This enabled the resist to act as a mask during the etching process which followed. The screen both applied the coating and also created the exact image of the tracks required on the wiring board.
In the 60’s and 70’s demand for PCBs grew exponentially and better ways of packing more components on to a board within less space.This led to double-sided boards and multilayered modules.The next step was to find a reliable method to connect boards side to side.Initially, this involved soldered wire links but eventually this gave way to the more reliable ‘plated through hole’ method which is prevalent method today for connecting one layer to another.
Wet Resists
Between the 70’s and early 2000’s track densities increased and circuits with more than 12 interconnected layers became commonplace. Track widths became increasingly smaller with 100 microns becoming achievable.
Traditionally, wet resists are used and still used as the defacto method of printing in the industry. However, the method is not perfect and has a number of disadvantages.
Wet resists are generally inexpensive but need pre curing before being printed and tend to be used in high volume production due to the capital cost of curing ovens. Handling also tends to be problematic as the wet resist cannot be touched. The partly cured resist can also attract dust particles due to its tacky surface. For these reasons, many in the industry are switching to the dry film method.
Basically, dry film is a partly dried wet resist sandwiched between two layers of Mylar. Upon application, one of the layers of mylar is removed on a purpose built laminator and the partly dried resist (dry to the touch) is laminated under heat and pressure on to the copper substrate.Despite dry film being more expensive than wet resist, the convenience of handling has made it the most popular material for imaging and etching PCBs.
Dry film manufacturing
The manufacture of dry films for PCB use is an exacting task and has to be carried out in strict cleanroom conditions using a purpose-built coating line.Such facilities have typically been dominated by a few large players such as DuPont, Morton Thiokol and Hitachi.
After coating, and part drying, the master rolls are cut to sizes required by PCB manufacturers. To maximise usage, the PCB firm must order the exact width needed for the panel size.If a different panel size is required the roll needs to be changed leading to down time and loss of production.In warmer climates the film has to be stored in a controlled storage facility which is an added cost.
The equipment used to laminate dry film (Cut Sheet Laminator) has become increasingly complex and is a significant cost to purchase and maintain.For example, CLS units can cost upwards of $200,000 and placement of the rubber laminating rollers after they become deformed can cost $600 every 6-8 weeks.
Another issue with dry film is that the PCB manufacturer has to specify the exact widths of film to match the size of the copper panels being used.Changing rolls of film for different widths of panel causes costly downtime.
However, perhaps one of the biggest drawbacks to thin dry film is the amount of adhesion to the copper panel.
The copper panel may appear flat to the naked eye but closer examination shows a rough surface with a dendritic (tree like) structure.Everything appears fine until the printed panel is in the etching bath.Etching chemistry can get underneath the dry film (micro fissures) and effectively eat through the thin copper creating an open circuit.Normally this is only discovered after etching and cannot be inspected for as there are no visible signs.
Over the past decade Rainbow has been working on chemistry and systems designed to overcome the issues with the wet film method.Rainbow’s proprietary liquid resist and coating system, launched at last year’s Productronica exhibition, addresses these issues giving PCB manufacturers complete control over handling and control of coating thickness.The liquid resist flows easily, adheres closely to the surface of the substrate and can be applied at ambient temperature.
Moreover, the resist does not require drying before it is imaged.
The panel can still be handled as the resist coating is protected by a layer of mylar.The resist is first applied to a reel of polyester and then laminated directly to the copper panel, leaving the protective polyester carrier in place over the resist transfer coating.
After lamination the panel is “singulated” separating it from the carrier web. The panel can then be exposed by any method, including DI/LDI.
Hybrid coating
To complement this new generation of resists, the company has now introduced its Hybrid Coating Unit (CL21 Hybrid Coating Laminator).The CL21 makes handling wet resists easy and puts the PCB manufacturer in complete control of the printing process.The resist is first applied to a reel of polyester and then laminated directly on to the copper panel, leaving the protective polyester sleeve in place over the resist-transfer coating.
After lamination the panel is “singulated” separating it from the carrier web. The singulated panel can then be exposed by any method, including DI/LDI.
The Coating thickness can be from 2-30 microns and thinner resists are ideal for fine line printing.Part numbers and thicknesses can be changed in as little as 2-3 minutes. The coating unit can be configured to coat panels, webs, single or double sided as both an etch or plating resist.
When the proprietary liquid resist is combined with the CL Coater, the aim is to save PCB manufacturers considerable amounts of time and money as well as improving the quality of the end product and reducing defects.
Author details: David Westwood, Sales and Marketing Manager, Rainbow Technology Systems.
