Making PCB manufacture more efficient

John Cunningham, Chief Chemist, Rainbow Technology Systems, Glasgow, Scotland

Rainbow_Process_UnitThis article looks at a new method of PCB processing which offers a genuine alternative to Laser Direct Imaging and is particularly adept at fine line printing under 20 microns.The fundamental principles of PCB manufacture have changed little since the invention of the process over 50 years ago. Now manufacturers are under increasing pressure to make PCBs more efficiently, increase yields and ensure high quality standards are achieved.

The fundamental principles of PCB manufacture have changed little since the invention of the process over 50 years ago. Now manufacturers are under increasing pressure to make PCBs more efficiently, increase yields and ensure high quality standards are achieved.

Current PCB production technology requires expensive clean rooms, and the equipment takes up considerable space. The process also consumes substantial amounts of energy and releases large amounts of solvent, which need scrubbed, that in turn produces large quantities of CO2. The process begins by either laminating the copper boards with dry film or roller coating with a solvent-based coating, which is then passed through a hot air drying oven at 375˚ F for 8-10 minutes or an infrared oven for 3-4 minutes. The panels are then covered with a protective Mylar coating and stored/transported in yellow light to the imaging unit. Imaging is usually an off-contact process using collimated light which can lead to occlusions and which currently can only achieve definition down to 50 microns. The imaged panel is then transported again in yellow light to the developing station where the 20 to 25 microns of film or coating spend 1-2 minutes being developed before storage or forwarding to the etching line.

For fine line imaging, the most common methods used are laser direct imaging (LDI) or digital mirror devices (DMD). Both technologies are simply imaging units that require panels preprocessed with a dry film laminate or a wet solvent based photoimageable resist. The panels need to be kept in yellow light conditions to prevent premature curing with UV light from the atmosphere. Like the older processing method,the laminating or coating lines take up considerable space, and both processes use a protective layer of Mylar to prevent stray debris forming occlusions on the substrate. Floor space becomes a premium commodity as both the laminating and wet coating processes take up considerable room. Both processes also consume large amounts of energy, as ovens from 20 kW to 70 kW are needed to remove the solvent carrying the chemistry. Laminators can also use up to 20 kW of energy bonding the dry film to the substrate and the heat generated by these processes can also lead to distortion of the panels. UV light sources for the imaging process can also use up to 20 kW of energy.

As the light source is off-contact, it has to be perfectly collimated to get the feature sizes required; however it does mean that any debris inclusions are free in the film and can create ‘holes’ in the image, which can lead to failures.

LDI and DMD technology can now image features down to 25 microns and are moving ever smaller, but there are issues with speed of production and cost of equipment. However, the key drivers are 1) increasing demand for ever-smaller features and 2) the need for higher volume production and greater yields. At the same time, the industry is under pressure to be greener by removing both solvent and the need to dispose of large quantities of Mylar in landfill sites.

Six years ago, we embarked on a journey to develop a system which would address the issues with the current technology and make the process easier, more cost-effective and greener. The result is the Rainbow Processing Unit—an automated PCB processing device that incorporates coating, imaging and development of the board. The unit is ideal for high volume production as it is capable of processing up to 200 double-sided boards per hour compared to 80 single-sided panels per hour using a LDI system. Total elapsed time is only one minute from beginning to end for producing a doubled-sided panel.

Central to the process is the use of a special wet resist developed by Rainbow that offers several benefits over traditional dry film. The resist is applied wet to the panel surface, and this means that the resist can be squeezed into all the contours of the copper surface. It is not possible to achieve this level of close contact with dry film as it tends to delaminate, particularly when printing finer detail. As the coating is 100% solids and solvent free, it does not require pre-drying before imaging, which dispenses with the need for a hot air or infrared ovens. Resist is cured using UV light at the moment of exposure—any resist not exposed remains in its liquid state and is washed off.

The unit is especially well suited to printing fine line detail as it can produce tracks features under 20 microns. As designers of equipment such as smartphones push to incorporate more functionality into ever-smaller devices we see this facility becoming increasingly important. At present traditional PCB manufacturing equipment offers minimum tracks and gaps of around 50 microns. Below this LDI equipment is used but there can be yield issues for tracks and gaps below 50 microns.

Key features of images produced using the Rainbow Process are their clean lines, perfect circles and absence of “stair casing” and “scalloping,” which frequently occur with inkjet images. With laser systems, collimation can occur, which highlights any trapped particles, which can lead to the board being scrapped. With the Rainbow process, collimation is only six degree half angle, and any particles are trapped in the cured resist. In addition, as there is only a nine second gap between the coating and imaging stages, and this operation is carried out in a vertical position there is very little opportunity for any debris to attach itself to the wet ink. By contrast, in a conventional system the panel spends several minutes in a horizontal position in a standard hot air oven drying system.

Today the electronics sector is very conscious of its environmental obligations and the need to save energy and wherever possible ensure that processes are green. The Rainbow unit has very low power consumption (under 3 kW) as it uses low power LEDs for exposing the film and does not require a drying oven.

The unit itself takes up very little production space, occupying under 12 sq m, and does not require a clean room environment as it has its own enclosure, which is kept at cleanroom air quality standards by HEPA filter units.

Although primarily designed for PCB production, we foresee other applications for the Rainbow Process. One of these is touchscreen display production. Fine conductive tracks, invisible to the human eye, can be printed on to a clear substrate. Grid patterns of five micron by 300-micron pitch offer more conductivity than ITO or conductive polymers. The process can be done additively using an imaged seed layer to create electroless plated nickel or copper with a Rainbow imaged plating resist to create tracks. Alternatively the process can be done subtractively using sputtered metal on plastic using a Rainbow imaged etch resist to create tracks. The key benefit will be the ability to produce much closer tracks and gaps (down to 10 micron spacing) and this will enable designers to introduce more features and functionality to wide screens.

The technology behind the Rainbow Process is the brainchild of chief executive and founder Jonathan Kennett who set up Rainbow Technology Systems in 2005 to develop fine line printing technologies for the PCB market. Jonathan pioneered the development of contact cleaning technology technology for the electronics and high-tech sectors 25 years ago when he founded Teknek which has become the global leader in contact cleaning and yield improvement equipment. Other founder members of the senior management team include John Cunningham (chief chemist) and Robert Gibson (chief mechanical designer).

Rainbow Process stages

1. Copper panel is fed by hand or automatically into the Rainbow Unit in a horizontal position.
2. Any contamination is removed from the board using a Teknek contact cleaning unit.
3. Panel is moved into a vertical position.
4. A 5 micron layer of the propri- etary wet resist is applied to the panel using Rainbow’s own  roller coater with specially machined rollers which ensure that the opti- mal amount of coating  fluid is applied to the board.
5. The panel is imaged in the LEP (laminate, expose and peel) unit using standard photo tools and  LEDs to provide UV light. The panel is dropped between the photo tools and brought together as a  “package,” which is then lami- nated, exposed to UV light and the photo tools are “peeled” off, ready to process the next panel.
6. Panel is developed using a stan- dard 1% sodium or potassium carbonate solution.
7. Panel is rinsed to remove any resist, which has not been cured.
8. Panel is moved back to a horizon- tal position and unloaded.
9. Panel comes out ready for etch- ing—there is no  need  to  store or further process them under yellow light

In conclusion the Rainbow Process offers a genuine alternative to LDI for PCB processing which is more efficient, more cost-effective, greener and ultimately more profitable for the electronics sector.

For further information please visit www.rainbow-technology.com or call +44(0)141 892 3320.

John_CunninghamJohn Cunningham is chief chemist at Rainbow Technology Systems Ltd. John has over 25 year’s experience as a Senior Manager and development chemist covering all the graphic art processes including ink jet and UV curable coatings. After studying his Chemistry degree at The University of Glasgow John spent 5 years teaching before entering the coatings industry with ICI Packaging Inks and Coatings in Hull where within 18 months he became Technical Manager. He was responsible for overseeing the development of new ranges of UV coatings for printing on paper plastics and metal decorating and looking after technical service to customers. He started his own business Printology in 1992 where he designed training course for UV formulating chemistry and carried out consultancy work solving UV projects around the world. Prior to coming to Rainbow John was Technical Director at IGM Resins travelling round the world solving technical problems and helping build the company as it grew to be the largest photoinitiator supplier in the world. He joined Rainbow in 2005 as Chief Chemist where he has developed the chemistries required for imaging within the Rainbow Process.
Current PCB production technology requires expensive clean rooms, and the equipment takes up considerable space. The process also consumes substantial amounts
of energy and releases large amounts of solvent, which need scrubbed, that in turn produces large quantities of CO2. The process begins by either laminating the copper
boards with dry film or roller coating with a solvent-based coating, which is then passed through a hot air drying oven at 375˚ F for 8-10 minutes or an infrared oven

for 3-4 minutes. The panels are then covered with a protective Mylar coating and stored/transported in yellow light to the imaging unit. Imaging is usually an off-

contact process using collimated light which can lead to occlusions and which currently can only achieve definition down to 50 microns. The imaged panel is then

transported again in yellow light to the developing station where the 20 to 25 microns of film or coating spend 1-2 minutes being developed before storage or

forwarding to the etching line.

For fine line imaging, the most common methods used are laser direct imaging (LDI) or digital mirror devices (DMD). Both technologies are simply imaging units that

require panels preprocessed with a dry film laminate or a wet solvent based photoimageable resist. The panels need to be kept in yellow light conditions to prevent

premature curing with UV light from the atmosphere. Like the older processing method,the laminating or coating lines take up considerable space, and both processes use
a protective layer of Mylar to prevent stray debris forming occlusions on the substrate. Floor space becomes a premium commodity as both the laminating and wet coating
processes take up considerable room. Both processes also consume large amounts of energy, as ovens from 20 kW to 70 kW are needed to remove the solvent carrying the
chemistry. Laminators can also use up to 20 kW of energy bonding the dry film to the substrate and the heat generated by these processes can also lead to distortion of

the panels. UV light sources for the imaging process can also use up to 20 kW of energy.

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