Boats and spitting

What causes spitting from evaporation boats?

ANSWER

Generally spitting is associated with changes to the pool shape and size.  The boat temperature needs to be stable and this not only means the current and voltage need to be stable but also the wire feed rate needs to be stable.

It can be easy to find that the thickness monitors respond to thickness changes and change the wire feed and this correction may be too much and the thickness monitor then re-corrects for this change with a further change. This can appear as an almost continuously changing wire feed rate. This will mean that the molten pool will be changing size and shape. The oxide from the surface of the aluminium wire, as well as any impurities, will collect as a skin or crud on the surface of the molten pool and often will collect around the edges of the pool. This collection of material is what usually is ejected and seen as spits. As the pool size changes this material either covers a new area of the evaporation boat or is left stranded on a drying part of the boat and can be thrown off as a spit.  Thus maintaining stability is regarded as the key factor in lowering the number of spits, assuming all other things are the same.

There can be other factors that also contribute such as the age of the wire, as older wire may have more oxide on the surface, and the purity of the wire.

One of you questions refers to MOC of the boat.  I take this to mean method of control?  If so you need to check the capabilities of the machines and possibly their history. If the machines are of different ages they may not have equivalent capabilities and so the same control process may not be available. If the machines are nominally identical it may be that they were used by different teams for completely different products that had different requirements and so different control methods were adopted for the different products on the different machines.  This could account for the original differences and if they now produce the same product these methods of control may simply have been continued on because that is what the operators are most familiar with.

The performance and feed rates can be calculated using the equations that are given in the AIMCAL Metallizing Technical Reference 4th Edn. book.  This is available from AIMCAL and most companies that are members of AIMCAL have copies. However if you need on if you contact AIMCAL at www.aimcal.org they sell then at ~$25 each + postage and packing.

Comments

[Andrew Mitchell]

Boat spitting can also be minimized by ensuring that a suitable cleaning regime is implemented after every process cycle. When removing excess aluminium shale from the source and shutter areas care must be taken that pieces do not do not bridge the live infeeds to the evaporator boats or connect the live infeeds to earth. If care is not taken in this area continuous localised spitting can occur.

Secondly as already mentioned, boat temperature needs to be maintained across the width of the machine on all evaporator boats. Boats for different machines and process are sold against a hot resitivity specification ( µ Ohm cm or MOC)
The hot resitivity required to give the required evaporation temperature will vary depending on the hardware installed on the vacuum coater (transformers, thyristers etc).
It is important that when installing the evaporator boats across the width of the machine that all of the boats are grouped within a tight MOC range. This ensures that all of the boats will reach the required evaporation temperature using similar power setpoints. This is also essential for maintaining uniform boat temerature and control when running the process in automatic deposition control modes.
If the boat MOC is not closely matched, variable power setpoints are required for each boat position to obtain the same boat temperature, this can also create instabilty when attempting to run the process in automatic deposition which can create spitting as the aluminium pool varies.

Regular cleaning of the boat after each cycle also helps to reduce spitting. The impurities and oxide deposits from the aluminium wire that accumulate on the boat should be removed after every run to minimize spitting.
Care must be taken that full clean contact is provided at all times on the ends of the evaporator boats to the faces of the copper end clamps.

The wire must be purchased against a set specification.It is recommended that the lowest quality grade to be used should be
1080A. It is also important that the hardness of the wire is specified.If the wire is too hard,resonance and vibration can occur along the wire from the stepper motors. The vibration of the wire onto the boat can generate spits.
It is essential that the wire is degreased.
Vibration of the wire at higher wire feed rates when running a machine hard can also cause spitting. If this is the case,the wire feed gears can be upgraded to accomodate a thicker wire (eg 1.6mm changing to 2.4mm.The stepper motor speed can then be reduced to a point where vibration is reduced whilst still maintaining the same rate of aluminium onto the boat.

It is worthwhile checking the evaporator source hardware for differences.For example some transformers supplied have multiple voltage tappings that can be selected depending the output requirements. It is possible that some of the hardware has been set to different settings and that this in turn can lead to variation and evaporation problems such as spitting.

Finally it is worth asessing the performance of different boat suppliers and products. Denka produce a boat which is individually hot pressed. The benefit of this is that the MOC is tightly controlled which results in more uniform boat temperature for the same power setpoints.They also have a "zebra stripe" groove feature along the length of the boat that provides uniform wetting of the boat across the width and along the length which certainly minimizes the occurence of spitting.