Pin Holes: 5 Ways to Eliminate Them

Pin holes, sometimes also referred as pin windows, are the areas where no metal has been deposited. These can be a big problem for quality Metallizing especially in yarn grade metalizing where a pin hole can cause the thread to break. 

Reasons  for  formation  of  pin  holes  and  how  to  avoid  them-  

1) If there was a particle on film that got removed after metallization, it will leave a pin hole. This can be easily identified by careful examination as this pin hole will also have an area of thinner metallization around it caused by 'shadow' of particle in the path of metal for deposition. 



Pin Holes in Metallizing (Image 1)

Solution: Storing the film roll properly, away from dust goes a long way in solving this problem.  For this, vertical stacking of rolls in custom made wooden pallets with core ends jutting out for handling of roll is the best possible way.

2) Spitting from boats may also cause pin holes; this is easily inferred by heat damage to film around pin hole. 

Solution:  Controlling the wire feed and adjusting current to boats can solve this problem. For details on the subject of correct wire feed, refer to my earlier blog post here.

3) Excessive tension while winding of film will also cause pin holes. This can be inferred by examining the film. Here, pin holes will be elongated in the direction of pull.

Pin Holes in Film Metallizing (Image 2)

Solution:  Avoiding this problem requires adjustment of winding tension to the appropriate value.

4) Residue of some solvent or masking oil on film can also cause pin holes. These pin holes are clean and are also called pick off. These are formed by removal of Metal from film because the solvent did not let metal adhere to the film.

Pin Holes in Roll to Roll Metallizing (Image 3)

Solution: Storing the film where solvent vapors are present should be avoided. Also, if in any application masking oil has to be put on the film; care should be taken for its proper removal as well.  

Hence, cleaning and properly storing the film solves this problem.

5) Aluminium wire that is oxidized on surface is another reason of pin holes as Aluminium oxide will not deposit properly. This is evident by a uniform pattern of pin holes.

 

Solution: Use of fresh Aluminium wire or ultrasonic cleaning of wire if wire is old, goes a long way in solving this problem.

Pin holes on metallized film can be studied effectively by placing metallized film on a flat surface below which a strong light source has been placed on a diffuser to spread the light uniformly. A large area of film can be quickly scanned in this way. 

                                                                                         

 

 

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 Written by: Anshuman Punj.                      

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Evaporation Boat: Get Your Money’s Worth Out Of It

In the present time of intense competition, it is natural that engineers are trying hard to optimize the use of every resource to reduce costs. In Metalizing, Evaporation Boats undoubtedly contribute most in variable cost. It is therefore important that maximum life is derived from Evaporation Boats without affecting the quality of Metalizing.

Based on my knowledge of Engineering, experience of nearly 20 years in vacuum line and interaction with actual users, I have compiled a list of measures which if implemented can ‘enhance’ the life of Evaporation boats by about 25% so that you get your money’s worth out of it.

 These have been summarized as under -

1)    Initial heating of the new boats also called “break in” must always be slow to get proper life from Boats. Reason? New boats will always have some absorbed moisture. If the boat is heated quickly in this condition, the moisture, converted into steam having much higher volume will have little time to escape and can cause cracks because of high pressure build up.

2)    At the end of the cycle it must be ensured that all the Aluminum or any other metal being metalized has evaporated completely before heating is switched off. Otherwise residual metal will seep into the pores of boats; now differential cooling rates of boat and metal will crack the boat.

3)    Care must be exercised that before the vacuum is broken, boats must have cooled down. This is because sudden contact with cold air can cause boat breakage. About 5 minutes is enough for boats to cool down.

4)    Worn out copper clamps also are a big reason for boats not performing to their full capacity. Non uniform wear of clamp causes sparking between clamp and boat; this overheats and cracks the boat.

5)    A good quality graphite foil is very important factor in enhancing the life of Boat. To save money, sometimes Metalizers use poor quality plumbing grade graphite foil. This foil contains many impurities and binders and offers resistance to the flow of current. This causes the ends of boat to overheat and break. Supervac supplies 100% pure graphite foil free of binders. This is made with the patented process of Exfoliation. This foil gives you the following outstanding benefits –

A)    Offers no resistance to the flow of current.

B)    Saves costly power.

C)    Enhances the life of Boats.

6)    There is one more major factor inter linking the life of evaporation boat with graphite foil. Good quality graphite foil has good compressibility. This allows a proper electrical contact in boats and clamp before heating begins and expansion of boats in heated condition. Poor quality graphite foil on the other hand will not allow expansion of boats in heated condition putting additional stress and subsequent breakage of boats.

7)    Aluminum of purity 99% or more is required to avoid premature cracking. Impure aluminum will have impurities which will have different boiling temperatures; this will lead to an unstable pool of metal in boat cavity. This in turn results in non uniform heating of Evaporation Boat leading to thermal stress and breakage.

8)    Age of aluminum wire also affects boat life. With age, aluminum oxidizes and Aluminum oxide thus formed causes spitting and excessive heating of Boats. Freshly made or “cleaned” wire works best.

9)  Hardness of Aluminum should be maintained in proper range if required by tempering as a very hard wire is likely to puncture the boats.

10)  Aluminum wire should be fed in such a way that it meets boat in the center widthwise and in between 1/3 to ½ lengthwise. This ensures proper pool formation and increases the life of boats. (I have written a blog post entirely on this topic in the past. You can read it here.)

11) Feed of aluminum wire must be regulated to ensure that some aluminum is always available as molten “pool” on the surface of Boat. This makes sure that boat is uniformly heated and increases the life of boat.

12) Power to boat must be so regulated that aluminum melts just before touching the boat , otherwise it will cause an indentation in boat cavity, hastening the failure of boat.

13) It has been observed that Boats made of Titanium Diboride and Boron Nitride (also called 2 components or dimet) work out to be more economical than boats where Aluminum nitride is also used in addition to the 2 ceramic powders mentioned above to reduce the cost. Three component boats, though cheaper in cost, have a markedly shorter life.

14)  Experience shows that normal rectangular boats with cavity work out cheaper then Triangular or elliptical boats or boats without cavity or laser grooves on surface.



15) Good quality boats, manufactured by reputed companies like Supervac, having density of the order of 98% of theoretical density have very low porosity. This reduces aluminum seepage in boats and enhances life of boat. These boats work out much more economical than those supplied by phony “importers”/ unknown companies having no quality standards.

16) Contrary to popular myth, using boats with initial resistivity higher then recommended by manufacturer does not give more life. In fact this wastes power, causes overheating and premature cracking of boats.

17) Experience proves that spring loaded end support for boats works best for optimizing the life of boats as it provides a proper contact and allows expansion of boats on heating.

18) Level of vacuum in system is also very important for getting proper life from boat. Working at vacuum above 1x 10^-4 Torr is detrimental to the health of Boats as they get oxidized and damaged.( For getting high level of vacuum in shortest possible time Supervac Sigma+™ Diffusion Pump oil and Supervac SV-77™ Rotary Vacuum Pump Oil can be used.)

Points mentioned above are quite easy to implement and best of all do not cost an extra rupee. This however can result in a saving of about Rs. 9, 00,000/- per year per metallizer (a typical metallizer uses about 300 boats per month, costing about Rs.1, 000 per boat. Saving is 300 x12 x1000 x25% = Rs.9, 00,000/-).  This is not all; implementation of these actions will also lead to better metalizing quality, better maintained plant and saving of costly power also. 

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Correct Wire Feed for Evaporation Boat

While running the metalizing plant, if we take a few seemingly small precautions, they can go a long way in ensuring optimum usage of energy, expensive materials and thereby money. And as we know, money saved is money earned.

fig.1: Evaporation Boat

Figure 1 shows the Evaporation Boat. Aluminum wire is fed into the cavity of the hot Evaporation Boat (heated up to 1500⁰C) where -

1.    It melts

2.    Forms a small pool to fill the cavity

3.    Evaporates to form vapor

4.    Vapors stick to the substrate to make a thin coating

Ideally the aluminum wire should touch the Evaporation Boat between the 1/3 part and the 1/2 part from the wire feed end; never before or after this. The following photos demonstrate the right as well as the wrong position of the feed wire.

                

  

Positioning the aluminum wire too close to the edge of the Boat will not make the cavity fill up uniformly with the molten metal. This leads to –

Ø Non-uniform coating of the aluminum on the substrate.

Ø Wastage of the Al wire since this will cause excess molten metal to fall off the boat from one end.

Ø Wastage of electricity.

Ø Shorter life span of the Evaporation Boat since improper feeding of the wire will cause variation of temperature across the Boat.

Ø Faulty metalizing: When a molten pool of the metal forms in one part of the Boat’s cavity while the other part remains relatively cold, SPITTING phenomenon takes place. In SPITTING, the liquid metal overheats and bursts forth or ‘jumps’ out of the cavity in small quantities and either sticks to the vacuum chamber walls or on to the substrate.

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Why Should Vacuum Oils be changed at specified intervals?

In the present time of cut throat competition, users are under great pressure to reduce the cost of production. To achieve this objective, many times vacuum oils are not changed at specified intervals to get more “life” from them.

This is a dangerous practice and can damage the vacuum system badly. Let us understand why-

Oils degenerate on account of oil oxidation. In Oxidation, oxygen reacts with the oil to form Hydro-peroxides. Hydro-peroxides are unstable and break down into Ketones. Ketones further oxidize to form Aldehydes and Carboxylic acid. Now Carboxylic acid attacks the pump metal parts as well as O rings, Oil seals etc. causing acid corrosion. That is not all. By products of oil oxidation in the presence of acid further react and combine to form larger molecular species which polymerize to form sludge.

 
Flow chart no. 1 - Degeneration of oil. 

Sludge makes oil very viscous and can cause –

·         Jamming and breakage of vanes in the Rotary Vacuum Pump.

·        Overheating and damage to the pump interiors in the Diffusion Pump.

What is even more damaging is that this process is like a nuclear chain reaction that keeps speeding up with time. With oxidation, oil thickens and this reduces lubrication, which in turn results in a higher temperature. At increased temperature the rate of oxidation further increases (Rate of oxidation doubles with every 10⁰C rise of temperature as per Arrhenius rate rule).  This damages costly vacuum pumps in a short time.

Flow chart no. 2 - Oil oxidation: chain cycle.

Oils therefore must be changed as per specified schedule.

Note: In this blog post I have explained the oxidation process of hydrocarbon/ mineral oils. In a different post, I shall explain how silicone oil degenerates.

                                                           

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Evaporation Boat: How it is made

GENERAL PROPERTIES

Evaporation Boats are ceramic components required for vacuum deposition of various types of metals e.g. aluminum, copper, silver etc. over various substrates.

ALFA – Evaporation Boat (Supervac Industries)

They are of two types –

A) - 2 Component Boat – This type of boat is made with Titanium Diboride (Conductor) and Boron Nitride (Insulator).

B) - 3 Component Boat – This is made with Titanium Diboride (Conductor), Boron Nitride (Insulator) and Aluminum Nitride (Insulator).

Two Component boat is longer lasting and of better quality. In three component boat, Aluminum Nitride is added to bring down the cost. But consequently, it is lower in life and quality as well. Hence, users prefer 2 component boats to 3 component ones.

Production of Evaporation Boats is very expensive, involving heavy, costly machinery, raw materials and running energy costs. Also it is protected by patent laws. Therefore, very few companies in the world are producing it.

THE PROCESS

There are 5 basic steps in the production of Evaporation Boats:

1.     Preparing the powders – First of all, the powder particles are brought down to the desired size through milling.

2.     Blending of the raw materials – The powders are mixed together in the desired ratio and blended thoroughly for the sake of uniformity in the raw material.

3.     Hot-pressing – The blended raw material is then hot-pressed at a very high temperature (More than 1500⁰C) with a pressure of more than 150 tons. This delivers a big block of ceramic.

 

4.     Machining – The hot-pressed ceramic block is then cut into boats with a diamond cutter. Each boat is ground for a smooth finish and dimensional accuracy.  A cavity is ground into each boat.  

5.     Quality control – Each and every boat is measured for resistivity. After final inspection, boats are packed for shipping.

ALFA Boats come in various sizes and resistivity.

Supervac Industries make Evaporation Boats known with the trade name – ALFA ONE - are made in various sizes and resistivity as per the customer’s requirement. ALFA ONE boats come with Supervac Industries’ stamp of world- class quality and reliability.



                                                                                                       Written by: Anshuman Punj.

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