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STEP 3: Solder Materials
December 31, 1969 |Estimated reading time: 9 minutes
Greater than 60% of end-of-line defects in SMT assembly can be traced to solder paste and the printing process. Another 15% occur during reflow. Using designed experiments and the measurement of critical solder paste-related process metrics, a solder paste evaluation procedure was developed to maximize information about the paste and its processability, while minimizing experimentation.
By Timothy Jensen and Ronald C. Lasky, Ph.D., PE
Studies show that more than 60% of end-of-line defects in SMT assembly can be traced to solder paste and the printing process.1 Another 15% of these defects occur during reflow. Despite this, it is surprising that no simplified procedure for solder paste evaluation has been documented. With the short time frame available to implement a RoHS program, there now exists a tremendous need for a simplified and highly effective procedure. Using designed experiments and measuring critical solder paste-related metrics, a solder paste evaluation procedure was developed to increase information about solder paste and its processability, while reducing experimentation.
Using 12 stencil-printed printed wiring boards (PWBs), statistically significant results were generated, allowing the ranking of solder pastes according to performance. Response metrics investigated included print volume and definition before and after pause, squeegee hang-up, slump, tack, release from aperture, and solder joint quality.
Solder paste expense represents only 0.05%2 of the value of finished electronics, yet it can have a tremendous impact on product performance and reliability. Given the importance of solder paste to the final assembled product, it is vital to evaluate solder paste performance in a systematic way. Printability, tack, reflow characteristics, surface insulation resistance (SIR), solder balling, and wetting form a minimum of solder paste performance metrics to consider. Testability and cleanability also may be metrics to assess in certain assembly processes.
Printability
A well-shaped printed brick with good volume consistency likely is the best predictor of high end-of-line yields. Too much solder paste in the printed brick could result in shorts, whereas too little may cause opens. Setting solder paste volume specifications and monitoring the printing process for conformance to these specifications can have the strongest positive effect on yields. An effective way to accomplish such control is a statistical process control (SPC) program that assures that the control limits of the printed brick volume are within the upper and lower specifications.3
Reflow Characteristics
Most modern lead-bearing solder pastes reflow relatively well. Reflow performance does not vary as much as printing performance. With lead-free solder pastes, however, reflow performance can vary greatly. For lead-free pastes, two reflow criteria are important:
The paste should perform well in a wide temperature and time above liquidus (TAL) window; post-reflow - the solder should exhibit good coalescence.4
Figure 1. Reflow profile matrix.
Due to the concern for components surviving higher reflow temperatures related to lead-free assembly, more discipline is needed to assure successful reflow at minimum temperatures. Figure 1 shows nine profiles for Sn3.8Ag0.7Cu solder paste (Tm=217°C). One solder paste exhibited good wetting and coalescence during reflow with all nine profiles.4
Tack
Tack is the ability to hold the component on the PWB after placement. Optimum tack holds the component with an acceptable amount of force that does remains consistent over time. Unfortunately, tack will vary with time. A useful rating scheme for tack has been proposed.5
SIR, Solder Balling, Slump, Wetting, and Electromigration
J-STD-004 and J-STD-005 (IPC-TM-650) cover a variety of tests related to surface insulation, solder balling, slump, wetting, and electromigration. It is not our intent to minimize the importance of these tests. However, most solder paste companies perform these tests with reasonable integrity, and the information that the solder paste data sheets provide can be used in a screening process for assessing pastes. After selecting the final candidates in any evaluation process, it may be wise to perform some of these tests on the final candidates yourself.
Proposed Screening Test for Solder Paste
Considering the importance of stencil printing, and the fact that most paste vendors test and report the results of their pastes for J-STD-004 and J-STD-005 (IPC-TM-650) faithfully, a screening test for printed-volume consistency with visual analysis of print characteristics such as slump, bridging, etc., can quickly separate top paste candidates from the “also-rans.” One approach was proposed, but it did not include measuring printed brick volume.6 It also required printing 27 boards. We propose an evaluation process requiring the printing of only 12 boards with print-volume consistency as its foundation (Figure 2).
Figure 2. The 12-board paste evaluator.
To follow the 12-board paste-evaluator process, start with enough paste for 12 prints. No kneading is done to the paste prior to printing. Four boards are then printed in Step 1 (Figure 2). No stencil wiping is done during the prints. Print volume, print definition, release from aperture, and squeegee hang-up are measured. Two of the four boards sit for two hours, and two of the four boards sit for six hours. Components are then placed. Then tack is measured. One of the first two sets of boards sits for one hour, and one for three hours prior to reflow. The same procedure is performed on the second set of two boards.
In Step 2 of Figure 2, the paste is left idle for one hour, and the process in the above paragraph is repeated. In Step 3, the paste is left idle for another hour and the process is repeated again. For initial screening, the process may stop at measuring print-volume consistency and definition. This approach may be reasonable as it minimizes work, and poor print-volume consistency or print definition may eliminate a paste candidate.
Paste candidates that do well in printed volume consistency, tack, coalescence, reflow window size (larger preferred), solder joint quality, and the J-STD-004 and J-STD-005 standard tests should be verified. One attribute that this evaluation technique does not consider is a solder paste’s resistance to shear thinning. This attribute typically is only important to those high-volume/low-mix operations in which the solder paste is being printed continually with little to no downtime. By design, solder paste is thixotropic in nature, meaning that the viscosity drops as shear is applied, and viscosity recovers when that shear is removed. In stencil printing, the squeegee blade imparts shear on the paste and lowers the viscosity, allowing it to fill stencil apertures better. At the end of the print stroke, the bead of paste recovers to its original viscosity. Continual printing overcomes some solder pastes’ recovery capabilities and does not allow sufficient time for their viscosity to recover. If this occurs, that solder paste will gradually become lower in viscosity, resulting eventually in slumping and bridging of its print deposits. To assess this phenomenon, several boards must be printed. However, it can be simulated by running the stencil printer’s knead function 50-100 times. Print deposits following this knead cycle should be inspected for print volume and visual print brick shape.
An analysis of three no-clean pastes was conducted to see how they performed using the 12-board paste evaluator. We printed through a 6-mil stencil using apertures for a 208-pin, 0.5-mm QFP. Twenty apertures were used for measuring the print deposits, and the average volume of the apertures was 7,968 mil3. The printed volume consistency yielded striking results (Figure 3). Each data point represents (for each print number) the average paste volume of the aforementioned 20 aperture sites.
Figure 3. Paste volume vs. number of prints.
As we see from Figure 3, the print-volume consistency of Paste 3 is poor. It varies from about 5,400-9,050 mil3. It also shows an unacceptable response to pause, the first printed volume decreased significantly after each one-hour pause. The average of this paste was 8,206 mil3 (or an average transfer efficiency [TE] of 1.03), and the standard deviation was 1,047 mil3. Paste 1 was the most consistent with an average of 8,616 mil3 (average TE of 1.08), but with a relatively low standard deviation of 279 mil3. Paste 2 finished second with an average printed volume of 8,745 mil3 (average TE of 1.10), and a standard deviation of 485 mil3.
Most SPC programs set control limits to ±3 standard deviations. Using these criteria, the best performer, Paste 1, would have control limits of 8,616 ±837 mil3, or less than ±10%. Typically, solder paste volume control of ±20-30% is needed. With these criteria, Paste 2 would still be a candidate at 8,745 ±1,455, or ±16.6%. From a screening perspective, we have eliminated one paste and can devote our resources to evaluating the other parameters for Pastes 1 and 2. In addition to print-volume consistency being the most important solder paste metric, it also may be the most variable among solder pastes. Therefore, using it as the first criteria can save time in screening pastes.
Voiding: A Greater Concern for Lead-free?
In a study conducted on the voiding of lead-free solders,7 it is clear that the higher molten surface tension and poorer wetting of lead-free increases the potential for voiding compared to tin/lead processes. Given this increased voiding, the question becomes whether this will impact product performance and/or reliability. The IPC Solder Products Value Council (SPVC) concluded that there was no direct correlation to quantity and size of voids and thermal-cycling reliability in products tested.8 This means that simple X-ray analysis is not sufficient to estimate a product’s reliability.
One scenario in which the increase in voiding may have a dramatic impact is in products that use µBGAs or chip-scale packages (CSPs) with microvia-in-pad technology. Because of the increase in lead-free voiding, it is possible that two adjacent balls could bridge and short if both have large voids. This phenomenon could create a significant increase in rework of costly components. Because optimizing the reflow profile can reduce this voiding, the importance of the profile window portion of the solder paste evaluation is critical when selecting the ideal solder paste.
Conclusion
A 12-board solder paste evaluator is proposed. Although the solder paste evaluator includes all important solder paste evaluation criteria, solder paste print-volume consistency is the first to be evaluated. Due to the fact that solder paste print-volume consistency is the most important criteria for high end-of-line yields, this first part of the 12-board evaluator can be used as a screening test.
The authors would like to thank Professor Daryl Santos and Aniket Bhave for their contributions to this article.
References
- Jensen, Timothy, “Solder Paste Printing and Reflow” workshop, 2003.
- Lasky, Ronald, “An Overview of the Electronics Industry,” 2003.
- Lasky, Ronald, “SPC Workshop,” 2003.
- Goudarzi, Vahid, “Lead-free Workshop,” Plantation, FL, 2003; con- tact rlasky@indium.com for a copy of the proceedings.
- Lee, Ning-Cheng, “R&D Test Requirement for Solder Pastes,” Indium Corporation, 2003.
- Herber, Rob, et al., “The 27 Board Challenge,” presented at SMT workshop, Toronto, Canada, 1998.
- Jo, Hyoroon, et al., “Voiding of Lead-free Soldering at Microvia,” SMTAI, 2003.
- “The Effect of Voiding in Solder Interconnections Formed from Pb-free Solder Pastes with Sn/Ag/Cu.” This paper is available for download at www.ipc.org.
Timothy Jensen, product specialist, Indium Corporation, may be contacted via e-mail: tjensen@indium.com. Ronald C. Lasky, Ph.D., PE, senior technologist, Indium Corporation of America, may be contacted via e-mail: rlasky@indium.com.