Optimizing batch cleaning for removing lead-free flux residues on PCAs
1210 chip cap resistors and eighteen 1825 Table 1 lists the factors used to process ranged from 40-60% flux residue removed
chip cap resistors. Both the 1210 and 1825 the three sets of test boards. under the chip caps. For the 1825 chip
chip caps are sealed on two sides with nine Seven sets of test boards were processed caps, cleaning was closer for the LFHR and
caps each placed with the opening in the in a programmable electronic assembly LFSR and ranged from 50-75% flux residue
horizontal position and nine caps each aqueous batch dishwasher cleaning ma- removed under the chip caps. The data find-
placed with the opening in the vertical posi- chine. The stainless steel chamber contains ings indicate that soft residues were more
tion. The strategic placement of the caps a heating element that elevates the wash easily removed, which is consistent with the
shields the egress of the cleaning material to cleaning material to desired operating tem- first research hypothesis.
the soil with six caps shielded on one side, peratures. Due to the limitations of shield- Inline Test 2 processed the boards at 0.7
six caps shielded on two sides and six chip ing and inconsistencies of spray impinge- FPM (4.28 minutes wash time). The mean
caps with no shielding. ment across all board surfaces, the variables value of the LFHR pastes cleaned under
During reflow, the surface tension of tested were wash temperature, wash time, 1210 chip caps ranged from 95-100% flux
the flux residue covers the entire Z-axis and wash concentration. One set of boards residue removed under the chip caps. The
under the 1210 chip cap. This forms a flux was placed in an oven to pre-heat the boards LFSR pastes cleaned under the 1210 chips
dam and prevents fluid flow under the cap at 200°F to determine if the pre-heat softens caps was 100% removal. For the 1825 chip
until the dam is removed from both the stat- the flux residue and promote easier removal caps, cleaning under the LFHR ranged from
ic and dynamic cleaning forces. The 1825 is during processing. 70-96% flux residue removed under the
a larger chip cap resistor that is packed with The wash cleaning solution took time chip caps. For the 1825 LFSR, 99% of flux
flux residue, but not all the caps are totally to reach the upper temperature set point. residue was removed under the chip caps.
filled. Some of the 1825s form a flux dam When transferring the wash material The data from Inline Test 2 correlates with
and others leave a small channel for clean- from the holding tank, five minutes were the second research hypothesis that infers
ing material to penetrate and flow. required to increase the wash temperature wash time and soft residues are critical
Of the five lead-free solder pastes se- from 130-150°F; 10 minutes to increase the variables for cleaning under the Z-axis.
lected, three form hard residues. Removal of wash temperature from 130-175°F, and 15 Inline Test 3 processed the boards at
hard residues typically requires longer wash minutes to increase the wash temperature 0.3 FPM (9.0 minutes wash time). There
times. Cleaning takes the form of concen- from 130-200°F. Table 2 lists the factors was an oversight when processing this set
tric cleaning action, similar to peeling an used to process the seven set of test boards. of test boards. The wash was not up to
onion. Two of the lead-free solder pastes temperature with the boards being pro-
form soft residues, which dissolve into the Data findings cessed at a temperature range of 130-140°F.
cleaning solution at a faster rate. Clean- All 1210 and 1825 chip cap resistors were This resulted in two changed variables of
ing takes the form of channeling, with the removed from the processed test boards. For wash time and wash temperature. The mean
dynamic energy pushing the cleaning fluid this paper, the mean values of the flux resi- value of the LFHR pastes cleaned under
through the soils, which promotes rapid dues left under the chip caps are reported. 1210 chips caps ranged from 55-99% flux
dissolution. The selection of hard and soft The boards were inspected with 10-30x and residue removed under the chip caps. The
residues is a criterion used when designing graded by a qualified expert. LFSR pastes cleaned under 1210 chip caps
for manufacturability. The six solder pastes use the follow acro- was 100% flux removal under chip caps.
The factorial experiment evaluated the nyms in the data sheets. For the 1825 chip caps, the LFHR removed
variables of wash time, wash temperature 69-92% and the LFSR removed 92-94%
Eutectic Low Residue: ELR
and wash concentration. The engineered flux residue under the chip caps. Based on
Lead-Free Hard Residue: LFHR
cleaning material was evaluated at a concen- the data findings from Inline Test 2, we
Lead-Free Soft Residue: LFSR
tration of range of 9-18% with 2% inhibi- would have anticipated 100% clean boards
tor added sump-side. The inhibitor design
spray-in-air control test boards
at the longer wash time. The impact of
prevents dulling of solder propagated when wash temperature correlates with the third
Inline Test 1 processed the boards at 1.5
exposing the circuit assembles to long wash research hypothesis, which suggests that the
FPM (two minutes wash time). The mean
times and high wash temperatures. rate of residue removal doubles with 18°F
value of the LFHR pastes cleaned under
As a baseline for removing all flux rise in wash temperature.
1210 chip caps ranged from 25-40% flux
residues under the Z-axis, three sets of test
residue removed under the chip caps. The
boards were processed as controls using an Batch-dishwasher-processed
LFSR pastes cleaned under 1210 chip caps
aqueous inline cleaning machine. The same
engineered cleaning material was fixed at
a concentration of 18%. No inhibitor was
Batch Pre-heat @ Wash Total
added. The inline wash used progressive
Test 200°F Wash temperature concentration wash time
energy dynamics designed to improve Z-axis
Test 1 130-150°F 18% 15 minutes
penetration (Figure 4).
Test 2 130-150°F 18% 40 minutes
Test 3 130-175°F 18% 25 minutes
Inline Wash Wash
Test temp. FPM time
Test 4 130-200°F 18% 40 minutes
Test 1 145-150°F 1.5 2.0 mins
Test 5 130-200°F 9% 40 minutes
Test 2 145-150°F 0.7 4.28 mins
Test 6 130-200°F 5% 40 minutes
Test 3 130-140°F 0.3 9.0 mins
Test 7 10 min. 130-200°F 18% 40 minutes
Table 1. Spray-in-air inline factors.
Table 2. Batch dishwasher factors.
14 – Global SMT & Packaging – September 2008
www.globalsmt.net
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