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STEP 10: Rework and Repair
December 31, 1969 |Estimated reading time: 7 minutes
The higher temperatures required for lead-free manufacturing pose challenges for electronic assemblies. Large, thick PCBs with area-array packages and thru-hole parts pose a particular challenge to the lead-free rework process.
By Jasbir Bath and Quyen Chu
The higher temperatures required for lead-free manufacturing pose many challenges for electronic assemblies, particularly rework processes. The additional heat excursions required for rework mean increased potential for damage to components and boards. Large, thick PCBs with area-array packages and thru-hole parts pose significant challenges for lead-free rework.
The International Electronics Manufacturing Initiative’s (iNEMI’s) Lead-free Assembly and Rework Project conducted several experiments over a three-year period to determine processing parameters for, and reliability of, large, thick-board assemblies subjected to lead-free rework. The group focused on developing a rework process for area-array packages using conventional hot-gas rework systems and investigated a pin-thru-hole (PTH) component-rework-attachment process. Work was done on thick boards, similar to those found in mid- to high-level system applications. This article summarizes results of the group’s efforts and highlights recommended best practices.
Area-array Packages
The area-array rework process included removal, site redressing and part replacement using current production rework equipment and tools. The project team attempted to follow the IPC/JEDEC J-STD-020B specification, which was the current standard when initial rework development was done. The following parameters were used for rework development:
- Equipment: conventional hot-gas rework equipment;
- Board thickness: 0.093" and 0.135" with 14 copper layers;
- Components: µBGA, PBGA, CBGA;
- Surface finish: immersion silver (Ag), electrolytic nickel/gold (NiAu);
- No-clean solder paste: Sn3.9Ag0.6Cu and Sn37Pb.
Multiple rework trials were required to develop satisfactory profiles for the PBGA, CBGA and µBGA. For the lead-free profile, minimum solder joint temperature was about 230°C, and maximum package temperature was about 245°C. Lead-free rework temperatures for three of the five sites conducting rework on the µBGA, PBGA and CBGA conformed to J-STD-020B; however, these results were achieved by collaborating with rework-equipment suppliers to optimize equipment and nozzles. The project team conducted rework-profile runs using thermocoupled boards over a period of several months to develop and verify the most suitable rework profiles. In production, rework profiles typically are developed in hours or days, and can be done only if thermocoupled profile boards are available. J-STD-020C, which has higher temperature limits, allows a wider, lead-free process-temperature window. The team recommends use of this standard.
Overall rework time was about eight minutes for lead-free and six minutes for SnPb profiles. In most cases, board temperature 150 mils from the reworked component was above the liquidus-reflow temperatures, which was also the case during SnPb rework. Time above liquidus (TAL), which was often close to 90 seconds, combined with higher peak temperatures for lead-free solder rework, let to increased solder joint voiding. More solder paste development work is needed to support the elevated lead-free solder-temperature profiles.
The team encountered three challenges during profile development:
- It was difficult to minimize top-package temperature while allowing sufficient heat to form solder connections.
- Unintended reflow temperatures occurred on adjacent and bottom-side components.
- Lead-free reflow parameters were near the limits of solder paste and package specifications.
With lead-free rework on 93-mil-thick boards, the bottom-side-heater set point needed to be elevated above the temperature used for typical SnPb rework. This adjustment was made to keep from overheating the top of the package beyond J-STD-020 limits. A higher heat was applied to thermally challenging 135-mil-thick boards.
Increasing bottom-side heaters to compensate for the reduced top-heater nozzle must be tightly controlled to prevent adverse effects. The concern is that when exposed to combined bottom-side and nozzle heating, bottom-side and adjacent components could exceed liquidus temperatures. During µBGA rework, the nearby CBGA was impacted by the heat flow, resulting in open connections. However, open connections did not occur on an adjacent µBGA that was a similar distance from the reworked µBGA as the CBGA. The team concluded that component construction and size contributed to the differences.
By shielding the CBGA during µBGA rework, the project team was able to avoid subsequent opens during post-rework. However, reliability of the CBGA decreased. This was attributed to the adjacent-rework process. This concern led to an additional experiment to better understand the thermal characteristics of adjacent heating.
Preliminary results show that the adjacent CBGA had joint-temperature ranges of 211° to 223°C (223°C being closest to the reworked µBGA). Thermocouples were placed at the bottom side of the PCB corresponding to CBGA-joint locations above registered temperatures ranging from 237° to 245°C. The adjacent µBGA had a solder joint temperature of 245°C. Table 1 and Figure 1 show the results and locations of the thermocouples. Additional work is needed to help reduce bottom-side and adjacent-component temperatures. The follow-up iNEMI Lead-free Rework Optimization Project will address this.
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Figure 1. Thermocouple-placement location of µBGA to CBGA adjacent rework study.
Test cells that were reworked on thicker boards (0.135") had the highest percent of failed CBGAs. This likely is due to higher bottom-side thermal loads used for thicker boards during the rework of an adjacent µBGA. The group also found that if a CBGA was reworked after the µBGA, there were no opens registered for the CBGA.
Thru-hole Rework Development
The team conducted preliminary lead-free rework evaluations on a thru-hole-soldered PDIP-16 component, with an SnPb thru-hole rework process used as a baseline. The following parameters were used:
- Equipment: mini-pot solder fountain;
- Board thickness: 0.135", 14 layers;
- Rework nozzle: 0.48" × 0.96";
- Board finish: NiAu;
- Solder alloy: Sn3.9Ag0.6Cu and Sn37Pb;
- Wave flux: no-clean, water-based VOC-free.
The experiment consisted of first-pass wave soldering on three PDIP locations on the test boards. The component was then removed and the thru-hole site redressed, followed by a second-pass rework soldering.
Both alloys had greater difficulties with hole fill and fillet forming during second-pass rework soldering; both alloys needed more dwell time for rework, especially the SnAgCu solder. For the SnAgCu wave process, different solder-pot-temperature settings were used in the evaluation: 260° and 274°C; 300°C at different dwell times with and without board preheat on the 135-mil board. An external BGA rework machine provided top-board preheat of 120°C because the mini-pot machine used did not have board-preheat capabilities.
During first-pass soldering at 260°C, no sign of topside hole fill was observed. When the pot temperature was raised to 274°C, acceptable hole fill was achieved with a dwell time of ten seconds. Good hole fill and fillet formation were seen on all PDIP-16 leads. With board preheat for first-pass PDIP-16 soldering, acceptable hole fill could be achieved at lower dwell times.
Removal of the assembled lead-free, tin-coated PDIP was successful with a lead-free SnAgCu pot temperature of 274°C using about 15 to 20 seconds dwell time. Excessive solder was removed and the site was redressed. Reattaching a new component was found to be more challenging.
Second-pass (rework) soldering found that a pot temperature of 274°C was needed to achieve close to 75% hole fill. Acceptable hole fill and fillet formation could be seen at dwell times of about 30 sec. at 274°C without board preheat. Tables 2 through 4 compare the results of SnPb and SnAgCu soldering of the PDIP component, showing pot temperatures and dwell times, and highlighting increased pot temperatures and dwell times needed for SnAgCu solder without board preheat.
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Comparing rework of DIP-16 components on 135-mil-thick boards, SnPb solder appeared to have greater flow than the SnAgCu solder up the thru-hole barrel during mini-pot rework operations. The time required for soldering and removing SnPb parts with SnPb solder was much shorter than lead-free parts with SnAgCu solder. The SnPb solder also gave better topside soldering. A preheat setup (which is uncommon in a production environment) is required for SnAgCu rework to achieve results similar to SnPb rework. Equipment upgrades must be made to provide the necessary preheat capability for thicker boards.
Visual, X-ray and cross-section analyses were performed on one of the lead-free, thru-hole reworked assemblies. Cross-section analysis showed that part of the copper-pad barrel had dissolved into the solder pot. Additional work is needed to define a workable process to characterize the integrity of the rework for thru-hole solder joints.
Conclusion
Based on test results, the team made the following recommendations.
For BGA rework:
- Minimize TAL for the solder joint. The TAL for all three reworked components (CBGA, PBGA, µBGA) was at the high end of the solder paste specification (close to 100 seconds).
- Minimize adjacent component and bottom-side temperatures during rework. The iNEMI Lead-Free Rework Optimization Project is investigating this.
- Improve nozzle design for more uniform heating. (Uneven flow rates occurred in certain cases for the rework nozzles.) Bottom-side heat and thermal uniformity also are critical to bring the board up to proper lead-free rework temperatures.
- Provide better thermal control of equipment for more repeatable rework performance at higher lead-free temperatures.
For thru-hole component rework:
- Preheat the board to improve hole fill.
- Minimize the problem of copper pad/trace dissolution caused by excessive solder-contact times when performing thru-hole rework using a SnAgCu mini-pot rework process on thicker boards.
- Rework equipment suppliers need to improve equipment to allow thicker boards to be reworked without using external preheat setup.
REFERENCES
For a list of references, contact the authors or visit www.smtmag.com.
Jasbir Bath, advisory process engineer, Solectron, e-mail: jabirbath@ca.slr.com; and Quyen Chu, advanced manufacturing engineer, Jabil Circuit; e-mail: quyen_chu@jabil.com; were co-leaders of the iNEMI Lead-Free Assembly and Rework Project’s Rework Process Development Group. For more information on iNEMI’s lead-free activities, visit www.nemi.org/projects/ese/lf_hottopics.html.