Beginning with chip capacitors and resistors on hybrid substrates in the 1960's, the process of Surface Mount Technology (SMT) has become the state of the art in electronics assembly. By placing the components on the board rather than through the board, PCB manufacturers realize significant production benefits and cost savings. Reduced component size and lead length increase circuit density, resulting in more applications per unit area or weight of board, which leads to lower production costs.
In the beginning, switching to SMT was an option for PCB manufacturers; the concern over component availability and questions over cost savings delayed the conversion for many assemblers. Today, SMT is not an option but a necessity, since many components are no longer available in a through-hole package.
The shift to SMT has focused a great deal of attention on solder joints. Now, in addition to providing the electrical pathway from the device to the board, the joint must also provide the mechanical connection. As the device sizes and lead pitches decrease, with a corresponding increase in lead count, the mechanical and electrical properties of the solder joint are critical.
Defect-free assembly demands choosing the proper materials and equipment to provide maximum control of the process.
To make the best cream, you start with the best powder. Only the highest quality solder powder is used in AMTECH products - solder powder manufactured by Advanced Metals Technology, Inc. Solder powder quality is determined by purity of the alloy, particle size, shape and distribution, oxide levels and lot-to-lot consistency. All of the alloys manufactured by AMT exceed the applicable IPC, QQS and J Standard specifications. AMT's proprietary processes allow the tightest control over size, shape and oxide levels in the industry.
The degree of sphericity is related to the oxide levels on the powder - generally, the more irregulars that exist, the higher the oxide level. Solder alloys cannot exist in free-flowing powder form without a thin oxide coat, but through a controlled production and classification process, AMT consistently produces the lowest oxide powder available.
One of the properties of finer size powder is that, as the particle diameter decreases, the ratio of surface area (and therefore oxide level) to mass or volume increases. However, a major contributor to the higher oxide levels typically found on smaller particles is the extensive processing most other manufacturers need to perform to produce these finer sizes. Excessive processing thickens the oxide layer. Through advanced technology, AMT makes ultra fine pitch powder available with less concern over oxide levels.
Advanced technology means working to develop better, stronger and safer alloys for the electronics industry. Our recent work with Hughes Aircraft has led to the development of a Fatigue Resistant Solder that can prolong the life of solder joints exposed to cyclic stress. Ongoing research and development projects will continue to bring the latest in soldering technology to the industry.
Classification of powders has been a much disputed area. A good deal of the controversy centers on the tools available for analysis. Until the advent of computers, determining the particle size distribution was carried out through the use of test sieves or screens. This technique was fairly quick and easily carried out by personnel with little training. With particle size distributions that ranged from 150µ - 15µ (-100/+625 mesh) the technique was adequate. However, the demand for more restrictive size distributions pointed out the lack of precision and resolution of test sieves. The development of supplemental sizing techniques (e.g., laser diffraction, image analysis, and/or volumetric resistivity modification) permits more precise characterization of powder distribution. However, the labeling of a given powder distribution is still tied to sieve designations. Unfortunately, these designations can be misleading since test sieves have defined tolerances from sieve to sieve as well as tolerances in the openings of a given sieve. It is much more prudent to specify a given distribution by stating the actual particle diameters required.
Saying that a powder contains particles between 45µ and 25µ (-325/+500 mesh) still does not define the distribution. Two powder samples may have the same average particle size (whether the average is calculated on a weight percent basis or population basis) but have widely different properties. Therefore, to properly specify a powder, it is necessary to recognize that it is a population of particles, and it is necessary to characterize that population in terms of the distribution of parameters that are important to the particular application.
The demand for finer pitch printing and the development of no-clean pastes require proper characterization of the powders used in solder pastes and creams. Considering Stokes Law (where F is the force on a spherical particle of radius a and velocity v in a medium of viscosity ) for the behavior of particles in a viscous fluid, it is seen that variations of size distribution in a solder cream will have a significant impact on the rheology with a corresponding effect on the paste's printing characteristics. Secondly, with no-clean creams, it is imperative that solder balling be nonexistent. This requirement demands that the printed solder cream come to the reflow temperature uniformly. This can only occur if the individual solder particles are heated at the same rate, which requires that they have nearly the same specific area. Hence, the size distribution must be precisely specified, and specification must be uni-modal with a small standard deviation. Under these conditions the cream will have a consistent rheology and the oxide content of the powder will be tightly controlled.
A beneficial side effect will be metallurgical uniformity in the powder particles themselves, which will promote a metallurgically sound solder joint. Of course, these specifications will preclude irregular particles and multi-modal distributions.
cream (sometimes refered to as paste) is made up of the powdered
alloy and the flux binder system. As the name implies, one of
the system's duties is to act as a flux.
The flux binder system has to perform many additional tasks during the life of the cream. It must:
RMA creams were the major type used for SMT applications. Rosin, along with the proper solvent, activator system and rheology modifiers, was able to meet all of the above criteria. The residue could be left on most assemblies without concern or could be easily removed with the then readily available chlorofluorocarbons (CFCs).
The adoption of the Montreal Protocol has phased out the production and use of CFCs with total elimination not too far away. The elimination of CFCs as a cleaning option presented a problem to manufacturers: customers had become spoiled with sparkling clean assemblies and did not want to settle for less. Drop-in replacements for CFCs were not perfected, so new formulations were required for customers who still demanded cleaned assemblies; water-washable assemblies were thereupon developed. The other alternative was to modify the RMAs to leave a less tacky and less noticeable, cosmetically acceptable residue. Today, there are two main options for post reflow residues: cleaning them off or leaving them on. Presently, a different flux binder must be chosen for each option.
The benefits of cleaning include better looking boards, easier visual and bed-of-nails inspection, better adhesion for conformal coatings and removal of residues and contamination from other manufacturing processes. Cleaning also allows the use of more aggressive fluxes to widen the process window, with less concern over the solderability of boards and components. Another concern has been raised over the effect residues will have on high frequency circuits.
The first water-washable creams had very short working lives, measured in minutes, and also left residues that had to be removed almost immediately to avoid corrosion problems. Developments in formulation technology have resulted in significant improvements in the succeeding generations of water-washable formulas. Today, formulations are available which offer excellent working life and activity, with reduced concern for corrosiveness prior to cleaning.
The main benefits of leave-on formulations are that they save manufacturing operations, saving on equipment and the associated expenses of labor, maintenance, power, chemicals and waste treatment. Many new leave-on formulations offer RMA activity and process window in a no-clean product. Newer formulations also have reduced residue levels, so the cosmetic problem also decreases. The value of visual inspection has raised concerns in that potentially damaging rework is being done on joints that were merely cosmetically unacceptable, and defective joints often pass. Also, as board complexity increases, the practicality and reliability of visual inspection decreases. With the increased use of J leads and BGAs, unaided inspection is impossible; the assembler must make the leap of faith that if his process is in control, the joints will be acceptable.
a leave-on formulation, the ultimate goal is no residue. The
drive for less residue is accomplished in two ways. The first
is to increase the percent metal. A small increase in weight
percent makes a large difference in volume percent. The second
way is to reduce the total non-volatiles in the flux. When one
or both of these modifications are used, all of the cream parameters
are affected. In the very low residue formulations, the flux
in no longer able to prevent re-oxidation of the cleaned surfaces
prior to reflow. To combat this situation, many ovens are being
designed with a nitrogen option. The nitrogen displaces the oxygen
and reduces both the potential for re-oxidation and the charring
of any remaining residue. The user must decide if the added expense
in nitrogen and process modifications is justified for his particular
The primary method for en masse depositing of solder cream on a circuit board is through the printing process. The increased complexity of board design has led to tighter tolerances for the printing process. This progression has resulted in the near-total shift from screens to stencils and the increasing popularity of polymer-coated metal blades.
The consistent deposition of solder cream is the first step in controlling the SMT process. Having the proper amount of cream will enable the formation of a joint with the proper fillet geometry, which will determine the thermal and mechanical properties of the joint. Too little solder cream can result in opens or mechanically and metallurgically weak joints. Too much cream can lead to bridging. Also, excess solder will make the joint less compliant and more prone to cracks due to component/substrate thermal coefficient of expansion (TCE) mismatch.
The printing process involves using a squeegee to roll the cream across the stencil surface. The cream fills the apertures corresponding to the pads, and releases onto the pads as the stencil is separated from the board. Many factors combine to determine the success of the printing process. These factors include the printer itself, the stencils or screens, the squeegees, the operators and their training, the environmental conditions, the board characteristics and the solder cream. Optimizing these is the key to printing consistently.
AMTECH has found excellent results in the lab and in the field using 4-6 mil metal stencils with metal blades in the on-contact printing mode. Squeegee pressure should be just enough to wipe the stencil clean, with speed in the 10-50 mm/sec range.
The design, quality and accuracy of the stencil becomes increasingly important; as the openings and spacings decrease, tolerances become much tighter. Recent improvements in the fabrication of stencils have resulted in less variation in apertures, less roughness in the walls, and more uniform prints.
An often-overlooked cause of print variation is the preparation of the bare boards. Uneven tinning on fine pitch pads can lead to voids or insufficient solder. This is one reason for the increased interest in organic solderability preservatives (OSPs) as an alternate to hot air solder leveling (HASL).
Solder cream factors for printing include powder size and shape, percent metal and viscosity of the cream. For applications down to 25 mil, AMTECH recommends our -325/+500 mesh powder. For 20 mil and below, the -400/+500 mesh powder gives increased resolution without excessive fines. For stencil printing the range of 89-91 percent metal is recommended, as well as a viscosity range of 180-340 Malcom (700-1400 Kcps Brookfield).
The conventional instrument for viscosity measurement has been the spindle type viscometer. AMTECH has found the spiral pump viscometer to be more repeatable and more representative of the shear actually experienced in the printing process. We currently test at 5, 10 and 20 rmp, with the 10 rpm reading being the reported measurement, also expressed in kcps.
Soldering can be defined as the joining of two metals by material heated above its melting point but below the melting points of the materials to be joined. The bond is formed in one of two ways: by the formation of intermetallic compounds, which is an irreversible chemical process, or by diffusion or absorption, which is a physical process.
When joining 63Sn/37Pb and other high tin alloys with copper, two intermetallic compounds are formed. On the copper side is Cu3Sn and on the solder side, the relatively rough and irregular Cu6Sn5. The total thickness of the intermetallic layers (IL) is usually 0.5-0.7 µm. The intermetallic compounds of copper and tin form crystalline grains, the structure of which is determined by the length and intensity of the thermal interaction. Short reaction times form fine equiaxed grains, which promote good solderability and solder joint strength. Long reaction times result in coarse grains, and a thick intermetallic layer. A thick IL gives poor solderability and poor joint strength, both in t = o shear and long-term reliability. The thickness of the IL depends on the temperature but will continue to grow even at ambient temperatures (which on the absolute (°K) scale approaches 60% of the 63/37 eutectic point). This is particularly important when parts or boards are solder coated or pre-tinned. Upon prolonged or improper storage, these ILs can grow through the surface, severely affecting the solderability.
The majority of parts and boards used today come to assemblers pre-tinned with the ILs already established. However, alternative lead finishes and passivated copper pads are becoming more common. These new lead finishes rely on the solubility of the metal in liquid Sn-Pb to form the bond. For some metals, e.g., Pd, Pt and Ni, the dissolution rate is very slow, requiring temperatures above the normal soldering temperatures and/or dwell times much longer than needed for normal soldering. The result is a joint that does not appear like the traditional solder fillet, causing concern during visual inspection.
The objective of the reflow process is to achieve high quality solder joints on all of the component leads of a particular assembly, and do to this consistently. The process involves heating the leads, pads and cream above the melting point of the alloy so that the solder on the leads and pads, and in the cream, reflows into a homogeneous fillet. Consistency in the process depends on the ability to control the application of heat and the variation of heat both across the board and from board to board. This controlled heating is called the profile. The typical profile includes a preheat, drying or soak, and reflow or spike zone. The preheat zone brings the assembly up to temperature uniformly, generally at a rate of 2°C/second or less. This will minimize the potential for thermal shock on the components due to varying heat capacities. The preheat zone also begins the volatilization of some of the solvents added to the cream for printing and releasing. The second zone continues the drying process to prevent out-gassing and possible spattering of the cream. This zone, sometimes called the soak zone, is also where the flux begins to remove the oxides from the surfaces of the leads, pads and the powder itself. The resins and/or higher boiling solvents remain as a cover to prevent the reoxidation that would readily occur at the elevated temperatures. In the reflow or spike zone, the temperature is quickly raised 20-40°C above the melting point of the alloy. It is here that the solder wets the surfaces and forms the intermetallic bonds. The period of time above reflow is called the dwell time, typically 30-60 seconds. The dwell should be long enough to allow for all of the joints to reach temperature and form the bonds. Too long a dwell time can lead to excessive intermetallic formation. Both of the intermetallics are brittle, and if they make up a large portion of the fillet, can lead to premature failure of the joint.
profile is not a line but a zone or band. The width of this band
is defined by the upper and lower temperatures that will still
give satisfactory results for the particular cream. This band
is also referred to as part of the process window; the larger
the band, the larger or more forgiving the window.
Besides variation across the board, you can also have variation across the oven. This is sometimes caused by the heat sinking of the conveyer system, air flow variations near the sides, or non-uniformity across the heating element. Another source of variation is the ability of an oven to hold temperature and recover after a board passes through. This is called the load factor of the oven. This will vary from oven to oven, but a starting point would be between one half and one board length between boards.
The actual method of heating is not as important as the ability to control the heating in a repeatable manner.
AMTECH Solder Paste Performance and Application Data
The following tests were performed in Kyzen Corporation's Cleaning Evaluation and Application Center. The Center is comprised of various installed PWA assembly, manufacturing and test equipment. The Cleaning Center maintains a complete inventory of batch, inline, ultrasonic, stencil cleaners and close-looped water treatment systems. Furthermore, the Cleaning Center retains the ability to screen paste, wavesolder and reflow assemblies for our customers. A full line of assembly cleanliness evaluation equipment is utilized to determine and document the ability of our products. The Cleaning Center retains dedicated personnel to complete vigorous testing on customers' products and to assist our customers with data answers to their commonly asked questions and concerns. In the interest to supply our present and future customers with the most recent and accurate process information available on our products compatibility with various solder pastes, Kyzen Corporation has subjected the following solder pastes and fluxes to a battery of cleaning tests.
Paste and Flux Tested:
Cleaning Chemistries Tested:
of Cleaning Equipment and Energy Utilized in Testing:
Inline Atomized Spray: Kyzen Aquanox and Lonox Products were tested at Kyzens' Application and Demonstration Center. Equipment utilized for the testing was Kyzens' High Pressure, high flow, conveyorized, atomized spray in-line cleaner.
PWA Testing Apparatus: All Kyzen cleaning chemistries outlined solder pastes and fluxes were tested utilizing a fixed front-end soldering manufacturing process. The PWA utilized is a 3.0" X 4.0" X .060" manufactured FR-4 with LPI solder mask. All boards are manufactured in the same lot.
The testing apparatus contains IPC specified pad geometry's and sizes. Components utilized in the reflow process were; 50 mill PLCC-64's, 50 mill PLCC-32's, SOT-23's, 6301 chip resistors, and 50 mill SOIC-16's.
Reflow Soldering Process: All the primary portions of the test board were screened, assembled and reflowed under the same parameters at the same time. The reflow oven utilized was a Heller 932 four zone forced air reflow oven. The reflow profile times and temperatures were based off manufactures specifications and industry standards. The solder paste was screened with a .020" thick stencil.
Test Format and Evaluated Aspects: The test boards were all evaluated for visual contaminates and solder tarnishing. Dedicated personnel evaluated visual results for all assemblies tested. A high standard of 30X was utilized under a lighted stereo microscope to determine residues and solder tarnishing if found.
The rating scheme used was a 0 - 4 scale with 4.0 being the best. All process parameters are to be defined as baseline applications. Final results can always be modified or improved with variations in time, temperature, mechanical energy and total chemical exposure.
I3330, FCR and HC-2 Test Results of Immersion Cleaning:
The following results were noted:
In addition, no solder tarnishing was noted on the sample assemblies.
A-4000, XJN and Lonox L5005 Test Results from Inline Cleaning:
Spray Inline Cleaning Equipment Parameters:
XJN, and A-4000 Atomized Spray Inline Cleaning Results:
Care must be taken as to not process in multiple passes with the A-4000. This is best suited for the Aquanox XJN.
It is generally felt that inline-cleaning applications are process specific due to the product diversity. Customers should take full advantage of the Kyzen Cleaning and Application Center to match the correct chemistry to the process and the variables that surround the soldering process.
L5005 Atomized Spray Inline Cleaning Results:
For this evaluation, one board was processed at the following parameters: Wash temperature 140F. Concentration of chemistry set at 15%. Mechanical energy of the cleaner is previously described above.
The following results were noted:
Paste Removal from Stencils and PWA Mis-Prints:
Single Chamber Equipment: Most stencil cleaners operate with a single chamber utilized for both washing and rinsing operations. Even though segregated wash and rinse tanks are utilized, singe chambers have a tendency to create extensive chemical drag-out (wash solution entering the rinse bank). This is most commonly an issue when the process chamber is large and when common plumbing between the wash and rinse is utilized. When a single process chamber is used, the customer should expect increased drag-out due to the surface area of the chamber. This increased drag out makes close-looping the stencil cleaner relatively and economically difficult. Care should be taken in choosing your equipment.
Re-deposition of Solder balls: Another area commonly overlooked, is the equipment's capability and capacity to remove the solder balls from the process tanks and chamber. This is not as easy as it sounds. Centrifuge and inline filtration are common methods utilized. In a dedicated stencil cleaner that is not necessarily an issue. In equipment used to clean Mis-prints, or especially double sided secondary side misprints, this is a major concern. The equipment capabilities to remove solder paste from the process chambers and spray manifolds are becoming an increased concern. Due to the higher applications of double-sided reflowed assemblies, solder balls must be removed. If solder balls are not removed from he process chambers, the high pressure and high flow created by the pumps will "whip-up" the solder paste and re-deposit the paste on to the circuit board. This can create reliability issue in the field.
Double Sided Reflowed Assemblies: The concern here is the double-edged sword. A lot of manufacturers have spent considerable resources to remove cleaning from the assembly process. It is commonly overlooked with double sided reflowed assemblies of how to clean the primary side of the assembly if the secondary side is misprinted. The concern is viable. Now that the primary side has been reflowed, and the secondary side has been Mis-printed, the primary side must either have no cosmetic effects after cleaning , or the cleaning equipment must have the ability to remove all the reflowed residues from the primary side of the assembly. Thus, we are cleaning again. If a PWA manufacturer has purchased equipment dedicated to ultrasonic energy, and the customer does not approve of components on the primary side being subjected to this energy, you will be hand cleaning these assemblies. If the equipment is of an atomized spray design, considerable pressure and flow may need to be used to remove there reflowed residues. Under most scenarios this was not taken into account during the purchasing of the equipment. Kyzen process engineers at the Cleaning and Application Center have considerable expertise in these areas and may be contacted for assistance.
Our experience here has shown the Aquanox and Ionox chemistry line to be considered for this application.
The above information in no way indicates or should be taken as Kyzen Corporation's endorsement of, or Kyzen's preference to any of the mentioned solder pastes, solder fluxes or cleaning equipment manufactures.
Reference the appropriate Kyzen manufacturer data sheet for the recommended operating parameters and conditions.
Evaluated and authored by: Charlie Pitarys (Process Development Manager)
Our customer service representatives are ready to discuss your SMT needs. We offer a very short turnaround time on orders and can ship to meet your just-in-time requirements.
To help us meet your particular needs, just provide us with the information on your alloy, flux type, powder size and packaging preferences.
If you are not sure which combination is best for your process or have any questions regarding soldering techniques, procedures or products, one of our technical support staff will assist you.
AMTECH is committed to bringing you the most advanced formulations available and we back that up with highly trained staff to provide for all of your present and future assembly needs.