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Flux selection for lead-free wave soldering: a DoE clarifies the best types, best available flux within each type, and process limits for each.

Ed.: The complete article can be found at circuitsassembly.com/cms/content/view/5895.

This article discusses an approach to selecting liquid wave solder fluxes. It includes the design of an internal company test vehicle with a range of component types and design characteristics, preliminary flux performance testing, and results verification on the top-performing fluxes.

Included is discussion of the gaps between the capability of existing flux chemistries and requirements of not only Pb-free assembly, but from industry standards and customer expectations with respect to flux residue and solder quality on more complex assembly designs.

Experimentation. The board used for flux testing was the internal company wave test 'vehicle (TV) (Figures 1 and 2). Table 1 lists general properties of the tested board. The board was available in thicknesses of 0.093", 0.125" and 0.180". Surface finishes could be selected based on the needs of the experiment. The most commonly used surface finishes have been organic solderability preservative (OSP) and immersion silver (1mm-Ag). The TV was for mechanical testing only and had no electrical functionality.

This board and related components (Table 2) were designed to demonstrate the limits of the wave soldering process.

The following sections summarize many of the process conditions considered critical to product quality and the significant factors contributing to each.

Results and Discussion

Table 8 shows how all fluxes performed in Phase 1 screening. The weighted score is based on defects, so the lower the value, the better the flux performance. In general, the water-soluble (OAWS) and high-solid rosin (HRNC) no-clean fluxes performed best, along with the low-VOC emulsion flux. Low-solid rosin no-clean fluxes (LRNC) showed the next best results. Low-solid organic acid VOC-free (OANC) fluxes, along with the high-VOC rosin emulsion no-clean fluxes, ranked lowest. Table Al in the Appendix shows detail with respect to specific CTQs/defects.

While the high-residue rosin VOC fluxes performed well, customer acceptance would be low because of cosmetic concerns and in-circuit test pin probability issues. Having identified the best of these fluxes, a possible solution for solder quality on the most difficult boards--particularly vertical fill--is identified if the customer is not concerned about the appearance of residue and has measures in place to remove flux residue for pin probe-ability.

One of the rosin emulsion fluxes (Flux P) showed promise; however, it did not pass Bellcore SIR testing and was removed from consideration. The other rosin emulsion flux, Flux Q, ranked among the lowest performers, and contained 70% VOC, in spite of being listed as a low-VOC flux. There was also evidence that some spray fluxers clogged as a result of the rosin solids of the LVRE type fluxes, which are suspended in the low-VOC solution and are sheared out of solution by the mechanical pump action of the spray fluxer.

The top fluxes in each of the remaining categories were used in the Phase 2 verification runs and are highlighted in Table 8 (Fluxes A, B, D, H).

As already mentioned, Phase 2 verification was performed according to the matrix shown in Table 5. Table 9 shows Phase 2 verification phase defect results. Each CTQ was again normalized and weighted, and the fluxes were ranked accordingly (Table 10).

As indicated, the lower the score, the better the flux performed. Given that the fluxes selected for verification in Phase 2 were among the best on the market, not only was the optimum flux determined for each category, but also the defect data were analyzed to determine process parameters having the greatest influence on the performance of the fluxes.

The following sections summarize many of the process conditions considered critical to product quality and the significant factors contributing to each.

Vertical Barrel Fill

Preheat effect. In general, the hotter the board, the better solder flows into the plated through-holes. The data show that x-ray defects decreased with higher preheat temperature when using water-soluble VOC (OAWS) or rosin low-VOC (LRNC) fluxes, while they increased with higher temperature when using the organic acid no-clean VOC-free fluxes (OANC). This indicated that OAWS and LRNC fluxes are more heat-tolerant than OANC fluxes.

Conveyor speed effect. The slower the conveyor speed, the greater the solder contact time so that the longer the solder has to wet to the barrel walls. This held true for the VOC OAWS and low-solid VOC LRNC fluxes. However, slower conveyor speed increased energy input to the board during preheating. As indicated previously, OANC VOC-free no-clean fluxes were less heat-tolerant. The data showed the lower conveyor speed was better for OANC fluxes, provided the preheat temperature was lower. Vertical barrel fill decreased significantly for OANC fluxes with higher preheat temperature on the 0.125"-thick OSP board for lead-free SnAgCu wave soldering.

Flux selection for improved Pb-free barrel fill. High activity fluxes (OAWS and LRNC) offered ~10% better barrel fill than VOC-free no-clean fluxes on the 0.125"-thick OSP boards. LRNC fill was slightly better than OAWS, but both were significantly better than OANC.

Bridges

Preheat effect. For the low VOC LRNC flux, lower board temperature resulted in lower solder bridging for Flux D. Preheat was a much more significant factor with respect to VOC-free OANC than OAWS VOC fluxes.

By the same reasoning applied with respect to vertical barrel fill, the OANC fluxes appeared to lose activity if overheated and could not perform their function of facilitating solder flow and smooth solder peel-off from the bottom surface of the PWB. This appeared to be a characteristic of organic acids, where the behavior was similar, but higher solid concentration in the OAWS fluxes survived longer than that of the OANC fluxes, leading to reduced bridging.

Conveyor speed effect. For the low VOC LRNC flux, higher conveyor speed resulted in less solder bridging for Flux D. High conveyor speed was better for the OANC VOC-free fluxes by similar argument with respect to vertical fill--more heat energy into the flux resulted in reduced effectiveness. Lower conveyor speed was better for the OAWS flux. Smooth peel-off from the board was more critical than thermal stability.

Flux selection for reduced solder bridges. On average, OAWS VOC fluxes exhibited less bridging than LRNC low VOC fluxes.

OANC VOC-free fluxes had significantly more solder bridging than the other categories under the high energy process conditions: low conveyor speed, low and high preheat temperature. It was only somewhat higher at low conveyor speed and low preheat temperature. LRNC low residue Flux E had an uncharacteristically high rate of bridging with the combination of low conveyor speed and high preheat temperature.

Solder Skips on Glued Bottom-Side Wave-Soldered SMDs

Preheat effect. Higher preheat increased solder skips when using a VOC-free OANC flux, while lowering the incidence of skips with low VOC LRNC and VOC OAWS fluxes. This again was likely due to the sublimation of flux solids during preheat. In general, preheat was a less significant factor.

Conveyor speed effect. As expected, solder skips were more numerous at higher conveyor speeds, but only with the no-clean fluxes, and not to a significant extent.

Flux selection for reduced solder skips. VOC-free OANC fluxes had the best results for solder skips. LRNC fluxes had very few skips. Higher preheat was worse for VOC-free OANC than VOC OAWS flux. By far, the OAWS fluxes generated more solder skips.

Solder Balls

Preheat and conveyor speed were not a factor in the presence or absence of solder balls with the organic acid fluxes. When using low-residue LRNC flux, there appeared to be fewer solder balls at higher conveyor speed.

Flux selection was the only critical factor. OAWS flux by its nature had zero solder balls due to washing of these boards. All boards with VOC-free OANC flux had solder balls. There were also significant differences between fluxes within each category.

Conclusions

As indicated, there are tradeoffs in characteristics within fluxes of the same type and among different flux type categories.

For Pb-free soldering 0.125"-thick OSP boards, the following conclusions can be drawn:

1. For increased Pb-free barrel fill, the water-soluble fluxes are the best to use, followed by low-VOC no-clean fluxes, followed by VOC-free no-clean fluxes.

2. For reduced Pb-free solder bridging for bottom-side wave-soldered SMDs, water-soluble fluxes are best, followed by low-VOC no-clean fluxes and VOC-free no-clean fluxes.

3. For reduced Pb-free solder balls on the bottom side of the wave-soldered board, water-soluble fluxes are best because solder balls are removed by washing, followed by low-VOC no-clean fluxes and VOC-free no-clean fluxes.

4. For reduced Pb-free solder skips on the board bottom-side, VOC-free no-clean fluxes are best, followed by low-VOC no-clean fluxes and water-soluble fluxes.

The data presented indicate much more work to do to improve Pb-free wave soldering results for organic acid VOC-free and low-residue rosin no-clean fluxes.

Future work should concentrate on the development of VOC-free and low-residue rosin no-clean fluxes for lead-free soldering, especially for boards with thicknesses of greater than 0.100". Also, pin probability evaluations need to be conducted more if low-residue, rosin no-clean fluxes are used, as the solids content may affect pin probe results.

References

1. J-STD-004A, Requirements for Soldering Fluxes, January 2004.

2. IPC-A-610D, Acceptability of Electronic Assemblies, February 2005.

Acknowledgments

The authors would like to acknowledge the flux suppliers for proving their wave soldering fluxes for this evaluation. They would also like to acknowledge their internal company colleagues who helped to support much of this work.

Ed.: This article was originally published at SMTA International in October 2007 and is used with permission.

The Appendix is online at circuitsassembly.com/cms/content/view/5895.

Douglas Watson is process development engineer, Jasbir Bath is lead engineer and Pan Wei Chih is process engineer at Flextronics International (flextronics.com); douglas.watson@flextronics.com.
Table 1. Wave Solder Text Vehicle Specifications

PCB Specifications

Board dimension 7" x 5.5"
Board thickness 0.125"
Total layers 16
Surface finish OSP (high-temperature rated)
Laminate material (core) FR-406
Solder mask PSR-4000 (GF5S)
Board Tg ~170[degrees]C (High Tg FR-4)

Table 2. Wave Solder Test Vehicle Components

Description Reference Designator Quantity per Assy.

PTH
64-pin header J2A, J4, J4A 3
DIP16 U40-U42 3
Axial resistor R1-R25 25
Bottom-side SMT
SO16 U1-U12, U30 13
SOT23 D1-D20 20
1206 resistor R50-R88 39
0805 resistor R100-R139 40
0603 capacitor C301-C320 20

Table 3. List of Fluxes Evaluated

Flux Category Carrier Solids J-STD-004 Des.

A OANC [H.sub.2]O 4.50% ORL0
B 3.70% ORL0
C 3.50% ORL1
D LRNC VOC 7.00% ROL1
E 7.00% ROL1
F 4.25% ROM0
G 6.00% ROL0
H 2.90% ORL0
I OAWS VOC 19.50% ORH1
J [H.sub.2]O 7.00% ORH1
K 20.0% ORH1
L 22.4% ORH1
M 19.5% ORH1
N HRNC VOC 15.0% ROM1
O 15.0% ORL1
P LVRE >50% [H.sub.2]O 6.20% ORL0
Q >50% VOC 2.40% ROL0

Table 4. Factors "Critical to Quality" and Impact Levels

CTQ Phase 1 Phase 2

Vertical barrel fill 5 5
Solder bridges 4 4
Bottom SMD skips 2 2
Solder balls 2 2
Residue 1 N/A

N/A -- Not assessed in this phase

Table 7. Flux Residue Score

Score Description

1 No residue
2 Evenly coated with thin residue (minimum white patches)
3 Mostly even but with some slightly brown residue
4 Even, thick residue
5 Uneven, thick residue, flaky brown residue, dross patches

Table 8. Summary of Phase 1 Results

Flux Weighted
Score Group Solids Base Phase 1

A OANC 4.50% [H.sub.2]O 1.072
B 3.70% 1.200
C 3.50% 1.228
D LRNC 7.00% VOC
E 7.00% 0.783
F 4.25% 1.075
G 6.00% 1.169
H 2.90% 1.461
I OAWS 19.50% VOC 0.617
J 7.00% [H.sub.2]O 0.656
K 20.00% 0.632
L 22.40% 0.670
M 19.50% 0.673
N HRNC 15.00% VOC 0.773
O 15.00% 0.767
P LVRE 6.20% VOC 0.918
Q 2.40% 1.326

Table 9. Flux Performance Relative to Each CTQ

 <85% Barrel
Flux Group Fill Solder Bridges Solder Balls SMD Skips

A VOC-free 1623 579 16 0
 OANC
B 1275 590 16 5
D VOC LRNC 484 222 6 6
H VOC OAWS 345 97 0

Table 10. Phase 2 Normalized and Weighted Score

Flux Group Weighted Score

A VOC-free OANC 3.875
B 3.274
D Low VOC LRNC 1.216
H VOC OAWS 0.956

Table 11. Detailed Results from Phase 1 Screening

 IPCJ-STD-004 <85% Solder
Flux Solids Designation Barrel Fill Bridges
 Impact Weight 5 4

A 4.50% ORL0 3420 353
B 3.70% ORL0 2828 568
C 3.50% ORL1 2980 725
D 7.00% ROL1 2759 280
E 4.25% ROM0 2871 616
F 6.00% ROL0 3400 545
G 2.90% ORL0 3395 784
H 19.50% ORH1 2712 338
I 22.40% ORH1 2882 298
J 15.00% ROM1 2605 289
K 15.00% ORL1 2626 367
L 6.20% ORL0 2599 352
M 2.40% ROL0 3450 904
Total Defects Possible 4640 7648

 Solder SMD
Flux Balls Skips Residue Weighted Score
 Lower is better
 2 2 1 Phase 1 Flux Cat.

A 15 39 24 1.072 OANC
B 16 46 28 1.200 OANC
C 16 29 24 1.228 OANC
D 4 30 52 0.783 LRNC
E 7 37 46 1.075 LRNC
F 12 35 48 1.169 LRNC
G 15 56 46 1.461 LRNC
H 0 22 8 0.617 OAWS
I 0 35 8 0.670 OAWS
J 6 35 60 0.773 HRNC
K 1 31 48 0.767 HRNC
L 16 21 24 0.918 LVRE
M 8 37 42 1.326 LVRE
Total Defects Possible 16 4096 64
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Comment:Flux selection for lead-free wave soldering: a DoE clarifies the best types, best available flux within each type, and process limits for each.
Author:Watson, Douglas; Bath, Jasbir; Chih, Pan Wei
Publication:Circuits Assembly
Date:Jan 1, 2008
Words:2297
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