12 Flexible Printed Circuit Boards
13.10 Solder Pastes for SMDS
Solder pastes, often called solder creams, are used in “reflow-soldering” of surface mount components, where the application of solder and the heat supply are separate steps in the fabrication process. A solder paste basically consists of solder in powder form and a flux with some additives necessary to produce the desired behaviour of the paste during or after its application.
Solder pastes are applied either by stencil or by screen-printing method. Pick-and-place throughput is an important issue in deciding the type of solder paste dispensing system. Erdmann (1991) brings out the stenciling technique including stencil development, stencil cleaning and printing etc. He points out that the demands of stencil are more rigid than that of ordinary SMT screen printing and stenciling, particularly for maintaining near perfect registration.
Large pick-and-place systems require volumes that only screen printers can provide. Pick-and-place throughput in the range of 1500 to 3000 compo- nents per hour, however, is ideal for today’s dis- pensing equipment, which can produce 16000 dots per hour in a typical production environment (Cavallaro and Marchitto, 1991).
A rotary positive displacement programmable pump as shown in Figure 13.23(a) can be effec- tively used for solder paste dispensing involving high speed application of a large number of very small dots. The pump is driven by a DC motor. An electro-magnetic clutch engages and disengages the Archimedian screw. Mounted above the screw is a bellows coupling that aligns the clutch and lead screw and reduces the impact of the Z-axis sensor by more than 60 per cent. The combination of con- stant motor speed, low air pressure, the software- controlled clutch, and the precise rotation of the Archimedian screw ensures a repeatability that is far superior to pulsed air or piston dispensing sys- tems (Cavallaro, 1994).
Mechanical foot Air feed to air cylinder Metal bellows
Electric clutch Electric
motor
Feed tube Linkage
Air cylinder 10-or 30-cc
syringe Constant air 10 –15 psi
Fig.13.23 (a) The dual-height rotary positive displacement pump, which can perform 25 and 50 mil dispensing within the same program. The pump is appropriate for SMD epoxy or solder paste
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The pump can be programmed in one-milliseconds (ms) increments from 10 to 10,000 ms. Most solder paste applications require a shot size of 15 to 20 ms for fine-pitch devices and 50 to 100 ms for 50-mil devices. This short cycle time allows the automated dispensing system to dispense up to 16,000 dots per hour, making it compatible with most pick-and-place machines in the marketplace.
The real advantage of dispensing over screen printing is programmability. For example, a 25-mil- pitch application, requires a dot diameter of 0.014 to 0.016 inch (0.35 to 0.40 mm) while a 50-mil- pitch application requires a 0.020 to 0.030 inch (0.50 to 0.75 mm) diameter dot. Dual-height dispensing systems are important to accommodate different dispensing requirements without the need to change either the needle or the programme. Dispensing systems are particularly effective for re-work and mixed technology applications.
13.10.1 Requirements of Solder Pastes Solder pastes must meet the following requirements:
a The individual powder particles of the solder alloy should have a homogeneous distribution of the metal within the paste as well as a fair equality of shape and surface roughness of the individual powder particles.
a It should develop an adhesive action in order to hold the components in place until the reflow operation has been finished.
a It must not tend to solder balling; if they become detached on the substrate, they may cause short circuits.
a It has to maintain its shape during curing and reflow and must remain on the pads, not leaking to unsolderable parts of the PCB.
a It must have sufficient activity as the solder paste is sometimes in contact with the parts for several hours.
13.10.2 Composition of Solder Pastes
Solder pastes are available with several fluxes and alloy compositions. Their consistency may vary from liquid cream to thick paste. Solder pastes usually contain a certain percentage of silver, most commonly 2 per cent, that gives a solder alloy of 62 % tin (Sn) + 36 % lead (Pb) + 2 % of Silver (Ag). This alloy has a melting temperature of 179 °C. The powder must contain granules of the alloy and not of the individual metals, which make up the solder. This will help to melt the solder at the temperature of the alloy used. Beside homogeneity, an important element in solder powder is the structure and shape of the alloy particles. Therefore, microscopic control of the solder particles is essential. An acceptable solder paste should contain alloy-particles only with the shape of a “sphere”, an “ellipsoid” a “tear” or a “dog bone”. Chilton and Gaugler (1990) describe the design of solder paste to meet the exacting requirements of fine pitch reflow.
A spherical shape minimizes the surface area and so reduces oxidation at best. However, on melting, the liquid flux flows outwards, also carrying with it the solder particles. The particles with a dog bone shape or a shape of an ellipsoid lock the solder better in place. Solder pastes having particles with extended irregular shapes, shapes of needles or even showing dust are not acceptable.
The solder particles are typically 20 mm to 80 mm in diameter. For screen printing, usually a smaller particle size is advisable.
Solder pastes can be manufactured either with rosin based or water soluble fluxes. The paste contains not only alloy powder and flux, but also organic solvents, thickeners and lubricants to determine paste rheology. With screen or stencil printing of solder paste, rheology is critical to ensure excellent print definition.
With the development of lead-free soldering alloys, solder pastes without lead are also available.
For low temperature soldering, a composition of 42 per cent tin, 42 per cent lead, 14 per cent bismuth and 2 per cent silver is commonly used.
13.10.3 Solder Paste Application
The reflow soldering process starts mostly with the application of solder paste to the specific areas of the circuit board where the components will be attached. Solder paste printing is commonly applied by stencil or screen-printing. It is wiped across the stencil or screen with a squeegee, which pushes the paste through the openings, depositing it on the lands at the right places. With solder paste printing, the entire amount of solder is deposited on the PCB in one operational step. Solder paste application with dispenser is generally used for laboratory applications because of its flexibility.
With a dispenser, the solder paste can be applied not only in form of dots, but also in stripes, which may be more useful and accurate in certain cases.
13.10.4 Handling of Solder Paste
Solder paste ages and changes with temperature, humidity and light. The following precautions may be taken while handling solder paste, keeping in view the recommendations of the paste supplier;
a Store solder paste in a clean, cool, dry and dark location.
a Before opening refrigerated solder paste, keep it for 24 hours at room temperature to avoid water condensation.
a Do not mix old and new pastes since the fluxes and thinners will evaporate at different rates.
a Stir solder paste at least 30 seconds before applying to the stencil/screen
a Use only clean and inert tools (Such as made of stainless steel, Teflon, Polyethylene) a Apply to the stencil/screen the amount of paste just required for printing.
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13.10.5 Stencil Printing of Solder Paste
In stencil printing as well as in screen printing, the entire amount of solder is deposited on the PCB in one operational step. While the squeegee moves over the stencil (or screen), the solder paste is pressed through openings in the stencil (or screen) to the lands on the circuit board. It is important that the stencil (or screen) openings match precisely with the locations of all the land patterns.
Therefore, for each circuit, the appropriate stencil or screen must be produced. The important parameters for solder paste printing are homogeneity of speed, squeegee speed, pressure, squeegee angle, snap-off distance and speed of board separation.
The solder paste must be thixotropic. It means, its viscosity should drop during the application process. Thixotropic pastes have an internal structure which breaks down when they are subjected to mechanical action and recover when the shearing force is removed. This property ensures that the paste will flow onto the board properly. The advantages of stencils versus screens are a considerably longer lifetime, higher and controlled paste depth as well as higher accuracy because of less snap- off distance. The snap-off is the distance between the screen/stencil and the surface of the board.
Stencil is usually made out of nickel plated metal or stainless steel. The land patterns in the stencil are mostly performed by means of laser-cutting (but it can also be done by chemically etching from both sides). The stencil is glued with an epoxy onto a sturdy cast aluminum or stainless steel frame which attaches to the screen printer. The stencil has to be accurately adjusted to be in precise alignment with the circuit board. Depending on the stencil thickness and of the hardness of the squeegee, a much higher wet layer thickness of solder paste can be obtained in comparison with screen printing by maintaining an excellent edge definition. The surface tension (adhesion) between the paste and the laminated board ensures that when the squeegee has passed over and the stencil/
screen has separated from the board, the paste remains on the board.
Board Fixture: The board fixture which holds the board during printing is mostly provided by means of a vacuum plate and is situated underneath the stencil. The purpose of the vacuum plate is to provide and keep a plane support to the board during the printing operation. If there are parts on the underside of the board, space or standoffs are placed in specified locations on the vacuum plate so that these components will be protected. The board fixture and the stencil will then have a proper alignment by using alignment marks (so called fiducials) on the board and on the stencil.
Squeegee: The squeegee may be made of thin metal. Setting the right and homogeneous pressure is done by putting paper underneath so that one can evaluate proper adjustment done by showing an even contribution of the solder paste. Best approach is to start always with too little pressure rather than with too much, because it may damage the stencil. The solder paste should always roll ahead of the squeegee during application and there should not be a film of solder paste left on the stencil. A film of solder paste left on the stencil indicates too low pressure of the squeegee. The diameter of the roll, rolling ahead of the squeegee, should be approximately 15 mm.
Polyurethane squeegees come as trailing edge or diamond-section (Figure 13.23b) and in several hardnesses. In all cases, the squeegee needs to present a sharp edge, and this is subject to wear and should be redressed periodically.
Trailing edge squeegee Diamond point squeegee
(b)
Fig. 13.23 (b) Trailing edge and diamond point squeegees (after Judd and Brindley, 1992)
Various references describe the best contact angles as being anywhere between 45-80°. In general greater print angles give poorer paste transfer through the stencil, while much shallower angles give degraded definition.
As the solder paste is being deposited, the stencil lifts immediately behind the squeegee (snaps off) and returns to its original snap off position, otherwise the stencil would smear the solder paste between the lands.
Fleck (1994) explains that using the laser cutting process to manufacture stencils provides more control over the amount of solder cream deposited on the pads of a printed circuit board. This control becomes more important when attempting to place fine pitch components using no-clean solder cream. There is no cleaning step to remove potential solder balls, making it necessary to control the printing deposit.
In the case of fine pitch printing with its characteristic narrow apertures, metal squeegees are the only practical solution. Their robustness overcomes the wear problem and they do not deform into apertures and scoop paste. The main danger comes from the tendency to ramp up the pressure which can lead to damaged stencils.
13.10.6 Screen Printing of Solder Paste
The advantages of screens over stencils are the lower cost and that they allow printing over a much large area. Disadvantages are limitations in accuracy and coating-thickness. In screen printing process, the solder paste is rolled over a mesh, which has been coated with an emulsion that closes the meshes of the screen where the solder paste is not required on the board. The screen, which may be of stainless steel, metallized polyester, polyester or nylon, is stretched across a strong metal (mostly aluminium) frame. This frame is held in the upper frame of a printing machine..
The action of the squeegee drives the solder paste through the open holes in the mesh, pressing the screen down so that the paste makes contact with the board. After the squeegee has passed, the screen springs back clear off the board, leaving on the board the paste that had been in the openings of the screen. The discrete particles of the solder paste flow together to yield a uniform coverage.
Screen Printing needs highly skilled and trained operators in order to get an accuracy of < 0.15 mm shifting for solder paste placement.
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Screen Fabrics Classification: Beside the selection of fabric-material, the classification is made according to the mesh counts of the screen that means the number of threads (or openings) it has per linear cm. For example a “55-T” screen printing fabric has 55 threads per linear cm ( or 140 threads per linear inch) in both directions. The materials used are:
a Stainless steel provides a long life excellent registration definition, good paste flow, prevents the build-up of static charge and needs the lowest snap-off distance.
a Metallized Polyester having a high resistance to abrasion, permits a good paste flow and the metallic surface prevents the build-up of static charge.
a Polyester has a better elasticity than stainless steel and is cheaper. The higher elasticity can become necessary if the bare board is unevenly soldered.
a Nylon having an excellent elasticity
The mesh used should have approximately 30 threads per cm and the opening between the threads should be minimum 3 times the diameter of the largest solder particle within the paste. Table 13.3 shows some selected fabrics for printing of solder paste.
Table 13.3 Some Selected Fabrics for Printing of Solder Paste (Courtesy Braun,2003)
Fabric number Material Mesh- Thread Theoretical Paste
cm inch openingmmmmmm -diameter (mmmmmm) thickness (mmmmmm)
43 110 Stainless steel 160 71 82
67 170 Stainless steel 100 50 49
24T 60T Polyester/metal 285 120 105
32T 82T Polyester/metal 195 105 70
36T 92T Polyester/metal 165 95 60
43T 110T Polyester/metal 144 84 55
20HD 51HD Polyester 300 200 144
24T 60T Polyester 275 150 111
27T 68T Polyester 250 120 65
34HD 85HD Polyester 175 125 73
40 HD 100HD Polyester 149 100 71
43 110 Nylon 128 88 51
T-normal grade; HD-heavy grade;
Frames: The necessary mesh tension for precision printing can only be obtained when strong metal frames are used. Aluminium is the most used metal for screen printing frames. Steel frames
are seldom used, as they rust and are nearly three times heavier than aluminium frames of the same size. However, one has to consider the fact that the thermal linear expansion coefficient of aluminium is twice as high as that of steel.
For example: Steel : 0.06 mm expansion/meter at 5 °C temperature rise Aluminium : 0.13 mm expansion/meter at 5 °C temperature rise
The frame size should be so large that the distance between the outer edge of the image and the inner edge of the frame is on all four sides at least 150 mm. Difficulties in adjustment arise if the distance is too small, since the distortion increases with the snap-off distance as well as with decreasing the distance of the squeegee to the frame.
Snap-off and Lift-off: The “snap-off ” is the distance between the screen/stencil and the surface of the board. High fabric tension allows a lower snap-off. The additional lifting of the frame at the edge where the squeegee started print is called “lift off”. This lifting is provided by advanced printing machines simultaneously with the movement of the squeegee in order to ensure the same snap-off by increased distance. So, the angle between board and screen will be equally maintained by moving the squeegee over the screen. In general, the snap-off should be set as low as possible. With high fabric tension, one can have a low snap-off.
Stretching of Fabrics: Screen printing fabrics are stretched either pneumatically or mechanically.
The loss of tension of a newly stretched fabric is usually 10 to 20 per cent within the first hours and is dependent on the type of fabric, the stretching equipment, the stability of the frame and also the rest period before gluing the fabric onto the frame. Therefore, it is recommended that for precision work, the stretched screens are left to rest for 12 hours before stencil making. The tension resistance of a fabric depends on the material used as well as by the thread diameter. The tensile strength of the thread rises by the square of the increase in thread diameter.
De-greasing: It is always advisable to degrease the stencil before every use. That can be done by commercial screen printing degreasing agents or with the help of 20 per cent caustic soda. But after rinsing, it is necessary to neutralize it with 5 per cent acetic acid.
Printing Speed: It is determined by thixotropy of solder paste and typically between 20 to 80 mm per second. It is usually recommended by the manufacturer of the paste. The more fluid the paste is when moved and rolled, the higher print speed can be achieved. The print start has to be at least 90 mm before the aperture pattern is reached so that the paste can roll nearly twice in order to get its thixotropic behaviour.
The mesh count in a screen refers to the number of openings or lines per linear inch (lpi); for printing solder pastes the mesh count is in the range 60-200. Typically maximum particle size should be no larger than one-third the mesh opening to prevent jamming. For example, an 80-mesh screen has openings of about 224 mm so the particle size should not exceed 75 mm. Finer mesh screens tend to be used for thinner deposits. A 180 mesh and a fine powder paste could be used to produce a deposit thickness of 100-150 mm, using an 80 mesh. Hall (1994) brings out the screen printer
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requirements for low defect process capability, while Noble and Moore (1992) illustrate methods for determining the accuracy of screen printing machines.
13.10.7 Pre-forms of Solder
A pre-form is an appropriately shaped layer of solder (or of solder paste), which contains the amount of solder required to make the joint. It is placed between the parts to be joined and then melted. Pre- forms provide solder in a carefully controlled shape as well as a controlled volume of solder alloy.
Typically, pre-forms are punched from a strip of solder alloy. Some pre-forms have flux as an integral part. In the case of flux containing pre-forms, it allows complete control over the placement of the solder alloy, which ensures that the joint is formed in areas specified by the designer and nowhere else. Usually, pre-forms may be applied when the circuit assembly is not plane or because of other reason that does not allow the application of the printing techniques.
13.10.8 No-clean Solder Paste
No-clean soldering processes are becoming the choice of many printed circuit board assemblers (Bauer, 1994). No-clean solder pastes are particularly important in the process. They can be divided into two main categories: Standard no-clean and low-residue no-clean.
Standard No-clean Paste: standard no-clean pastes are typically rosin-based, and have solid content of 35 to 50 per cent in the flux form and 3.5 to 5.0 per cent in the paste form. Most of these pastes do not require a special atmosphere such as nitrogen, because reflow and joint quality is typically good to excellent. Wetting may be an issue with no-clean pastes, but this is often traced to component or board solderability. Excessive residue quantities can interfere with testing.
Low-residue No-clean Paste: low-residue no-clean pastes can be rosin or synthetic-based. Typically, the ultra-low residue pastes use synthetic non-rosin-based ingredients. The main advantage of the low-residue pastes is the reduced residue levels. Low-residue solder pastes usually require an inert atmosphere such as nitrogen.