Evidence shows tight specifications on the two response variables for the temperature profiles can do the trick.

An initial study was conducted to determine if just three thermocouples could verify a PCB’s profile during reflow. In doing so, a thermal recorder, with the capability to measure up to 20 channels was used to thoroughly map a test vehicle. Three of the 20 channels were used to record the temperatures on the leading edge of the PWB laminate, while the other 17 were distributed around the PCB in critical and noncritical locations. Two techniques were conducted: “characterization” and “verification.”

In the characterization technique, a T/C is applied to a low-mass component (L), an intermediate/sensitive component (S) and a high-mass component (H). In the verification technique, the three T/Cs (L1-L3) are placed along the top, leading edge surface of the PCB. The characterization technique can be represented graphically (Figure 1). This figure is representative; component locations conceivably could be anywhere on the PCB. Similarly, Figure 2 demonstrates locations of the three T/Cs in a verification technique. Here, the locations – left, center and right – are fixed.

The logic behind the placement/evaluation of these three locations is that the components with the smallest thermal mass – i.e., the most thermally sensitive components on the board – will not see profiles significantly different from these leading-edge locations.

Experimental

Thermocoupling the TV. The test cehicle (TV) is a desktop motherboard. The board measures 9.5" x 9.5". One goal of this project is to permit the profiling to be done in a quick fashion; a “lick-and-stick” approach was taken for connecting the T/Cs to the TV. The T/Cs, in other words, were attached solely via Kapton tape either on the surface of the board, or on or under components of interest. A more thorough R&D type of T/C attach – for example, drilling into a BGA sphere – was intentionally avoided to better reflect current practice in many line-side situations. Figure 3 shows a schematic of the 20 T/C locations; a photographic image of the TV appears in the Appendix.

Because a major focus of this work is to determine if three T/Cs located along the top, leading edge of the PCB is sufficient for verifying product temperatures (profiles), the other 17 T/Cs available (via use of the 20-channel M.O.L.E. Temperature Recorder) were attached to various locations on the TV.

T/Cs used for “characterization” technique. Locations/components identified for the characterization technique were:

• Location 18 – Identified as a component with low thermal mass (L).
• Location 8 – Identified as a component with intermediate/sensitive thermal mass (S).
• Location 19 – Identified as a component with large thermal mass (H). This location is arguably the most thermally sensitive on the assembly with the narrowest process window for acceptably reliable assembly.

T/Cs used for “verification” technique. The three T/Cs placed along the top, leading edge of the PCB are those identified for the “verification” technique as described (Figure 2). They are numbered locations 1, 2 and 3.

Reflow profiles. To perform this initial study, four different Pb-free reflow profiles were planned: a baseline profile and three other profiles that represent changes to the stability of the oven. A reflow oven with six heating zones and one cooling zone was used. A brief description of each of the profile settings is provided below; diagrams of each of the temperature profiles are in the literature.¹

Profile 1: This profile is referred to as the baseline. It was selected as it provided a good Pb-free reflow profile for three critical components (4, 5 and 19). Locations 4, 5 and 19 are considered the most critical of the components, as they correspond to the largest thermal masses on the board: the corner joints of a large ball grid array and the processor socket joints. For this profile and for comparison purposes for the other profiles, the zone temperature settings (top zones and bottom zones set to be the same) are shown in the Appendix; the belt speed for Profile 1 is 30 cm/min.

Profile 2: This profile is identical to Profile 1 in zone temperature settings; belt speed is 35 cm/min.

Profile 3: This profile is identical to Profile 1 in zone temperature settings; belt speed is 40 cm/min.

Profile 4: This profile is identical to Profile 1 except for one significant exception. The reflow zone (6) temperature was set much lower than it was in the other profiles. It was set at 220˚C, whereas the reflow zone temperature was 260˚C for all other profiles.

The logic for changing the belt speed with regard to Profiles 2 and 3 was to determine if the three top leading edge T/Cs alone would be sufficient to depict a change in temperature recordings with regard to the response variables of peak temperature and time above liquidus (TAL). Profile 4 was an attempt to simulate a bad temperature zone without, for example, actually shutting down a fan and compromising the oven.
Procedure. The following steps were followed for each of the four profiles:

• Set machine parameters (zone settings and belt speed).
• Permit oven to stabilize.
• Set mole to record.
• Place TV on center of the conveyor belt and allow assembly to go through oven.
• Upon exiting oven, stop recording on the mole.
• Set TV aside to allow it to cool/stabilize – while keeping the oven running at current profile. (This is done to mimica production environment.)
• Disconnect mole from T/Cs.
• Connect mole to computer and download the temperature readings.
• After TV has cooled/stabilized for a one-hour period, reattach mole to the T/Cs.
• Repeat Steps 3 through 9 until a total of five replicates have been accumulated for the current profile.

Results

As two studies are conducted, one for characterization and one for verification, the results will be separated accordingly. Each study consists of a group of three T/Cs.

For each experiment/technique (characterization and verification), the data were analyzed first by looking at each individual T/C in the group, and second, by looking at the grouped behavior of the three T/Cs. In evaluating the profiles, the following two response variables are of interest:

• Peak temperature.
• Time above 217˚C; aka TAL.

In analyzing the response variables, a Oneway Analysis of Variance (Oneway ANOVA) was conducted. The purpose was to determine if reflow profile has an effect on the response variables. A Tukey-Kramer multiple comparisons test (MCT) was also conducted. The MCT was used to determine if differences in the performance (readings) of the T/Cs between the different profiles are significant. The MCT is conducted at an α = 5% value (i.e., providing a 95% confidence).¹

Before the results are summarized, it is important the reader have a fundamental understanding of how the data were analyzed.

Consider the following discussion with regard to peak temperature and T/C 1 – one of the T/Cs used in the verification experiment. In comparing the baseline (Profile 1) to Profiles 2 and 3, which have the same zone temperature settings, but increasingly quicker belt speeds, we would expect peak temperature would reduce from baseline to Profile 2 to Profile 3. We should also expect that in comparing the baseline to Profile 4 (where the belt speed is the same as the baseline, but the reflow zone (6) is significantly reduced) peak temperature will also reduce. Given this discussion, Figure 4 is a graph for T/C 1 (L1) and peak temperature.¹

Figure 4 indicates the mean values of peak temperature (designated by the horizontal lines in each diamond figure) have decreased, as expected, from baseline (256.1˚C) to all other profiles, respectively (253.64˚, 252.00˚, and 242.18˚C). The circles on the right-most portion of the figure indicate the significance of the differences across the different profiles. For example, the circle related to the baseline (Profile 1) is separate from the circle of Profile 2. Since they do not overlap, the Peak Temperature of Profile 1 for T/C 1 can be said to be significantly different from that of all the other profiles.

A similar understanding/analysis can be made with the TAL data appearing in Santos et al. (2008). We should expect the TAL, in moving to Profiles 2 and 3, as compared to the baseline, will decrease. Profile 4, however, is a little more interesting. As Profile 4 is only different in one zone (6), but because that setting is still high (220oC) and the liquidus value is below that (217oC), there may not be as large a difference in TAL when comparing the baseline to Profile 4 as when comparing the baseline to Profiles 2 and 3. All this is evidenced in Figure 5.

Figure 5 indicates significant differences between the mean TAL values for the baseline, as compared to all other profiles. (The circle for the baseline does not intersect with any others.) The mean values of TAL - T/C 1 for Profiles 1-4, respectively, can be found in Santos, et al1 and are 142.12, 119.58, 101.46, and 124.26 sec.
Now that an understanding of how some of the data/graphs were analyzed, summary results of the two experiments are now presented, beginning with the characterization experiment.

Characterization experiment summary results. The three characterization T/Cs are locations 18, 8 and 19. These represent a low thermal mass component (L), an intermediate/sensitive component (S) and a high thermal mass component (H).

Tables 1 and 2 in the Appendix present the mean peak temperature values and mean TAL values for each of the components in the characterization group. The values are presented for each of the reflow profiles (RP1-RP4). In addition, percent changes in moving from the baseline (RP1) to each of the other profiles (RP2, RP3, or RP4) are noted.

Verification experiment summary results. The three verification T/Cs are those numbered 1-3 and are the three located on the laminate across the top, leading edge of the TV. The white paper1 provides the following for each of these T/Cs: an ANOVA analysis for peak temperature and an ANOVA analysis for TAL. The white paper also provides an ANOVA analysis for peak temperature for the combined (1, 2 and 3) thermocouples, as well as an ANOVA analysis for TAL for the combined (1, 2 and 3) thermocouples.

Tables 3 and 4 in the Appendix present the mean peak temperature values and mean TAL values for each of the components in the verification group. The values are presented for each of the reflow profiles (RP1-RP4). In addition, percent changes in moving from the baseline (RP1) to each of the other profiles (RP2, RP3, or RP4) are noted.

Conclusions

The two response variables of importance in this work are peak temperature and time above liquidus. Pb-free guidelines for these two variables are typically listed as:

• Peak temperature: min. 235˚C, max. 260˚C.
• TAL: 60-120 sec.

In looking at the two groups (characterization and verification groups), Tables 5 and 6 in the Appendix of this paper provide the mean values of Peak Temperature and TAL.

Response variables and baseline profile. Concerning the baseline profile (RP1), we see that regardless of group, mean peak temperature does not exceed the 260˚C specification, as desired. For TAL, the characterization group does not exceed 120 sec., which is also desired. However, the verification group does exceed the 120 sec. threshold. We offer that this is not necessarily a bad situation. The reader should keep in mind the baseline profile was developed while considering the three locations of highest thermal mass (locations 4, 5, and 19). To get those three locations to temperature and for sustained (60-120 sec.) duration, it is not surprising that three thermocouples simply placed along the leading edge of the substrate have a TAL that exceeds 120 sec. Further, and to restate, the mean peak temperature does not exceed 260˚C in this group; nor does the mean peak temperature of any individual thermocouple in this group exceed this value (Table 3).

To further support that this is not a necessarily bad situation, consider one of the most thermally sensitive components on the TV as measured by T/C 18 (see Tables 1 and 2). T/C 18’s mean peak temperature is comfortably below 260˚C, and its TAL is only slightly above 120 seconds.

Effect of changing from baseline profile on the response variables. Even a casual evaluation of Tables 5 and 6 reveals that when changing to Profiles 2 or 3 – where the belt speed is increasingly quickened – both the characterization group and the verification group see decreases in peak temperature and TAL. These results are expected. In changing to Profile 4 – that simulates a bad reflow zone, both the characterization and verification groups also see decreases in peak temperature and TAL. Again, these results are expected, but it is even more important that the data support these expectations.

This work represents but a subset of a 35+ page white paper1 that contains a wealth of additional statistical analysis, graphs, and tables. Readers are invited to contact the authors for a copy.

Interesting ending observation. In fact, by the very conducting of this study and focusing on only three T/Cs, there is evidence to support (studying the performance of T/C 3 alone – Table 3) that the reflow oven used in this experiment may need to be serviced soon! In fact, an evaluation of T/Cs 14 and 15, relatively in the same plane of travel as T/C 3, also show (but are not presented herein) non-statistically-separable performance (as did T/C 3) in peak temperature between Profile 1 (baseline) and Profile 2.

Acknowledgments

We would like to extend our sincere appreciation to Unovis Solutions for allowing Larry Harvilchuck to participate in this study. Thanks also go out to Ashok Pachamuthu, graduate research assistant and student lab manager of Binghamton University’s surface mount assembly laboratories. The authors would also like to thank ECD Inc. for use of the 20-channel temperature profiler in this study. Finally, the authors would like to thank the Integrated Electronics Engineering Center (IEEC) and the S3IP Center at Binghamton University.

References

1. D.L. Santos, A. Ramasubramanian and L. Harvilchuck, “On the Use of 3 Thermocouples to Verify a Printed Circuit Board Profile During the Reflow Operation,” White Paper Technical Document, Department of Systems Science and Industrial Engineering, Binghamton University, Binghamton, NY, September 2008.

Ed: This article was originally published in the Proceedings of the 2009 SMTA Pan Pacific Microelectronics Symposium, with minor additions herein, including to the title, and is published with permission.

Dr. Daryl L. Santos is professor in Systems Science and Industrial Engineering (SSIE) Department of Binghamton (NY) University (ssie.binghamton.edu); santos@binghamton.edu. Arun Ramasubramanian was at at Binghamton (NY) University. Laurence A. Harvilchuck is a process research engineer at Unovis Solutions (unovis-solutions.com).

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