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Beiträge vom October, 2011

The Laminating Resin

Monday, 31. October 2011 2:47

Some of the key properties of a material that will determine which laminating resin is chosen are:
 
       operating temperature
 
       coefficient of thermal expansion
 
       dielectric constant
 
       water absorption
 
       thermal conductivity
 
       flexibility
 
For the first part of this unit, we are deliberately looking at just two resins, but these are different in performance as well as chemical structure. Key differences between the two are their maximum continuous working temperature and insulation resistance.
 
For a laminate the maximum continuous working temperature is around the value of the glass transition temperature of the impregnation resin. For FR-2, this is typically 105°C; for FR-4, in the range 130–140°C. Whilst these temperatures are not the maximum temperatures which the laminate will survive in the short-term, of these two laminates only FR-4 will withstand reflow soldering conditions for one/two cycles, and even FR-4 would degrade after repeated exposure.
 
Phenolics also have a low insulation resistance. Coupled with their greater water absorption and sensitivity of electrical properties to humid environments than epoxies, the use of phenolics has generally been restricted to lower-cost paper-based applications.

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Punched or Drilled

Monday, 31. October 2011 2:46

Whilst most of our attention in this module is on materials such as FR-4, intended for professional applications, in fact over half of the area of laminate sold world-wide is for materials which can be punched, most of which are still paper-based. Punching allows all the holes, slots and other shapes to be produced in one or two press strokes, using a tool with multiple punches (‘perforators’). The process is similar to metal stamping, using punches and matching die openings: every perforating punch must have a hole below it so that the pierced slug can pass through and be collected as waste. With printed circuit boards, however, much tighter clearance is needed between perforator and die opening in order to produce a clean hole that is free of fibres.
 
Again as with metal stamping, you will come across three different die types:
 
       ‘Pierce’ dies, which create just holes and slots
 
       ‘Compound blank and pierce’ dies which both pierce and blank a part in a single operation
 
       Progressive dies, where several stages of piercing are completed by a final stage which blanks the part.
 
A critical factor in producing punched parts is the alignment of the perforator with the die blocks: misalignment results in punch breakage. Punch and die sets are expensive, and their detailed design and clearances depends on the exact properties of the laminate being punched.

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Basic Board Materials

Friday, 28. October 2011 1:35

 We are considering the main materials of which a printed circuit board is made. Whole books have been written about the choices available, but to give too much information immediately would confuse! We are therefore restricting our focus to the main laminates that you will encounter in commercial and professional electronics, two materials which share an FR- designation, in that they are Fire Retardant:

FR-4laminates are constructed on multiple plies of epoxy-resin impregnated woven glass cloth. FR-4 is the most widely used material in the printed circuit board industry because its properties satisfy the electrical and mechanical and thermal needs of most applications, and its performance can also be adequate for high-technology requirements. FR-4 is used in aerospace, communications, computers and peripherals, industrial controls, and automotive applications. Actually translucent, FR-4 is normally thought of as green in colour, the colour coming from the solder mask on the finished board.

FR-2 laminates are composed of multiple plies of cellulose (‘Kraft’) paper that have been impregnated with a flame-retardant phenolic resin. FR-2 laminate is less expensive than FR-4, and the cost difference becomes even greater for the finished board, because holes and profile can be created by punching. FR-2 is typically used in applications where tight dimensional stability is not required, such as in radios, calculators, toys, and television games. FR-2 is an opaque brown in colour.

For both materials, we will be looking at a conductive foil of copper: virtually every circuit board uses this material, although there will be differences in the final surface finish. 

 

The board consists of resin, reinforcement, copper foil, and of course a lamination process. Although broadly similar to the manufacture of multilayer boards, the production of base laminate is generally carried out by specialists, who supply board fabrication houses with process blanks. ‘Rolling your own’ laminate is a possibility, but one that few will consider – there are already quite enough variables in the process!

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Increasing Demands on LPISM

Friday, 28. October 2011 1:25

 Reduced feature size

      50–75µm solder-dams are a common requirement as fine pitch (0.28–0.48mm), ultra-fine pitch (<0.28 mm) and BGA technology increases.

       High density circuitry puts greater demand on the accuracy of PCB imaging processes.

       By reducing the width of solder dams, PCB fabricators are able to compensate for the registration challenges of existing surface mount features.

Reduced hole diameter

       Requirement for reduced diameter (<0.2 and 0.3mm) via holes to be washed clean of solder mask.

Aggressive surface finish chemistry

       Requirement for narrower pitch and flatter pads are driving plated surface finishes as alternatives to hot-air-solder-level in turn putting greater demands on LPISM chemical resistance and track-coverage.

       Electroless nickel – immersion gold (ENIG)

       Immersion tin

No-clean and organic flux assembly

       Flux composition offers less protection to the LPISM and assembly conditions put new demands on LPISM performance:

       Increased chemical resistance and track-edge coverage

       Reduced risk of solder balling through the use of extra matt solder masks.

Reduced cost

       Constant demand for PCB cost reductions mean that solder mask processes must address technological needs as well as maintain sustainable operating costs.

These demands can be used to compile a range of key success factors for a LPISM and its application.

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Air Bubble Inclusion

Thursday, 27. October 2011 1:36

Process control can yield minimal undercut but if bubbles are present in the base of coating they can increase the risk of solder-dam adhesion loss during developing or hot-air-solder-level. The low risk of air-inclusion offered by the spray systems offers a distinct capability advantage when imaging solder-dams of 100µm and below. 

Table 6 shows the results of an experiment to investigate the practical differences in imaging of LPISM when applied by screen-print and air-spray. The tests were carried out using a purpose designed board with the following requirements:

       Maintain minimum 10 µm on track edges

       Wash clean 0.2 mm via holes

       Maintain 50 µm solder dams

Table 6 : LPISM imaging characteristics – screen print and air spray  
  Green matt High resolution Dark green matt High resolution  
Screen print Air spray  
Developing speed:
clean 0.2mm holes
90s dwell 90s dwell 30s dwell 30s dwell  
Note: Holes were plugged after printing  
Exposure energy:
hold 50µm dams
822 mJcm−2 583 mJcm−2 270 mJcm−2 134 mJcm−2  

From the above results it is evident that benefits attributed to reduced ink deposit in holes and between pads allow significant improvements in capability and, in turn, productivity.

Transfer-efficiency refers to the percentage of material leaving the sump that is actually deposited on the PCB.

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Solder Mask in Holes

Thursday, 27. October 2011 1:33

As pointed out in Table 4, differing amounts of LPISM are deposited in the holes by the four application methods, requiring different developer dwell times to ensure they are washed clean.
 
The increased undercut caused by longer developer dwell times adversely affects resolution capability and although this can be compensated for by increased exposure energy it leads to reduced productivity.
 
Despite elaborate methods to prevent the alignment of holes on successive prints or scraping excessive LPISM from the back-side of the mesh, screen-printing tends to deposit large amounts of ink into both component and via holes. Unlike curtain-coating which tends to deposit ink in larger tooling holes or slots, these smaller component or via holes are difficult to wash clean due to reduced impingement by the developing solution.
 
Table 5 demonstrates this difference and highlights the gain in line speed delivered by the spray applications.
Table 5 : Typical LPISM developing speeds
Typical developing speed
for 2m chamber
Screen print 1.3–2.0 m·min−1
Curtain coat 2.0–2.5 m·min−1
Spray 3.5–4.0 m·min−1
 
These differences in line speed and capability become even more apparent when using very matt LPISM formulations which, because of their overall solubility, are prone to being stubborn to remove from via holes.

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Solder Mask Thickness

Wednesday, 26. October 2011 2:23

Table 3 and Table 4 identified the differences in thickness typically deposited by the alternative application methods. Ink deposit around tracks and pads is key to imaging capability.
 
The greater the solder mask thickness, the more difficult it is for the ultraviolet light to penetrate to the base of the coating. leading to a cure differential between the top of the LPISM and the base and potentially excessive undercut.
 
Figure 2: Effect of LPISM thickness on light penetration

 

Greater thickness can result in insufficient polymerisation at the base of the coating manifesting itself as poor resolution capability.

To compensate for a higher LPISM thickness it is necessary to increase the exposure energy. This reduces productivity, impairs artwork stability and, unless the light source is highly collimated, causes image growth at the LPISM surface.
 
Figure 4: Diagrammatic view of undercut & image growth

Diagrammatic view of undercut & image growth

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Drying Stage

Wednesday, 26. October 2011 2:19

The objective of the drying stage is to remove the solvent from the coating with minimal activation of the curing reaction. Excessive activation of the cross-linking process will cause reduced hold-time capability and ultimately solder mask residues from poor developing. 

The main differences between the various application methods that affect the drying process are:
 
a) Single or double-sided coating
 
b) Solder mask thickness
 
c) Solvent level
 
d) Solder mask in/around holes
 
e) Air bubble inclusion
Table 4 : Factors and characteristics affecting LPISM drying
  Screen–print Curtain coat Electrostatic spray Air spray
1- or 2-sided All the application methods discussed, with the exception of curtain coating and horizontal electrostatic spray, allow both sides to be coated and dried in a single stage (double-sided coating). This avoids the trade-off between drying the second side coated sufficiently to prevent artwork marking or increased undercut and not over-drying the first side coated.
Solder mask thickness Approx. 25–50µmm over most of the PCB.

Thickness between tracks can easily reach 60–80 m m depending on track height/gap.

Approx. 80–90µm over most of the PCB.

Coating speed means that thickness between tracks is lower than screen printed LPISM.

Approx. 50–75µm over most of the PCB.

Thickness between tracks is minimised. Certain copper areas are prone to thicker ink deposits caused by electrostatic effect.

Approx. 50–65µm over most of PCB.

Thickness between tracks minimised.

Solvent level Approx. 25% (w/w) solvent on PCB.

Equates to ≈16 g·mm−2 needing evaporation

Approx. 42% (w/w) solvent on PCB.

Equates to ≈46 g·mm−2 needing evaporation

Approx. 30% (w/w) solvent on PCB.

Equates to ≈28 g·mm−2 needing evaporation

Approx. 35% (w/w) solvent on PCB.

Equates to ≈28 g·mm−2 needing evaporation

Ink in/around holes Significant LPISM deposition in holes which must be dried to prevent artwork damage. Tends to lead to increased drying times depending on level of fill. Large tooling holes and slots tend to have heavy LPISM deposition. Under certain conditions thick ‘tear–drops’ may form on the underside of the PCB. Generally both spray methods leave the holes almost free of LPISM eliminating the need for extended drying times.

However, a thin deposit of LPISM does coat the barrel of the holes which is quick to dry and can be a source of residues.

Air bubble inclusion Mixing and shearing action during printing induces air into the LPISM.

Viscosity and thixotropy of SP materials slow the release of these bubbles.

Slow solvents and reduced thixotropy can be introduced to aid the release from between high tracks but this results in a trade–off with edge coverage.

Pumping and coating action induces air into the LPISM.

The low viscosity and thixotropy of CC materials allows quick release from between high tracks. Temperature ramp up must be controlled to prevent the LPISM surface ‘skinning–over’ and trapping bubbles in the coating.

Both electrostatic–spray and air–spray applications apply LPISM via a fine mist.
As a result, neither method is prone to air entrapment.

The low risk of air inclusion allows the use of faster temperature ramp–up during drying leading to reduced cycle–times.

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Coating: Track Encapsulation and Coverage

Tuesday, 25. October 2011 1:36

Spray solder masks’ rheological properties are optimised to prevent slumping from track-edges.
Skips can generally be overcome with slower print speeds and/or multiple print strokes. Once track/gaps fall to certain levels (typically 100–150µm depending on track height) full encapsulation becomes impractical. Skips tend not to be a problem unless viscosity is too high or the solder mask has poor atomising characteristics.
Both spray technologies allow encapsulation below 100 m m track/gap configurations.
 
Optimised rheological characteristics are key to achieving the best possible performance but the nature of some application methods imposes considerable formulation boundaries. This often results in a trade-off between encapsulation and coverage.
 
Curtain coat solder masks must be long flowing at low shear stresses to ensure that the materials flow between tracks during and immediately after coating. Apart from affecting edge coverage this also imposes restrictions regarding very matt formulations. This is because the overall level and oil absorption properties of the fillers used in matt solder masks tend to decrease material flow, particularly at low shear stresses.
 
Screen print processes involve much higher shear stresses and slower coating speeds which enable the use of more thixotropic materials.
 
The spray processes permits very high levels of thixotropy resulting in excellent track coverage.
 
Note that there are substantial differences in rheological characteristics between materials that have been optimised for a different application methods: under typical conditions, the viscosity of the three materials spans two decades. That is, spray process materials are two orders of magnitude higher in viscosity than those for curtain coating, whilst screen print materials are intermediate in their resistance to flow.

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Coating: Cosmetics and Uniformity

Tuesday, 25. October 2011 1:34

Providing parts are clean and undamaged, both screen-printing and curtain-coating systems generally apply a uniform coating to the board surface.
 
Multiple gun spray systems are generally more difficult to set up and have a tendency to form stripes across the PCB. Striping is caused by overlap or interference between adjacent guns and as such is not experienced with single-gun designs.
 
Electrostatic spray systems are prone to non-uniform coatings because the electrostatic effect preferentially attracts the LPISM to the copper areas. This poses two problems:
 
The copper coating characteristics will vary depending on circuitry layout, leading to certain part numbers requiring special voltage settings.
 
Areas of bare laminate coat with a thinner deposit, because they are ‘robbed’ of LPISM by larger areas of copper.
 
The above problems can be overcome by reducing the voltage potential or increasing the LPISM conductivity by use of alternative diluents. However, these simply reduce the electrostatic effect and in turn lower the transfer efficiency.
 
When spraying it is important that the LPISM has good atomisation characteristics. If difficult to atomise, the solder mask will form a mottled surface or require excessive atomisation pressure and/or dilution, leading to lower transfer efficiency and inferior track coverage.

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