Optimizing batch cleaning for removing lead-free flux residues on PCAs
High-energy inline cleaners have typical- a wide compatibility window on materials ing residues from tight gaps. Lead-free and
ly been successful in removing flux in filled of construction; break surface foam at rate ‘no-clean’ fluxes can be particularly chal-
gaps at belt speeds of 0.6fpm
1
to 1.5fpm
2
. greater than foam build; are low in toxicity lenging. The key may lie in the thermody-
Batch cleaners typically have not proven as and odor; and protect metal alloys during namic nature of the residue itself.
successful
3
due to an inherent lower level of the cleaning process. Removing the residues in a batch clean-
physical cleaning energy in comparison to The dynamic rate is the energy force er format requires a different approach.
inline cleaners. applied from the machine and its fluid The research question asked ‘What can be
Establishing process equivalence delivery system. The dynamic cleaning com- done to change the nature of the residues
between inline cleaners and batch clean- ponent is directly related to fluid flow, fluid themselves to further optimize batch clean-
ers assures an equal result in both clean- pressure at the board surface and directional ing rates?’ Of course, we could not refor-
ing processes. This is highly desirable if forces delivered to the surfaces and gaps to mulate the solder paste, but we can change
a company is manufacturing in multiple be cleaned. the modulus of the flux matrix by heating
assembly locations or with different contract Spray-in-air inline cleaning equipment it beyond its softening point. This paper
manufacturers. This leap in batch process provides a platform delivering spray im- describes the results of testing performed to
performance requires rethinking the clean- pingement perpendicular or angled to the evaluate this concept.
ing rate fundamentals. circuit board being cleaned. Batch cleaning
The data findings indicate the benefit designs use spray impingement, spray under hypotheses
of increased wash temperature and time. immersion and ultrasonic energy forces. H : Soft residues require less time to remove
1
Increasing wash temperature approaches The batch cleaning machine dynamic rate flux under the Z-axis
rosin and resin melting points. Approaching commonly applies less energy forces over the H : Wash time is a critical variable when
2
rosin and resin softening points expands surface of the circuit board than does the removing flux under the Z-axis
the residue under the Z-axis. Surface tension inline cleaning machine. H : The rate of residue removal under
3
and temperature effects create a set of forces The dynamic cleaning rate decreases the the Z-axis doubles with 18°F rise in wash
that allow the flux to seep out from under process cleaning rate. In a typical spray- temperature
the component. The cleaning material in-air cleaning machine, the time needed H : Pre-heating the circuit cards before
4
rapidly dissolves and penetrates the Z-axis to clean all residues under the Z-axis is cleaning softens the flux residue and in-
in the absence of high impingement energy. commonly less than 10 minutes of direct creases the cleaning rate
These forces combine to clean flux residues spray impingement. In the absence of fluid
under the Z-axis when processed in batch force, fluid pressure and directional forces Methodology
style dishwasher cleaning equipment. consistently applied to the substrate, residue The research design compared one eutectic
Process cleaning rate removal is inconsistent at best. Additionally, low residue solder paste and five lead-free
The inferences from the cleaning rate flux residues trapped under low standoff low residue solder pastes. Figure 3 illustrates
theory
4
predict two parts to the total clean- components create a flux dam and require the test vehicle populated with eighteen
ing rate; one component is the static rate, energy consistently applied to develop a
the other is the dynamic rate. The static rate wide process window.
plus the dynamic rate equals the process Batch dishwasher cleaning equipment
cleaning rate. This relationship is expressed applies pump pressure and flow to power
in Equation 1. dynamic energy through rotating and fixed
The static cleaning rate is the rate at spray jets. Racking and board placement
which the cleaning material dissolves flux commonly shield some of the assemblies
residues in the absence of impingement en- from spray impingement. The inconsistent
ergy. The static rate is determined by placing dynamic forces applied within the cleaning
the test assemblies in an uncirculated dip chamber create cleaning variability under
tank and calculating the time required to Z-axis components.
dissolve surface flux residues. The static rate Process equivalence
depends upon the residue and the clean- Most batch cleaning processes are
ing agent being used. It is influenced by capable of meeting IPC visual standards on
temperature and, in aqueous solutions, the the exposed surfaces. This has been accom-
engineered cleaning fluid composition and plished by optimizing the cleaning fluids
in-use concentration. and delivery systems. Reaching flux residues
The cleaning fluid design influences trapped under tightly spaced components in
the static cleaning rate. Aqueous engineered a batch cleaner remains a daunting task.
cleaning materials are formulated with sol- The search is on to bring batch cleaners
Figure 3. Test vehicle design.
vating materials, builders that soften or re- to an inline level of performance in remov-
act with the flux residue, wetting agents that
drop surface tension, and minor ingredients
Conveyor
to control foam and protect metal alloys. Direction
Cleaning material design influences the dis-
7
9
5
3
solution rate, saponification, foam propaga-
1
tion, material compatibility, bath life and
10
metal inhibition. Best in class cleaning
8
4
materials dissolve all types of flux residues, 2
including polymerized and charred residues;
penetrate and wet under low standoffs; offer
Figure 4. Progressive energy dynamics.
www.globalsmt.net Global SMT & Packaging – September 2008 – 13
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64