All solder pastes are not created equal: several in-house tests will reveal if you are using the right solder paste for your application.

One of the most important and critical materials the surface mount process engineer will work with is solder paste. Interestingly, solder paste is often taken for granted with the attitude that "all solder pastes are the same." However, solder pastes are not all created equal. Over 20 solder paste manufacturers are currently marketing in North America, many with several formulations each of water soluble (OA) and no-clean. Yes, they all work, but some pastes work so much better than others.

When was the last time a solder paste analysis was performed in your facility? How were the current pastes you use evaluated and selected, and who was involved in those tests? If you cannot answer these questions, you are not alone. With all of the research and development recently conducted on water soluble and no-clean solder pastes, if your facility has not evaluated solder pastes within the last year or two, you are not using a state-of-the-art formulation.

Technically, solder paste is not a commodity. A paste that works best in one facility with a certain set of conditions may work differently at another facility. Factors affecting solder performance include equipment and set-up parameters, stencil fabrication methodology, component lead density, operator skills, component and board solderability as well as the ambient temperature and humidity in the printing environment. For example, a solder paste that performs well in a Guadalajara facility may not work as well at a sister facility in Juarez.

Of course, the "ideal" solder paste will work consistently the same anywhere under a range of conditions, but very few solder pastes approach this ideal. The solder paste you use must perform as well as possible within your range of applications and within your printing environment. But, the only way to determine which solder paste works best is to test it in your facility under the conditions you normally build printed circuit board assemblies (PCBAs).

Various test procedures can be used, including those published by the IPC and by solder paste manufacturers. A few consultants well versed in this area can also help you. Some procedures use existing production equipment while others may require elaborate laboratory equipment. Personally, I prefer to test the paste as it will be actually used rather than conduct "beaker tests."

Following are some tests that can be used with minimal equipment and materials. These tests will allow you to specify the best solder paste for your application.

Quantitative Solder Ball Test

One common soldering defect that occurs in surface-mount assembly is the formation of solder balls. They can be especially deadly to circuit functionality in a no-clean process because they can cause electrical shorts. Numerous factors in the soldering process can cause solder fines to separate from the main mass (fillet). These factors include improper reflow profile, lack of solderability of components or lands, and misalignments of solder mask or solder print. Solder paste may be a major contributor to these problems.

Sadly, many solder pastes exhibit a tendency toward solder balling. Whether the PCBAs are being cleaned or a no-clean process is used, the solder paste itself should not be prone to solder balling. Therefore, a test should be used that isolates whether the solder paste is contributing.

The most straightforward method is to print a circle of solder paste onto a ceramic substrate. Here, the solder can only interact with itself. Be sure that the ceramic substrate has been profiled in the reflow oven to the time/temperature duration that each solder paste manufacturer specifies. Using a stainless steel stencil as thick as you normally use, manually print a circle of paste 0.250-in. in diameter. After reflowing the substrate, look at the resulting fillet under 10X magnification. Evaluate the number and size of solder balls that satellite the fillet; ideally, no solder balls should occur.

Residue Examination

This particular test also provides the opportunity to visually examine the residue of a no-clean formulation. Because the alumina substrate is white, the clarity (vs. yellowness) and thickness of the resulting residue surrounding the fillet can be assessed.

One major issue with very-low and ultra-low residue no-clean pastes is the ability to penetrate the residue with test probes to accommodate reliable in-circuit testing. The residue should not impede contact with the test pads nor should it gum up the probe; both will cause false readings. One test method is uses a 5-volt power supply, a digital voltmeter and a force gauge, such as a Chatillon DFG-2 with motorized stand. This set-up provides comparative measurements of the force required to penetrate the flux residue to accommodate good contact.

Solder Wetting

The typical wetting test is to print the solder paste onto a substrate of bare copper clad laminate. Of course, no one really solders to bare copper, so this test is one of "relativity." The substrates should be equally prepared by deoxidizing them as much as possible either with chemicals or abrasion.

If you can cross-section the resulting fillets and measure the angle that way, print the 0.250-in. diameter by 0.006-in. thick circles. Alternatively, obtain a stencil comprised of a series of 0.025 x 0.050-in. apertures, arranged in rows with spacing between the apertures varying as follows: 0.050 in., 0.040 in., 0.025 in., 0.015 in., 0.010 in., then 0.010 in., 0.015 in., 0.020 in., 0.025 in., 0.040 in. and 0.050 in., respectively. These stencils are readily available from most stencil suppliers.

Print the paste onto the copper-clad laminate, and reflow to the manufacturer's requirements as profiled for the substrate. After reflowing, clean the substrates in isopropyl alcohol and examine under 10X magnification. Record the smallest unbridged gap between two adjacent pads within a row, on either side of the test pattern. The gap is considered "unbridged" if the reflowed solder is not in contact, whereas a simple contact between two tinning layers is considered "bridged."

If you are using or considering organic solder protectant (OSP) coated substrates instead of hot air solder leveling (HASL), this test is extremely important. Use copper-clad laminates coated with the appropriate OSP and perform the test as described. Recent evaluations have demonstrated that many solder paste formulations, both OA and no-clean, are not compatible with OSPs, including several that specifically claim they are. Of course, if you are using another circuit board finish such as nickel-gold or immersion tin, you should have PCB coupons prepared by your PCB fabricator to your specifications with that finish on it for the wetting test.

Slump

Solder paste performance during the printing operation can profoundly affect the resulting solder joints. One factor that must be considered is the solder paste's ability to stay well defined once released from the stencil. The material can collapse on itself or slump and spill over, causing bridging. While many circumstances can contribute to bridging including printing and stencil parameters, the paste's theology must not contribute to this defect.

Use a stencil similar to those designed to IPC-A-21 or IPC-A-20 (Figure 1). Carefully print onto ceramic substrates or glass microscope slides. After carefully releasing the stencil, examine under 30X magnification and record the incidence of slump, particularly at which pitch it became prevalent.

[FIGURE 1 OMITTED]

Most pastes perform fairly well. However, after reflow, occasional bridging may still result because of slumping. In this case, the paste's rheology changes during the reflow cycle. As the solvents are driven off, the paste's chemistry is altered and slump occurs. Thus, these conditions must be simulated during paste evaluation. This simulation can be done by placing the previously printed and examined slump test printings that were evaluated at ambient temperature in a box oven set at 150[degrees]C for two minutes. Remove the substrates from the oven and evaluate the occurrence of slump. Most mainstream solder pastes perform well in ambient (cold slump) temperatures, but several do not perform well during reflow (hot slump).

Tack

The solder paste must act as an adhesive to hold the components in place until the soldering cycle is completed. Tack should be evaluated in pre-placement and post-placement. For pre-placement evaluation, print several boards with each solder paste and evaluate the tack at one-hour intervals. Use a force gauge or populate the boards at these intervals with passives such as 1206s and 0805s. Then invert the boards and count the components that fall off at each respective solder paste. Go at least four hours out, but consider whether you are printing in-line or in batch mode, We are also simulating placement machine breakdown.

For post-placement tack, use the same inverted board test but instead populate several boards with each paste and wait for one-hour intervals before inverting and assessing component fall-off. Again, go four to six hours out, depending on your manufacturing scheme and how long boards are likely to sit after placement before they are reflowed.

If you are using a recent vintage high-speed turret type chip-shooter, you should perform tack tests with the machine. Users have reported some pastes lacking adequate tack, resulting in components that "skid" off the pastes during placement.

Worklife

Solder paste worklife should also be assessed. How long can the paste remain on the stencil and still print adequately? In this case, print a board at one-hour intervals with the range of component pitches you normally encounter, leaving the paste on the stencil in the interim. Evaluate the paste quality at these intervals by examining the shape of the deposit--it should be a brick--and watching for skips and bridges.

You must do a blind test when evaluating solder paste. Have a person not affiliated with the testing program mark the samples with a letter designator and hide with tape all logos and identifying nomenclature from the samples. The letters assigned to each sample are marked on a sheet of paper and sealed away until the testing is concluded. During the tests, the solder paste samples are only known by their letter designator. This procedure helps eliminate bias, so the testing is objective.

Conclusion

The tests described here can be accomplished within a day and use existing production equipment, so little investment is required. Less than 1,000 gm of each sample solder paste will be used. Solder paste samples can usually be obtained at no charge from the manufacturers.

Other tests can also be performed as required. For example, if surface insulation resistance is critical to your application, you should have soldered samples evaluated for compliance to the solder paste manufacturer's specifications. You might also consider accelerated age testing, particularly in no-clean process applications if reliability issues are in question.

Production yields can be greatly improved just by assuring that the proper solder paste for the application has been qualified. Solder paste selection is one area you definitely do not want to take for granted.

Phil Zarrow is president of ITM Consulting, Durham, NH; [phone]: e-mail: [e-mail]: www.ITMconsulting.org.