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Practical solder paste issues

Practical pastes

Designing a viable product is complex, and some of the performance parameters are trade-offs. for example:

Every ingredient plays a role in the final performance of the paste and, most of the time, all the ingredients interact in determining paste behaviour. Be aware that materials which may appear similar may have totally different characteristics in the application!

Standard paste identifiers

To identify solder paste products, suppliers often use codes which are based on the American specification ANSI/J-STD-005 Amendment 1, which uses a code of the form AAAAABCCCCDEEFFFFGGG, where the elements have the following meanings.

Symbol Solder paste characteristic
AAAAA Alloy short name from ANSI/J-STD-006
(see Solder materials, Table 1)
B Solder form – P for solder paste
CCCC Flux designator from ANSI/J-STD-004
(see How joints are made, Table 1)
D Powder type designation (see Solder paste basics, Table 2)
EE Metal content in percent by weight
FFFF Viscosity in kilocentipoise
GGG Package contents in kilogram

Thus Pb36APREM039108000.5 would be the code for a 500g container of solder paste, containing 91% by weight of 62:36:2 Sn:Pb:Ag solder as type 3 powder (45–25µm diameter balls), with moderate flux activity but zero halide content, supplied at 800kcps viscosity.

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Testing solder pastes

Solder powder purity

The raw metals from which the solder alloy starts are of high purity – lower limits are typically tin (99.95% pure) and lead (99.99% pure). The alloys will be weighed under clean conditions and melted under nitrogen or vacuum, to reduce oxidation.

A technique called differential scanning calorimetry (DSC) measures the heat associated with the transition of materials and is very useful in determining boiling point, melting point, softening point, glass transition, phase transition and heat of reaction, and is a convenient tool to monitor thermal stability of materials. DSC is used as a quality control tool for the melting range of solder powder, which also reflects its composition. Where phase transitions occur, these can also be detected.

Solder powder size and shape

Particle size distribution can be determined by several techniques based on light scattering, sedimentation, electrical zone sensing, permeability, and specific surface area. Each technique has its inherent characteristics, and the results may vary from one technique to another. Understanding the principle behind the techniques is thus important in correlating data.

The comparison between the average particle size identified from a surface area measurement with that found by sedimentation, light scattering, or electrical zone sensing gives a useful indication of the degree of particle agglomeration.

Solder powder oxide content

The oxide content of solder powder has a direct impact on the solderability of solder paste, particularly on solder balling. The objective has always been to achieve an oxide-free or minimal oxide formation during solder powder production, handling and storage.

To estimate oxide content, a wet chemical method has been used to measure the weight difference between before-fusion and after-fusion of a specified weight of solder powder in a rosin solution. The result obtained by this method is usually an over-estimate, but will reflect the cleanliness or purity of the powder, and is a useable indication of the level of oxide and other contamination.

Oxide-free solder has a smooth, shiny appearance, but excessive oxides appear as rough, dull growths on the surface. A quick (though non-standardised) test method is to reflow a small amount of paste on a clean alumina substrate, when a good paste will form a single, shiny ball.

Flux

The normal tests used to qualify the fluxes in solder paste are specified in ANSI/J-STD-004. However, an extra test of washability is required, because fluxes used in reflow soldering processes tend to produce residues which are more difficult to remove than those of the liquid fluxes associated with wave soldering. This is attributed to the presence of thixotropic agents and plasticizers and flux activators, and the parameters of the reflow process; high rates of heating and longer times at elevated temperatures can cause increased isomerisation of the flux and a more stable, viscous, tenacious, less soluble residue.

There are a number of tests for ease of flux removal within a defined time (usually 24 hours) of it being deposited, by cold cleaning, steam cleaning, ultrasonic cleaning and cleaning by hand. Unfortunately, as with most aspects of cleaning, it is very difficult to create a standard test which gives consistent results.

Melting behaviour

In a test for wetting, four circular spots of solder paste are deposited on a copper plate, aged, melted and visually assessed for evenness of wetting. This test can be extended also to examine:

However, these factors are as much a function of the underlying surface as of the paste.

Viscosity

The generic techniques available to measure flow behaviour are described in Viscosity and flow. The basis of many commercial instruments is the configuration shown in Figure 1, where the solder paste can be kept in its own container and a spindle immersed to a known depth. In the more common configuration of the right-hand diagram, the spindle with a paddle attached executes a number of axial rotations on a helical path, so that the paste being contacted is constantly changing.

Figure 1: Schematic configurations of viscometers suitable for solder paste

Figure 1: Schematic configurations of viscometers suitable for solder paste

Viscosity changes with temperature, and it is therefore crucial that viscosity measurements are made at a constant temperature; 25ºC is the usual choice for reference temperature, although this may not be representative of conditions in Far Eastern assembly plants.

Dispensability

An entirely different approach for comparing viscosities of solder pastes is the nozzle flow method in which the paste is dispensed at different defined pressures onto a glass slide. The weight of paste deposited is an arbitrary but relevant measure of viscosity and the test is appropriate to pastes intended for dispensing rather than screen printing.

In a typical test: the glass slide is weighed; the needle length and internal diameter are defined (15mm and 840µm or 510µm respectively being standard); a set pressure (1, 2, 3, or 4 bar) is exerted on the piston for ten seconds; the slide is then reweighed.

Printability

A paste that is designed to be printed has good printability (or ‘screenability’) if the proper wet height, coverage and edge resolution are maintained on the printed substrate. Printability of a paste includes such subjective evaluations as how easy it is to print, and how well the solder paste spreads.

Although in many ways a qualitative assessment of the solder paste, attempts have been made to define the properties that ensure good screenability.

The shearing rate of the paste can be calculated from the squeegee speed, and the properties of the screen and its emulsion. Knowing the shear rate, the shear stress can be determined using a viscometer. For the paste to screen well, the shear stress needs to be less than 3Ncm−2.

Slump

While there is no quantifiable measure of slump, it is useful to have a comparative standard test that can be used to rank different pastes. The ANSI/J-STD-005 test, which references IPC-TM-650 Method 2.4.35, estimates spread using a well defined deposit of paste on a flat ceramic glass or glass-epoxy laminate substrate. The stencil has a pattern similar to that in Figure 2, with several arrays of pads separated at decreasing spacings. Specimens are examined both after room temperature storage (for ‘cold slump’) and storage at 150oC (for ‘hot slump’).

Figure 2: ANSI/J-STD-005 slump test pattern

Figure 2: ANSI/J-STD-005 slump test pattern

Tackiness

The ANSI/J-STD-005 standard gives a tack test procedure (IPC-TM-650 Method 2.4.44) in which a stainless steel test probe of defined dimension (which is a substitute for the component) is brought into contact with the printed sample. The rate of approach (slow), the force applied, and the time of application are all specified. The probe is then pulled away from the sample, and the tack force measured as the peak force required to break the contact.

The tack time is the time which has elapsed between the print being printed and the measurement – this should be broadly representative of the worst case process. Clearly this is a parameter which varies very considerably between different applications. IPC-TM-650 Method 2.4.44 suggests that the maximum tack time for a paste is when the tack force has reduced to 80% of its initial value.

There are also two qualitative tests for tack, which are generally easier to apply and require less equipment.

  1. Print an appropriate paste pattern and expose it for different times but under constant conditions. Place the part on the paste, applying a set load for approximately the same time on each occasion; turn the board upside down and record the number of times the component drops off under gravity and the time taken for the component to drop off. This test is best carried out with components such as PLCCs which are large and heavy, relative to their contact area.
  2. Print a pattern and expose it for a set time as before, but measure the angle at which the component starts to fall off.

In both these cases, comparative performance is given as a ranking of different materials, although the measurement techniques are not quantifiable as such. However, the test can be carried out in a factory and will allow effects such as a change in storage conditions to be assessed without going to the expense of buying laboratory standard equipment.

Self Assessment Questions

You have been given a sample of an unknown paste. Given full laboratory facilities, how might you set about:

a) measuring its slump, viscosity and tackiness?

b) determining the shape and size of its solder powder?

c) assessing the compatibility of its flux with your application?

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Self Assessment Questions

The storekeeper has discovered a large quantity of time-expired solder paste which the production manager would like to use.

a) What might be the problems in doing so?

b) Given that only simple equipment is available to you, what tests would you attempt to carry out, to reassure management that yield and quality will not be affected?

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Paste management

Health and safety aspects

Regarding paste (as distinct from during subsequent reflow) the health and safety issues are confined to:

In order to reduce the hazards, bulk users are generally supplied with solder pastes pre-packaged in 500g or 1kg cartridges, and only normal standards of hygiene and handling are needed. Recommendations are as follows.

Storage and preparation for use

Solder paste is a homogeneous, stable suspension of solder powder particles in a flux binder, but all suspensions will eventually precipitate, the rate of deterioration depending primarily on temperature. This segregation is not necessarily totally detrimental, but is obviously not acceptable where pastes are stored and intended for use direct from a cartridge without pre-mixing. Fluxes will also lose activity, especially in unsealed containers where they are exposed to air. Solder pastes therefore have a defined shelf life, after which their use is not recommended.

Performance can be improved by the way that paste is handled and stored. A common option is refrigerated storage, but care has to be taken to ensure that paste containers are only opened when they have regained room temperature, otherwise:

Self Assessment Questions

What health and safety aspects are important in the handling of solder paste?

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Self Assessment Questions

What advice would you give the storeman as to how to extend the storage life of paste?

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Author: Martin Tarr


 

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