regions of high strain that will lead to cracking of the coating and gaps will appear in these regions. If you then look at where these cracks might appear a good starting point will be areas of existing damage. Thus a pinhole with a scratch, particularly if the orientation is aligned, may already provide a gap that just expends and widens.

Thus if I were to have to look for areas of poor barrier I would look to the final package and look around the areas where the web has most likely been deformed the greatest during the packaging process.  This may also include temporary stresses, such as if the neck of a pouch were stretched open during filling and the stretch may have been too much for the type and thickness of the polymer and thus putting a permanent deformation into the film. Heat-sealing often uses both heat and pressure and can cause some film distortion. I would also test the metal adhesion to make sure the adhesion was being optimised. If the adhesion is lower than optimum I would check to see if the barrier improved on the existing package when the adhesion was improved to the optimum.

For the pinholes, if the cost and specification justifies it I would add a tacky roll cleaning process for both the front and back surfaces immediately before the rewind before the film enters the metallization process including a positive pressure filtered hood to prevent recontamination. Or consider how I could include the tacky roll inside the vacuum deposition process before a plasma pre-treatment. I know that a couple of companies have done this but it has been for much higher technology applications were the running speeds were lower and roll lengths shorter and so I am not sure the tacky roll technology is yet suitable for inclusion in vacuum for metallizing speeds and lengths.    

I hope these thoughts help.

Film storage conditions

Can you provide us any information on the shelf life and proper storage conditions of vacuum metallized films (in rolls)? And what will deteriorate in the laminate over time, what are the factors that can cause the deterioration of the vacuum metallization?

Answer

To define proper storage conditions can be difficult. Different areas in the World have different problems and hence requirements. However there are some basic principles that can be applied. It is preferable to have the temperature and humidity regulated. The reason for this is that many polymers will absorb and desorb moisture and all will expand and contract with temperature. Any relative movement can induce surface damage such as microscratches as the layers of film move very slightly against each other. This can be due to any protruding filler or because of the residual debris on the surface that may also lead to pinholes.

Large temperature changes can also lead to film handling problems as if the roll is moved from one temperature to an area with a very different temperature part of the roll may have expanded but not the rest and if moved the roll will be prone to telescoping. If the temperature becomes very low the cores can be crushed particularly after metallizing where the roll will already be hard because the rewind in vacuum does not provide any interleaving air to soften the roll.

High temperature can also be a contributing cause of delamination problems in some materials.  Many films contain oligomers and additives such as slip agents. These low molecular weight materials will migrate from the bulk of the polymer to the surface. If the metal is not well adhered this low molecular weight material will appear at the interface and the adhesion will fall. This may well also occur with laminates but it does depend on the material adhesion and the wetting characteristics of the any coating.  Thus the aim is always for optimised adhesion where possible so that the coating wets the surface well and the adhesion is high. This will lead to a more stable product long term as there are fewer points where the low molecular weight materials can exude into.

In controlling humidity this can lead to other problems. It would be nice to have very dry conditions as this would keep the moisture level in the polymers low and this could help the vacuum metallizing process and possibly reduce the speed of post deposition aluminium oxidation.  However if there is winding processes in the same environment there will be significant electrostatic charging and the concomitant winding problems and possible film damage from uncontrolled discharges. Thus the humidity needs to be sufficient to keep these electrostatic problems under control.

Hence in some parts of the World the controlled atmosphere usually means cooling and drying the atmosphere whereas in others it is preferable to heat and moisten the atmosphere.

I would monitor your seasonal changes in temperature and humidity and pick a suitable mid-point (allowing for a workable humidity level) and then look to stabilise the temperature and humidity around that.

I am sorry it is not a definitive answer but hopefully it is one that you can work with reasonably easily.

Deposition of conducting polymers

I need to know

1.         What are the methods used to deposit conducting polymers?

2.         Can we use vacuum coating deposition to deposit conducting polymers on glass surface?

Answer.

To some extent the deposition method depends on the type of conducting polymer and the formulation used.

Many of the conducting polymers have other materials added to provide other properties such as a polymer binder to better adhere to the substrate where there will be extreme flexing of the device or the addition of solvents including water to reduce the final coating thickness.

H.C.Stark gave this description of application methods in the paper they presented at the 2005 AIMCAL Fall Conference.

For small and irregularly shaped parts, spray coating or dip coating is often preferred. For larger substrates, such as plastic sheets or wide-web plastic rolls, other application methods such as gravure printing, roller coating, etc. may be used.  For deposition of patterned PEDT films, ink-jet, screen or nozzle printing can be used.   In addition to this small discrete substrates are often spin coated.

If the polymer has been diluted using a solvent there could be problems in trying to deposit the coating in a vacuum system because of the different vapour pressures.

The OLED polymers are regularly evaporated in vacuum systems and Kurt J. Lesker sell a slot source specifically for the deposition of the OLED polymers in vacuum.  In principle if the conducting polymer does not fractionate it too could be directly evaporated in vacuum using a similar source.  The trick with this type of deposition appears to be using the lowest possible temperature to prevent damage to the polymer and to keep the temperature as even and constant as possible to keep the deposition rate and hence coating thickness constant.

Problem with bond strength.

Is VMCPP with high optical density (2.4 - 2.6) will have lower bonding strength compared to VMCPP with lower density (1.8 - 2.0)?

If yes, please explain why and how to overcome this problem?

Answer.

It is most likely that the adhesion is the same for both coatings but the stresses on the thicker coating means that it appears to fail more readily.  Essentially you have two dissimilar materials and they have very different coefficients of thermal expansion/contraction so that there is always likely to be some stress between the coating and the substrate. The thinner the coating the less noticeable this is likely to be. As the coating thickness increases there is likely to be an increase in the tendency for the material to curl and in keeping the web under tension to keep the web flat will increase the load at the substrate/coating interface. Also as the metal does not stretch like the polymer the tension will also add a load to the interface. If the adhesion has any weakness the coating will fail at this point.

Many polymers have a surface that is slightly different to the bulk polymer. It is possible that the failure is occurring within the polymer and not at the metal/polymer interface. This surface layer may be due to material exuding out of the surface such as slip agents or other additives or it could be due to unpolymerised residues.  These are a source of weakness and it is these that pretreatments are used to try to minimise.  So it is worth checking on the pre-treatment, find out what has been done, if any, and then as some of these treatments can be temporary check to see how old the treatment is and ideally make sure there is a plasma treatment in the metallizer immediately before the metal deposition and make sure this final treatment has been optimised.  It is possible to over treat the surface and this can degrade the adhesion too hence it is worth checking the treatment is optimised.

Pinholes in foils

Please tell me the international standard of pinholes in Aluminium Foil 7 microns and 6 microns.

ANSWER

I do not know if there is any standard count of pinhole levels in aluminium foils.

Can anyone help with this request? 

Checking the resistivity of lacquered film.

Request: How do I check the Surface resistivity of 23-micron metallised and
both side lacquered coated film?

Answer.

The conductivity or resistivity of a metallized film does not require the film to be in intimate contact with any probes. Using an eddy current monitoring system the resistivity can be measured. The process uses a signal coil and a receiver coil, these can either be positioned one either side of the film or both concentrically on the same side of the film. The signal coil has a current passed though the coil that has a magnetic field that can induce a voltage in the metallised layer, which in turn can be detected by the receiver coil.  These coils have to be designed to be used for particular conductivity ranges and if they are used for coatings outside the designed range the system will lose some sensitivity.  This should not be a problem, as generally the system will read well for a resistivity over approximately 2 - 3 orders of magnitude.

I suspect that you may find that this type of system is used within your metallizers as part of the feed back control for getting the deposition from each metallizing source uniform.

If this is so then you can always check that the lacquered material resistivity is similar to when it was metallized.

Counting pinholes & type of illumination

1.You mentioned that we can count the pinholes by using a lightbox and counting method. We have tried to check for pinholes using a dark room and by putting the VM sample against a 2,000,000 candlepower lamp and were able to detect these pinholes. However, our supplier just uses a light box with 4 fluorescent lamps and a glass top. With this kind of light box, pinholes cannot be detected. We also counted pinholes with an area of 17 cm x17 cm and then just converted it to count per square meter. Is that the proper was to count pinholes? I would also like to ask what is the standard or specifications of the lightbox and what counting method should we use, what is the procedure?

Answer.

As far as I am aware there is not a standard for the measurement of pinholes.  As such everybody makes or buys their own light box and decides on the area for measurement and works accordingly.

In theory the number of pinholes should be the independent of the light source but this is not true. This is because high intensity lights will make areas that have had some
coating but are of reduced optical density appears as pinholes that with lower intensity illumination are not classes as pinholes.

If you consider how pinholes are produced you will see how the various shapes and types of pinholes are created.  The first and generally by far the largest cause of pinholes is the dust/debris on the surface of the film that is present during the
metallization process.  It is impossible, even with the best cleaning techniques, to remove debris of less than 0.3 microns in size due to the Van der Waals forces present. If you consider that this debris of up to 300nm size is considerably larger than the thickness of the aluminium coating that is generally around the 15nm - 25nm type of thickness.  If the film has not been cleaned, which is often the case; there will be plenty of debris that is in the range 1-10 microns, considerably larger than the coating
thickness.  If any of this debris is moved following deposition it will leave behind and area that is not coated with aluminium that is seen as a pinhole.  If any of the debris is moved, or drops off, during deposition there may be a reduced deposition at that point that may not be classed as a pinhole with a low illumination light but will be with a high intensity light.

The second major source of pinholes is due to 'pick-off'. This is where the adhesion may be lower that ideal between the aluminium and the substrate.  Usually the substrate is either through filled or is a coextruded film with the back surface layer filled. The filler is used to reduce the coefficient of friction by allowing the filler to roughen the polymer surface. These peaks where the filler pushes out the polymer surface will be in hard contact with the freshly metallized front surface as the film is rewound. As the winding is in vacuum it will always be a hard wound roll. The high peaks press hardest against the coating and in some cases will stick to the coating batter that the aluminium adheres to the front surface of the substrate and so when the roll is next unwound the aluminium is picked off and remains on the back surface leaving a pinhole in the metallized film.  This type of pinhole can generally be reduced in number by the appropriate optimised plasma treatment that gives the maximum aluminium adhesion of the metal to the front surface.

A source of thinner coating spots that with high intensity lights might be classed as pinholes can be created in the following manner. Polymer films are not 100%
polymerised and will often have a proportion of short chain oligomer that is mobile and this can migrate to the surface. This oligomer is low molecular weight and may also reduce the surface energy. In other materials it is common to add a slip agent, or many other additives, these too may be of low surface energy or low molecular weight.  The slip agents, as their title suggests, are added to improve the film handling characteristics by reducing the coefficient of friction. Even with a filled polymer where the filler reduces the coefficient of friction to some level, slip agents can be added to reduce the friction further.  This oligomer or the additives may have a lower melting point than the polymer and the material may be vaporized during the deposition process locally reducing the sticking coefficient of the aluminium and so locally reducing the thickness of the aluminium deposited.

High intensity sources will also tend to allow smaller diameter pinholes to become bright enough to be included.  I did some work in counting defects and I used and image capture and image analysis to automate the process but this needed to have thresholds set for the minimum contrast difference that would count as a pinhole. Thus there is a margin of interpretation between what is a pinhole and what is not. This will change with intensity of illumination as the sensitivity on small diameter pinholes is improved.  It can also mean that the diameter of the larger pinholes is over estimated with high intensity lights as the detector can become saturated and there will be more of the light diffraction included in the measurement.

Hence suppliers may wish to use a lower intensity illumination because it will reduce the number and size of the pinholes counted. End users may wish to have higher intensity illumination because this gives a more realistic, if more frightening, count of the pinholes present.   At some point an agreement and a correlation between the supplier and customer measurement equipment and technique must be made. Otherwise there will be an ongoing battle over the number of pinholes and the film quality.