I do not get it. Help me out on this one. With soldering being the dominant source of assembly defects and solder paste being the key component of the process, how come a majority of SMT assembly facilities have no idea how they came to be using the solder paste they use? While a few assemblers perform comprehensive and frequent solder paste evaluations, the vast majority of users either do not know how or do not bother. Ask the typical manufacturing engineer why they are using their current brand of paste and you will get answers ranging from “we have always used this paste” (the 'Legacy Syndrome'), “I dunno”, to “the sales rep gives good lunch".
This is a deplorable fact. Solder paste testing should be carried out on a regular basis. The correct paste should be qualified for your applications in your facility. If you think that solder paste is a commodity in that 'they all perform the same', you are in for a major surprise. At ITM Consulting, we perform many solder paste evaluations for clients every year and I can attest to the fact that not all solder pastes are created equal.
Neither are all applications. Factors affecting solder performance include equipment and setup parameters, stencil fabrication methodology, component lead density, operator skills, component and board solderability as well as ambient temperature and humidity comprising the printing environment. In fact, up here in the Great White North, a solder paste that performs well in the summer, when the facility is airconditioned, may be defect prone in the winter simply because the facility is now running at less than 20% relative humidity due to the heating of the plant. The ideal solder paste must perform as well as possible in the range of applications present and within the printing environment variation range. You cannot take someone else’s word for it either, you have to test them yourself. Remember, 'In God we trust, all others bring data'.
All right, how does one go about testing solder pastes? There are a number of procedures out there, including those published by the IPC and by solder paste manufacturers and there are a few consultants out there well versed in this area that can help you. Some of these procedures use existing production equipment while others may require elaborate laboratory equipment. I personally prefer the former as opposed to ‘beaker tests’. Test the paste as it will actually be used.
Here are some of the tests that can be used with the minimum of equipment and materials and yet get you on your way towards specifying the best solder paste for your application. While space does not allow for elaboration, this should give you a shove in the right direction.
Quantitative solder ball test
One of the most common soldering defects that occurs in SMT assembly is the formation of solder balls. These can be especially deadly to circuit functionality (as they have a propensity for causing electrical shorts) in a no-clean process. Here we do not have the benefit of cleaning to 'wash our sins away'. There are numerous factors in the soldering process that can cause fines of solder to separate from the main mass (fillet). These include improper reflow profile, lack of solderability of components and/or lands, misalignment of solder mask, misalignment of solder print, and many others; but solder paste must not be a contributor to this problem. Sadly, many formulations do indeed exhibit a tendency towards solder balling. Regardless of whether or not we are cleaning the assemblies or using a no-clean process, the solder paste itself should not be solder ball prone. Therefore, we utilise a test that eliminates all the other factors from the 'stew'.
The most straightforward method is to print a circle of solder paste onto a ceramic substrate. Here the solder has nothing to interact with but 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 6,35 mm in diameter. After reflowing the substrate, take a look at the resulting fillet under 10x magnification. Evaluate the number and size of solder balls that satellite the fillet (ideally there should be none).
Residue examination
This particular test also provides the opportunity to visually examine the residue of a no-clean formulation. Since the alumina substrate is white, one can assess the clarity (versus yellowness) and thickness of the resulting residue surrounding the fillet.
Of course, one of the major issues with very-low and ultra-low residue no-cleans 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 of which will lead to false readings. One means of testing this is to hook up a 5 Volt power supply and digital voltmeter in conjunction with a force gauge. This would allow for comparative measurement of the force required to penetrate the flux residue to accommodate good contact.
Solder wetting
The typical test for the ability of the paste to wet 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. It is best to equally prep the substrates by deoxidising them as much as possible, either chemically or with abrasion. If you have the facilities for cross-sectioning the resulting fillets and measuring the angle that way, print the 6,35 mm diameter by 0,15 mm thick circles. Failing this, obtain a stencil comprised of a series of 0,635 x 1,27 mm apertures, arranged in rows with spacing between the apertures varying as follows: 1,27, 1, 0,635, 0,38, 0,25 then 0,25, 0,38, 0,5, 0,635, 1 and 1,27 mm respectively.
Print the paste onto the copper-clad laminate, reflow to manufacturer’s requirements as profiled for the substrate. After reflowing, clean the substrates in isopropyl alcohol and examine under 10x magnification. Determine and record the smallest unbridged gap between two adjacent pads within a row, on either side of the test pattern. Note that 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 thinking of using, Organic Solder Protectant (OSP) coated substrates, instead of HASL, this test becomes particularly pertinent and extremely important. Use copper-clad laminates coated with the appropriate OSP and perform the test as described. Evaluations performed by ITM with our clients has demonstrated that many solder paste formulations, both OA and no-clean, are not compatible with OSPs, including several that specifically claim they are.
Slump
Solder paste performance during the printing operation can have a profound impact on the resulting solder joints. One of the factors 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 a number of circumstances can contribute to this, including printing parameters, stencil parameters and others, the paste’s rheology must be such that it is not a contributor to this defect.
Use the stencil described in the wetting test or one similar such as those designed to IPC-A-21 or IPC-A-20.3. 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.
Now comes the part that separates the 'men from the milquetoast'. Most pastes perform fairly well here yet, to the assembler’s surprise, there is still occasional bridging as a result of slumping. What happens is that the rheology of the paste changes during the reflow cycle. As the solvents are driven off, particularly during the Preflow Soak stage, the chemistry of the paste is altered. This is typically where slump occurs.
Thus it is important that these conditions be simulated during paste evaluation. This can be done by taking the previously printed and examined slump test printings that were evaluated at ambient temperature and placing them in a box oven set at 150°C for two minutes. Remove the substrates from the oven and evaluate the occurrence of slump. ITM has found that most mainstream solder pastes in the market perform well in ambient conditions but several leave a lot to be desired with regard to their performance during reflow.
Tack
The solder paste has to act as an adhesive to hold the components in place until the soldering cycle is completed. Tack should be evaluated as pre-placement and post-placement. For pre-placement evaluation, print several boards with each solder paste and evaluate the tack at one hour intervals. You can use a force gauge to do this or you can populate the boards at these intervals with discrete components (ie, 1206s, 0805s, etc), invert the boards and count the components that fall off for each respective solder paste. I recommend you do this for at least four hours but take into account whether you are printing in-line or in batch mode. We are also simulating placement machine breakdown – something, of course, that rarely occurs.
For post-placement tack, you can use the same inverted board test but here, you populate several boards (of each paste) and wait for one hour intervals before inverting and assessing component fall-off. Again, consider doing this for four to six hours, depending upon your manufacturing scheme and how long boards are likely going to sit after placement before they are reflowed.
If you are using a recent vintage high-speed turret type chip-shooter, it is recommended that you perform tack tests with the machine. Users have reported some pastes lacking adequate tack, resulting in components literally skidding 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? This is a matter of printing a board with the range of component pitches you normally encounter at one hour intervals, leaving the paste on the stencil in the interim. Evaluate the quality of paste at these intervals, examining the shape of the deposit (should be a 'brick') and watching for skips and bridges.
It is incredible how yields can be improved just by assuring that the proper solder paste for the application has been qualified. This is one area you definitely do not want to take for granted. We are all in this together.
For more information visit www.itmconsulting.org
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