Develop Outerlayers
Aim of Process
To chemically remove all the unexposed ( monomers ) resist and leave the circuit image, with a good side wall profile.
Process
The mylar protective sheet is then removed ( this is here to help with the lamination of the resist, to provide damage to the resist, and provide oxidation of the resist. ) The cores are then put onto a horizontal conveyorised processor, which will
  • Spray a heated ( 28 degrees C ) solution of Sodium or Potassium Carbonate ( 1% ) at the core developing the image within 100% of the first chamber, leaving the other developing chamber for providing vertical side walls.
  • The core is then transported onto the next chambers, which are 5 independent water rinses, which are sprayed at the dry film resist to ensure that all dry film and carbonate residues are fully rinsed off.
  • The core is then passed through a drying stage and then automatically unloaded for inspection.
    The BGA device now looks like the following
    Now we must inspect the circuit image alignment to the dsrilled pattern to ensure it passes IPC, at this point we need to check what Class the board is being inspected to whether Class 2 (default) or Class 3
    This is the same for Class 2 or Class 3, what changes is what is acceptable and what is not dependant on the release Class of the PCB, this is displayed on the front of the job card
    For IPC Class 2 it is as follows
    For Class 3 it is as follows
    The basic difference is for IPC Class 2 , we can accept 90 degree or less breakout, however for Class 3, there must be  aminimum of 0.002”(0.050mm) annular ring.
    Now for the theory………
    DEVELOPMENT
    Development is the removal of unexposed portions of the negative working resist. The development stage is critical as it determines the quality of the resist remaining on the surface in terms of track profile, adhesion, etc.
    As circuit density increases, the track width becomes smaller and more closely packed. As such, the development process becomes more important.
    On exposure the resist is polymerised and this alters the dissolution kinetics between the exposed and unexposed resist.
    The development mechanism is a diffusion controlled process, that is the developing solution penetrates the unexposed resist and partially removes the resist in the form of a colloid of binder polymer carboxylate salts. This layer must then be removed by mass transfer of developing solution on the resist surface and mechanical action by spray pressure before the next layer can be attacked.
    Eight key factors must be taken into consideration to achieve the correct development action, cleanliness of the developed surface and optimum track profile.
    · pH concentration of the developing solution
    · Resist loading within the developing solution
    · Temperature of the developing solution
    · Development Time (breakpoint): removal of the resist
    · Spray nozzles (type, volume of solution, spray impact)
    · Antifoam type and quantity
    · Filtration
    · Water rinsing (time and temperature)
    The above factors demonstrate how the mechanical aspects of the development machine and the chemical control of the process are extremely important. Equipment design is very important and often the dry film resist supplier has to use equipment that is already installed. However, the resist supplier must ensure the following points so that resist performance is not jeopardised.
    · The spray from the nozzles is effective and even over the entire board surface. Development is even on the upper and lower board surfaces.
    · The spay impact pressure is sufficiently high to remove unexposed resist from within fine lines and spaces and yet is not too high to break tents, particularly those over oblong holes.
    · There is no puddling on the upper surfaces of the panel.
    · If more than one chamber is incorporated in the development part of the machine, the space between the chambers should be clean and the conveyor mechanism should remain wet. This can be achieved by using “misting “nozzles.
    · Rinsing is an integral part of the development mechanism. Ideally, the rinsing time should be 50 % of the development time.
    · Drying reduces swelling of the resist that occurs in the development stage.
    If wet resist passes directly to a cupric chloride etching solution, increased organic contamination may occur in the etchant.
    The development mechanism is a diffusion controlled reaction, The sodium carbonate reacts with the carboxylic (acid) radicals in the unexposed resist and solubilises the resist. This  carbonate resist mixture must be removed with fresh sodium carbonate before the reaction may proceed.
    Dry film resist is not dissolved in the developing solution, but is held in suspension in a colloid form. Any mechanism by which this colloid is broken down can lead to scum formation on the panel surface. It may also appear as an “oily“ substance on the solution surface. These mechanisms include mechanical stress created by sheer forces in the pumps, spray nozzles, antifoams, dry film resist load in the development solution, etc.
    If the colloid is destroyed then a scum will deposit in the developing solution and an oily substance may be seen floating on the surface of the developing solution.
    pH Concentration
    The concentration of sodium carbonate used for development must be within the range specified for each dry film resist. If the concentration is too low the unexposed resist will not be completely removed and a scum remaining on the surface will lead to etching or electroplating problems. If the concentration is too high the resist will be attacked at the resist substrate interface leading to poor track profile, and in severe cases, lifting of the resist. This in turn will lead to electroplating deposits under the resist or track width reduction during
    etching.
    The dry film resist has acid radicals within its chemical structure. As the resist reacts with the sodium carbonate, the acid radical is neutralised and the pH of the developing solution will fall. If the pH of the developing solution falls below a specified pH, the developing mechanism will cease. Therefore, it is essential to replenish the developing solution by either pH control, conductivity, or area of resists developed etc.
    Resist Loading
    The quantity of dry film resist within the developing solution is defined as resist loading. As resist loading increases, the dissolution kinetics is changed and therefore the speed of development changes. There is also a greater tendency for the colloid resist particles to become unstable and precipitate back onto the panel. This leads to open circuits in the case of pattern plating and short circuits in the case of innerlayer production or circuits produced by the tent and etch technique.
    The specific recommendation for resist loading will depend on the type of
    circuit being produced. Circuits with lines and spaces greater than 150 microns a loading of 0,4 square metres of 40 micron thick resist per litre of developing solution is recommended. As the lines and spaces become smaller, the resist loading becomes correspondingly lower. For lines and spaces of 100 microns and lower, the resist loading should be less than 0,1 square metres per litre of developing solution. (See Appendix 1 for method of analysis for resist loading in developing solutions.)
    The dry film resist has acid radicals within the chemical structure. As the resist is dissolved in the sodium carbonate solution this acid radical is neutralised and the pH of the developing solution will fall. A 10-gpl Sodium Carbonate solution will have a pH of about 11,2.
    Temperature
    Most dry film resist specifications require that the temperature of the developing solution be controlled within close limits. Normally this is the optimum temperature plus or minus two degrees Celsius.
    Thermostatically controlled heaters are required to control the temperature. If the action of the pump mechanism generates heat the developing solution temperature will rise and could exceed that set on the heater thermostat. In this case cooling coils are also necessary to remove this excess heat. If persistent problems arise always check the actual temperature of the solution with a thermometer; never solely rely on the temperature indicated on the
    development machine.
    When the developing solution temperature is too high the speed of development is greater and will lead to over development unless the total development time is reduced. It will also give rise to a poor resist side wall profile. A high temperature may lead to breakdown of the colloid and give rise to scum on the panel surface. If the temperature of the developing solution is too low then under development can occur and leave resist residues on the panel surface and also give a larger “ foot “ to the resist profile.
    Development Time (Breakpoint)
    The minimum development time is the time taken for the resist to be removed from the copper surface. Actual time in seconds will depend on a number of factors including concentration and temperature of the developing solution, volume of solution being sprayed onto the surface, type of nozzle, height of the nozzle above the surface, etc.
    For PHOTEC dry film photoresists, a breakpoint is required when the resist is 50-60% developed through the total effective spray length of the developing machine. Note: The effective spray length may not be the overall length of the developing machine. It is essential to inspect the machine and check the actual length where the developing solution is sprayed onto the panel.
    If the breakpoint is less than 50% of the total effective spray length over development will occur. This in turn will lead to a poor resist sidewall profile and undercut at the resist to copper interface. This results in under plating in the case of pattern plating or over etching when innerlayers are produced or the tent and etch technique is used.
    If the breakpoint is greater than 60% then under development will occur.
    Under development will lead to track width reduction in the case of pattern plating and in severe cases will lead to plating inhibition or plating adhesion failures.
    In the case of etching innerlayers under development will lead to a track width increase. In severe cases this may also lead to organic contamination in the etching solution. Organic contamination in the etching solution will give small areas of copper not etched on the surface (copper spots).
    Spray Nozzles
    Development is a diffusion-controlled mechanism. This means that the developing solution must diffuse through the unexposed resist and through the interaction of the sodium ion in the sodium carbonate solution react with the carboxylic acid groups within the resist to form a colloidal particle. These colloidal particles are then washed from the resist surface by fresh developing solution. As the colloids are removed it leaves the surface in a condition for more sodium carbonate to diffuse into the resist surface. This mechanism is repeated until all the unexposed resist is removed.
    Because resist removal is a diffusion controlled mechanism, a large volume of low pressure sodium carbonate to reach the resist surface is required during the first stage of development. To achieve, spray nozzles are used in the first part of the development machine that spray a cone shaped pattern onto the surface (i.e. cone nozzles). These cone nozzles give a low pressure on the resist surface. It is recommended that each nozzle deliver in excess of 4 litres per minute of developing solution. (A typical cone shaped nozzle will have a spray impact of 0,4-0,5 Bar on the panel surface.)
    To ensure the complete removal of resist at the base of fine lines and spaces a higher pressure is required so that the developing solution will reach the base of resist at the resist to copper interface. In fluid dynamic terms this is a deep recess. A nozzle that delivers a fan shaped spray pattern is used for this purpose.
    The nozzle with a fan shaped spray is referred to as a high impact nozzle and will have a typical spray impact of 8-9 Bar on the panel surface. Equipment design is important to achieve the correct development of the resist across the entire panel. Nozzle height and the angle of the nozzle jet must be such that all the entire panel is sprayed at equal volume and pressure and that no overlapping of the sprayed solution occurs. Any overlapping of the sprays will reduce the effective pressure that impinges onto the surface. Prevention of development solution “puddling” on the top of the panel must be prevented, otherwise this will affect the replenishment of fresh solution on the resist surface.
    To even out the differences in solution replenishment on the top and lower panel surfaces different spray pressures are used between top and bottom.
    Normally a 0,2 – 0,3 Bar higher pressure on the top spray bar is used. Typical spray pressures for development are Top 1,5 Bar and Lower 1,3 Bar.
    However, the exact spray pressure is equipment related and should be arrived at experimentally.
    Antifoam
    The type and quantity of any antifoam used in the development solution will determine the stability of the colloid formed. If the incorrect antifoam is used precipitation within the development solution will occur. This may lead to scum formation on the panel surface that will interfere with the subsequent plating
    or etching operations. To eliminate these problems, an antifoam that is a polyethylene oxide or polypropylene oxide block copolymer based should be used.
    Siloxane based antifoams are known to cause instability of the colloids in the development solution and give rise to scum or sludge. Silicone based antifoams should not be used as they may provide problems in subsequent processing operations such as electroplating. Trials should be made with all new antifoams to check for colloid stability prior to use on production.
    The concentration of antifoam used should be minimised to limit the foam build-up in the development solution. Actual concentration used should be within the range recommended by the supplier.
    Filtration
    To prevent the possibility of resists particles or large colloidal particles being re-deposited back onto the panel it is essential to filter the solution. The filter should be in line between the pump and the spray bar manifold and equipped with filter to remove particles greater than 30 microns. The filters should be changed regularly to prevent loss of developer solution volume being supplied to the spray nozzles. Pressure indicators should be fitted into the spray manifold after the filters; this will give a more accurate indication of the spray pressure.
    Water Rinsing
    Water rinsing after development is an integral part of the process. The type, volume and temperature of the water are important to ensure dry film resist performance. Water with a hardness of 8 – 12 on the DIN scale should be used. (See Appendix 4 for water hardness conversion.) The reason for using slightly hard water for rinsing is that the divalent cautions. Calcium and magnesium converts the soluble sodium form of the polymers present on the resist sidewalls after development into less soluble carboxylate salts. This effectively stops further development while improving both resolution and resist sidewall profile.
    Soft water or softened water is not an effective rinsing medium. The pH of soft water is easily affected by drag-in of developer solution and in severe circumstances may continue the development action.
    The pH of the rinse water should be less than pH 9,0 and the temperature of this rinse water should be less than 30 0C to stop development action and prevent attack on the resist surface.
    Some of the detail of this was provided by Enthone.