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A NEW DEFINITION OF LOW STAND OFF CLEANING
Harald Wack, Ph.D., Umut Tosun, and Naveen Ravindran ZESTRON America Manassas, VA, USA
[email protected], [email protected], and [email protected],
Joachim Becht, Ph.D., and Helmut Schweigart, Ph.D. ZESTRON Europe
Ingolstadt, Germany [email protected], [email protected]
Dirk Ellis
Speedline Technologies Camdenton, MO, USA
ABSTRACT: The associated increase in the complexity of components in the electronics industry results in a continuous decrease in stand off spacing between the components and the substrate’s surface. At the same time, the requirements for the product reliability and life expectation are continuously increasing, especially in the case of RF Technology.iThis in turn makes cleaning mandatory and the question arises as to which cleaning process can provide the required cleanliness levels under narrow capillary spaces. Furthermore, the contacts that are present under BGAs, micro-BGAs or CSPs pose additional mechanical barriers. As a result the capillary penetration of the cleaning and rinsing agent is hampered. A suitable cleaning process should not only allow the cleaning media ample access to capillary spaces, but it also has to remove contamination and discourage re-contamination. New innovative approaches are now being introduced to further address this increase in cleaning demand. These include innovations on the mechanical as well as on the chemical side. Specific cleaning products have been developed to support lowest cleaning process parameters (i.e. temperature, concentration) but at the same token address overall costs per cleaned part. INTRODUCTION: The emergence of smaller components is placing significantly more strain on all cleaning requirements. Chip components are currently placing the highest demand on removability as gaps are being reduced to less than 1 MIL.
The presence of the latest chip components (i.e. 0201 and 01005) combined with other geometric obstacles has been noticed in the cleaning community. As such challenges emerge, users must therefore first determine that residues are cleanable by the cleaning agent of choice before even addressing this new geometric challenge. This paper is the second in a series of experimental data to be presented that will help users to overcome their respective cleaning issues. These findings are based on specifically designed cleaning fluids (i.e. product technologies) for the highest currently known cleaning requirements. Latest product technologies available in the market were compared to benchmark our findings. Worst-case conditions were used to allow this data to be valid over a period of time. Customer evaluations were also included as an independent verification of our findings and to assure the validity of the data obtained. HYPOTHESES: Extensive internal research and literature review allowed the authors to define effective research hypotheses which will be addressed herein as well as in future studies:
I. Component density can obstruct cleaning ability II. Surface tension is not the only key physical
function of the cleaning agent III. High pressure high volume systems provide the
best results
Theory of Fluid Mechanics: Due to the complexity of the overall topic, with virtually limitless variables (process settings, cleaning agents, soil contaminations, reflow conditions, etc), the authors argue that systematically designed experiments hold most value and can surely be supported by theoretical calculations based on theories such as Navier-Stokes. Fluid flow can be either laminar or turbulent. The factor that determines which type of flow is present is the ratio of inertial forces to viscous forces within the fluid, expressed by the non-dimensional Reynolds Number, where V and D are fluid characteristic velocity and distance. For example, for fluid flowing in a pipe (i.e. spray manifold), V could be the average fluid velocity, and D would be the pipe diameter.
ρVD
R = µ
Figure 1: Equation for the Reynolds Number calculation It is believed that turbulent flows obey the Navier-Stokes equations. Direct Numerical Simulation (DNS), based on the incompressible Navier-Stokes equations, makes it possible to simulate turbulent flows with moderate Reynolds numbers (restrictions depend on the computing
power and efficiency of the solution algorithm). The results of DNS agree with the experimental data. Typically, viscous stresses within a fluid tend to stabilize and organize the flow, whereas excessive fluid inertia tends to disrupt organized flow leading to chaotic turbulent behavior. Fluid flows are laminar for Reynolds numbers up to 2000. Beyond a Reynolds number of 4000, the flow is completely turbulent. Between 2000 and 4000, the flow is in transition between laminar and turbulent, and it is possible to find sub-regions of both flow types within a given flow field. During this study, free-stream fluid velocities (V = 4 m/s), pipe diameter (D = 0.02 m), fluid density (� = 1032 kg/m3) and fluid viscosity (µ =1 x 10-3 kg/m.s) result in a Reynold’s number R = 8 x 104 which is in line with our assumption of “turbulent flow”. Experimental Setup and Data Analysis: Each and every cleaning study should demonstrate meaningful results. Subsequently, the authors applied a methodical approach, by conducting a detailed DOE study in a conveyorized spray-in-air inline cleaner (Figure 2). Also a strong emphasis was placed on presenting findings which are supported by the use of the latest product, hardware, and tool technologies, i.e. cleaning, reflow and inspection equipment.
Figure 2: Schematic of an inline cleaning process
2
1
Dosage Station
3
4 5
6
7
Pre- Wash
Chemical Isolation
Rinse Final Rinse Dryer Dryer Wash
Treatment system Treatment system
�
Phase I
Phase II
Phase III
Phase IV
Phase V
50 � � � �� ��
70 � � � �� ��Spray
pressure (psi) 90 �� �� �� � �
4 (9) �� �� � � ��
Spray manifolds
(spray nozzles)
16 (19) � � �� �� �
15 �� � � � �
18 � �� � � �
24 � �� � � �
Fixe
d P
aram
eter
s
0603 and 0805
component density per
board 30 � �� �� �� ��
Lead-free �� �� �� �� ��Pastes
Leaded �� �� �� �� ��
A �� �� �� �� ��Cleaning agent B �� �� �� �� ��
10 �� �� �� �� ��Concentration (%) 15 �� �� �� �� ��
140 �� � �� � �
150 �� �� �� �� ��Temperature
(°F) 160 �� �� �� �� ��
3.5
�� � � �
1.7
� �
1
�
Var
iabl
e P
aram
eter
s
Conveyor belt speed
(ft./min.)
0.6 � �
Chart 1: Overall experimental overview
Initial cleanability needed to be confirmed for tested residues and given stand offs. The component type was chosen as 0603 and 0805. The stand off was measured to be less than 1 MIL (Figure 3).ii
Figure 3: 0603 component stand off measurement The initial test series (Chart 2) was used to determine best and worst-case settings, which in turn provided the overall basis for all subsequent phases. Results were evaluated by three different chemical engineers to establish repeatability. Results were classified as clean (+), not-clean (-) or partially clean (0) to eliminate any further interpretation. They were then averaged to provide final results. Tests boards were chosen to be industry accepted SABER® boards, as well as internal test boards. Both were found to be interchangeable, as only capacitor areas of the boards were populated. The authors differentiated between fixed and variable parameters for each phase. For example, in phase 1 the spray pressure and number of spray bars was kept constant. Fifteen components (Figure 4) were aligned perpendicular to the media flow, with one space (unpopulated pad area) in between. As previous studies by other companies had aligned fewer (5 or less) and larger components (1210, 1825), the authors argued that an increase of up to 30 components would provide useful data to the reader.
Figure 4: Test board area with 15 sets of 0603 and 0805 components Two different cleaning agents were used in conjunction with two solder paste formulations. The solder pastes were specifically chosen based on the highest level of difficulty to clean (Figure 5). The authors argued that this would not only eliminate lengthy testing with over 100 different paste formulations but at the same token simulate the worst case scenarios.iii,iv Wash temperature and belt speed settings were adjusted as well. A spray-in-air conveyorized inline cleaner was used to provide the most efficient mechanical agitation possible. Two different spray manifolds, one with 4 spray manifolds each having 9 V-Jet nozzles and the other with 16
spray manifolds each having 19 JIC solid stream nozzles were used for the study. Soldering was performed in a 10 stage reflow oven under air-atmosphere.v Reflow under nitrogen had previously been demonstrated to provide significantly better cleaning results. The authors therefore opted for reflow with air to resemble worst-case scenarios. During all experiments only one parameter was changed at a time and the results recorded before the next experiment was conducted.
0603 component before cleaning
0805 component before cleaning
Figure 5: 0603 and 0805 components showing flux residues before cleaning Findings Phase 1: The initial results for lead-free paste formulations demonstrated that the stand off height provided a very difficult, initial challenge to overcome. Tests were started at 10% for cleaning agent A and then temperature and belt speed were adjusted, in a stepwise manner. Interestingly, none of the results provided fully cleaned substrates. The experiments were then conducted at a higher operating concentration to address surface tension considerations. It is commonly assumed that lower surface tension provides better cleaning results.
0805 x15 0603 x15
Fixed Parameters
Equipment Specification Board Specification (0603 and 0805)
Spray Pressure (psi) 90
Spray manifolds
(spray nozzles)
4 (9)
Component density 15
Variable Parameters
Cleaning agent
Board #
Conc. (%)
Temperature (°F)
Conveyor belt speed
(ft./min.)
Waiting time between
reflow and cleaning (hours)
Cleaning Result
2 10 140 3.5 40 -
12 10 150 1.0 18 -
34 10 150 0.6 40 -
18 15 150 2.6 22 -
26 15 150 1 1 -
35 15 160 1 20 -
Lea
d-fr
ee
A
37 15 160 0.6 20 +
6 10 150 3.5 40 -
10 10 150 1 18 0
28 10 160 1 17 0
29 10 160 0.6 18 +
13 15 150 3.5 65 -
Lea
ded
A
17 15 150 1 22 0
104 10 160 0.6 1 0
43 15 150 1 24 -
111 15 160 1 2 - Lea
d-fr
ee
B
45 15 150 0.6 60 +
108 10 150 0.6 1 +
B 48 15 150 1.0 1 0
Lea
ded
49 15 150 0.6 1 + +: Clean 0: Partially cleaned -: Not clean Chart 2: Excerpt of the phase 1 experimental results
Interestingly, at 150°F, higher concentrations and slower belt speed did not change the outcome. Time delays after reflow were addressed as well (boards 13, 17, 18 and 26) and cleaning immediately after reflow also left residues. Increasing the temperature to 160°F then provided satisfactory results at a belt speed of 0.6 fpm. Increasing the time delay after reflow to 20 hours from 2 hours, confirmed the positive findings. Using cleaning agent A, this iterative set of experiments was also conducted for leaded formulations. Again a 10% solution was the starting point, using 150°F and 3.5 feet per minute, which turned out negative. At 1 fpm the flux residues were only partially cleaned (0). Iterations in delay after reflow made little to no difference. The process parameters were slightly adjusted to lower the belt speed, which indeed improved cleaning results significantly. Interestingly for this product technology, a concentration increase to 15% did not improve cleaning results. This confirms that pure surface tension considerations for low stand off cleaning trials are not the only physical variable for cleaning agents to consider (Hypothesis 2). The final belt speed of 0.6 feet per minute at 160°F and 10% provided satisfactory results. All experiments were then repeated with cleaning agent B, which led to slightly different results. Full cleaning could be confirmed at 150°F and 15% for lead-free solder. Time after reflow varied between 2 and 60 hours. Under worst case conditions the obtained process parameters were able to clean products after 60 hours of reflow. Concentrations of 10% and 160°F only provided partial cleanliness (0), even at belt speeds of 0.6 fpm. Cleaning agent B again was able to lower the process parameters for leaded formulations. Temperatures of 150°F with a belt speed of 0.6 fpm and 15% concentration gave satisfactory results. Lowering concentrations to 10% did not require a temperature increase. Best process settings are summarized in chart 3.
Chart 3: Cleaning agent A and B removing lead-free and leaded under low stand off – Phase 1
+ Clean 0 Partially cleaned - Not clean
Conclusions after Phase 1: • The cleaning agent’s ability to clean was fully
confirmed. • Stand off heights of <1 MIL for 0603/0805
components were cleanable at elevated temperatures and extended exposure times.
• Leaded formulation required same process parameters as lead-free, in terms of exposure time.
Findings Phase 2: Having established the overall best process parameters during phase 1, the authors now wanted to determine the impact of increase in component density. Using the lead-free formulation, the process parameters for cleaning agent A were 15%, 160°F and 0.6 fpm. The component density for 0603’s and 0805’s was then increased to 18, 24, and 30, respectively (Figure 6). All results were found to be 100% clean. Switching to leaded formulations further confirmed the cleaning trend with cleaning agent A. Worst case conditions with 30 components (0603 and 0805) in a row formation were cleanable using 10%, 160°F and 0.6 fpm. A time delay after reflow of up to 22 hours made no difference. Fewer components (18 and 24) also showed positive results with the same settings. Partially cleaned results were found with higher belt speeds.
Figure 6: Test boards with 18, 24 and 30 sets of 0603 and 0805 components
0805 x15 0603 x15 0805 x18 0603 x18 0805 x24 0603 x24
0805 x30 0603 x30
Fixed Parameters
Equipment Specification Board Specification
(0603 and 0805)
Spray Pressure (psi) 90
Spray manifolds
(spray nozzles)
4 (9)
Component Density 18, 24 and 30
Variable Parameters
Cleaning agent
Board #
Conc. (%)
Temperature (°F)
Conveyor belt speed (ft./min.)
Waiting time between
reflow and cleaning (hours)
Cleaning Result
Lea
d-fr
ee
A
38 39 40
15 160 0.6 2 +
Lea
ded
A 30 31 32
10 160 0.6 22 +
Lea
d-fr
ee
B
56 57 58
15 150 0.6 4 +
Lea
ded
B 120 121 122
10 150 0.6 4 +
+: Clean 0: Partially cleaned -: Not clean Chart 4: Excerpt of the phase 2 experimental results
Using cleaning agent B, comparable results were obtained. Again, the temperature could be lowered to 150°F without affecting the cleaning results. For leaded formulation, the positive parameters obtained in phase 1 were sufficient for cleaning agent B to remove all residues. Higher belt speeds however did not provide fully cleaned assemblies. Again time delay after reflow played no role in the overall cleaning results. In summary, the authors were surprised to establish that component density did not contribute to the difficulty of the cleaning application. On the contrary, the authors were able to use the same settings while reducing the gap between components to 0.12 inches. The authors were able to conclude that not only was the mechanical energy provided during these experiments fully sufficient, but spray deflection due to less spacing in between components did not affect the positive cleaning results (Hypothesis 1). Additional iterative experiments in this context will be of great interest to determine the precise spacing limits, and should also include the component height by itself. Findings Phase 3: During phase 1 and 2, the authors established satisfactory cleaning windows for various experiments. Different cleaning agents and process parameters however did not allow for an increase in belt speed above 0.6 feet per minute. Therefore, the authors designed experiments to further increase the mechanical energies and improve the throughput.vi All previous tests had been conduced with a 4 spray bar manifold (Figure 7A). Each spray bar is equipped with 9 V-Jet nozzles distanced 2 inches apart from each other. In this phase, the number of spray bars were increased by a factor of 4 (Figure 7B).
Figure 7A: 4 spray bars in cleaning section
Figure 7B: 16 spray bars in cleaning section After confirming successful cleaning results with cleaning agent A the authors used more demanding process parameters for the lead-free solder. Concentrations of 15% combined with a operating temperature of 150°F and 1 fpm did not provide 100% cleanliness. An increase in temperature to 160°F still left white residues under both component sizes. Overall the belt speed could not be increased to 1 fpm even with the 16 spray bar design (Chart 5). However, the concentration could be lowered by 5%. Contrary to lead-free, the leaded formulation did provide better belt speeds for cleaning agent A. Belt speeds of up to 1.2 fpm were achieved. This particular result was obtained with a time delay after reflow of 2 weeks. Immediate cleaning after reflow however did not allow for higher belt speeds than 1.2 fpm. Partially cleaned results were found at 1.5 and 1.7 fpm.
Fixed Parameters
Equipment Specification Board Specification
(0603 and 0805)
Spray Pressure (psi) 90
Spray manifolds
(spray nozzles)
16 (19)
Component density 30
Variable Parameters
Cleaning agent
Board #
Conc. (%)
Temperature (°F)
Conveyor belt speed (ft./min.)
Waiting time after reflow
(hours)
Cleaning Result
66 15 160 1.0 2 0
Lea
d-fr
ee
A
67 10 160 0.6 2 +
71 10 160 1.2 4 weeks 0
Lea
ded
A 73 10 160 1.2 2 weeks +
86 10 160 1 4 -
60 15 150 1 2 0
Lea
d-fr
ee
B
62 15 160 1 2 +
84 10 150 1 3 -
83 10 150 0.6 3 +
Lea
ded
B
59 15 150 1 2 +
+: Clean 0: Partially cleaned -: Not clean Chart 5: Excerpt of the phase 3 experimental results
All tests results in phase 3 were also conducted for cleaning agent B. Again process improvements could be achieved. The desired belt speed of 1fpm at 15% removed all lead-free contamination under both component types. Faster belt speeds left residues under components. A reduction in concentration to 10% only furnished insufficient cleaning results. Process settings for leaded solder could be improved to 1fpm at 150°F for all component sizes. Reducing the concentration by 5% did not allow for higher belt speeds. Belt speed of 1.2 fpm only furnished partially cleaned components. Positive results at 10% for cleaning agent B were only obtained with 0.6 fpm, identical to the 4 spray bar design in phase 1 and 2.
+: Clean 0: Partially cleaned -: Not clean Chart 6: Cleaning agent A and B removing lead-free and leaded under low stand off in phase 3 Conclusions after phase 3 compared to phase 1:
• Cleaning agent A, lead-free � lower concentration, same belt speed and wash temperature
• Cleaning agent A. leaded � higher belt speed, same concentration and wash temperature
• Cleaning agent B, lead-free � higher belt speed and wash temperature, same concentration
• Cleaning agent B, leaded � higher belt speed, same concentration and wash temperature
Findings Phase 4: Since the advent of spray-in-air cleaning equipment, a debate has been ongoing with various philosophies on the relationship between spray pressure to flow volume. Some companies argue that a low pressure provides a better ability for the cleaning agent to penetrate under components, whereas higher pressures are mostly reflected and not always needed. On the other hand though, arguments are also presented that a flow pattern that is highly pressurized with a high liquid volume is the best overall solution. While the authors will not attempt to completely address the overall topic due to its complexity, it was nevertheless interesting to include this discussion in our study. During phase 4, test boards similar to phase 3 (30 components, 16 spray manifolds) were subjected to lower pressures while maintaining high volumetric flow rate.
Fixed Parameters
Equipment Specification Board Specification
(0603 and 0805)
Spray Pressure (psi) 50-70
Spray manifolds
(spray nozzles)
16 (19) Component Density 30
Process Specification Variable Parameters
Cleaning agent
Board #
Conc. (%)
Temperature (°F)
Spray Pressure
(psi)
Conveyor belt speed (ft./min.)
Waiting time after
reflow (hours)
Cleaning Result
78 10 160 70 1 72 -
81 10 160 70 0.6 2 weeks +
77 10 160 50 1 15 -
Lea
d-fr
ee
A
82 10 160 50 0.6 96 0
80 10 160 70 1.2 20 +
Lea
ded
A
76 10 160 50 1.2 18 +
95 15 160 70 1 20 +
103 10 160 70 1 24 0
Lea
d-fr
ee
B
98 15 160 50 0.6 24 +
102 10 160 70 0.6 1 0
94 15 160 70 1.0 20 +
Lea
ded
B
99 15 160 50 0.6 1 + +: Clean 0: Partially cleaned -: Not clean Chart 7: Excerpt of the phase 4 experimental results
For cleaning agent A and lead-free solder, the authors were able to decrease the wash pressure by 20 psi to 70 psi, while cleaning 100% under both components. A subsequent increase in belt speed to 1 fpm was unfortunately not achieved. Best results were found with a top spray pressure of 70 psi. Belt speeds remained at 0.6 fpm with temperatures of 160°F and 10% concentration. At pressures of 50 psi, and 0.6 fpm, residues remained, although only slightly. Switching to leaded solder, the belt speed of 1.2 fpm provided satisfactory results at 70 psi. Here the authors were able to also lower the pressures to 50 psi, while the volume remained high (103 gal./min.). Temperatures in both cases were 160°F with 10% concentration. An increase to 1.5 fpm was not successfully accomplished during this set of experiments. For cleaning agent B, the lead-free formulation was more sensitive to pressure changes. Based on the successful experiments in phase 3 the authors noticed that pressure drops from 90 psi to 70 psi resulted in higher temperatures (160°F) and higher concentrations (15%) to successfully clean under both components types. Lowering the pressures to 50 psi required a belt speed of 0.6 fpm, a concentration of 15% and 160°F for positive results. Similar results were found using cleaning agent B and leaded solder. In order to lower spray pressures to 70 psi, the concentration could not be lowered below 15%, and temperature increase to 160°F was necessary. While 10% concentration and 150°F and 0.6 fpm were cleaning at 90 psi, the pressure drop to 70 psi left behind residues. Lowering the pressures to 50 psi further eroded the cleaning ability in all experiments conducted. Findings Phase 5 During this test phase the board preparation was again identical to phase 3 (30 components, 16 spray manifold). The assemblies were subjected not only to lower pressures but also to lower volumes. To simulate lower spray volumes and pressures, the number of spray arms was subsequently reduced to 4 and spray pressures of 70 psi and 50 psi were used, respectively. Starting at 70 psi and a belt speed of 0.6 fpm, 15% and 160°F was not able to provide good cleaning results for cleaning agent A using lead-free formulations. Lower pressure did not improve findings. Somewhat better results were found with the use of eutectic solder. However slight residues remained under the components at all times. The authors were able to establish under these conditions that a lower pressure and lower flow stream did not support cleaning efficiency (Hypothesis 3). The use of cleaning agent B showed more encouraging results. For example the spray volume reduction provided good cleaning results at 0.6 fpm, 150°F and 15%
concentration (at 70 psi). Reducing the pressures further to 50 psi did not furnish positive results, as residues remained. An increase to 160°F did not affect the latter result. Overall best results for a lowered spray volume were found for leaded pastes again. Spray pressures of 70 psi were able to clean at 1 fpm, 15% concentration and 160°F. A reduction to 50 psi also demonstrated positive results under the same conditions. Attempts to decrease the concentration to 10% were however unsuccessful.
Fixed Parameters
Equipment Specification Board Specification
(0603 and 0805)
Spray Pressure (psi) 50-70
Spray manifolds
(spray nozzles)
4 (9) Component Density 30
Variable Parameters
Cleaning agent
Board #
Conc. (%)
Temperature (°F)
Spray Pressure
(psi)
Conveyor belt speed (ft./min.)
Waiting time after
reflow (hours)
Cleaning Result
125 15 160 70 0.6 1 -
Lea
d-fr
ee
A
128 15 160 50 0.6 1 -
130 10 160 70 0.6 24 0
Lea
ded
A
129 10 160 50 0.6 24 0
113 15 150 70 0.6 20 +
Lea
d-fr
ee
B
119 15 150 50 0.6 24 -
114 15 160 70 1 20 +
Lea
ded
B
116 15 160 50 1 24 + +: Clean 0: Partially cleaned -: Not clean Chart 8: Excerpt of the phase 5 experimental results
Conclusions after Phase 4/5: • High spray pressure and high volume play a very
significant role to ensure wide process window. • Lead-based flux residues are generally easier to
clean. • Sufficient mechanical energy is essential to provide
effective penetration Findings Phase 6: As a final phase, the authors were able to include actual customer data in this study. The leaded solder paste used was identical, while the component sizes were reduced to 0201 and 0603 components, respectively. Stand off heights were measured to be 1-2 MIL. Since these were specifically prepared boards and screened with different stencil, the stand off height varied slightly. Reflow parameters were comparable without the use of nitrogen.
Figure 8A : 0201 chip resistor components before cleaning
Figure 8B : Flux residues under 0201 chip resistor components before cleaning
Figure 8C : 0201 chip resistor components after cleaning with agent A
Figure 8D : Under 0201 chip resistor after cleaning with agent A Distances between each chip resistor component was measured at 2 mm. The number of components was limited to 4 in each row. Two rows of 0201 components were used on a specifically designed test board for this customer (Figures 8A to 8D). For 0603 chip resistor array, a 2x3 matrix was chosen. Overall experimental results were able to confirm fully cleaned assemblies under 0201 chip resistors and 0603 chip resistor arrays. A total of 10 iterative cleaning experiments were conducted with the optimal process settings, which are summarized below:
• 10% concentration of cleaning agent A • Temperatures of 160°F • Belt speed of 1fpm • Time delay after reflow of 3 hours. • High pressures and low flow (4 regular spray
manifolds V-jet spray nozzles) All data obtained during phase 6 confirmed previous observations made. Interestingly, a slight improvement was feasible while using a low volume and high pressure setting. This can be explained by the fact that stand off was more for the components on these boards.
Recommendations With the emergence of harder to clean geometries, the industry has to adapt to the new challenges. This study has clearly highlighted the currently existing challenges, which can be addressed through various means. Cleaning technology providers have to continue to adapt to the changing needs, either on the cleaning agent or on the cleaning equipment side. We have witnessed for example that a large process window was obtained where pressures and flows were the highest. Given this data, new innovations should follow to further increase the cleaning process window. At the same token, specific, new cleaning agents are being developed to further facilitate the penetration under low stand off components. The authors feel that the current overall cleaning window is simply not wide enough. OVERALL CONCLUSIONS Extensive experiments were conducted to address current user concerns related to cleaning under low stand off components. A large variety of variables was chosen which led to over 400 experiments conducted during this study. The authors were able to furnish useful experimental data that allows successful cleaning under frequently used components with very low stand off heights. Worst case scenarios were specifically chosen to allow current users a broad process window with acceptable conditions. This study generally demonstrated an increased level of cleaning difficulty with increasing geometrical densities. It also shown that time between reflow and cleaning was found to be inconsequential up to a time period of 2 weeks. Tests were concluded in an inline equipment at ZESTRON’s Global Inline Center of Competence and Excellence. Cleaning agent A and cleaning agent B are FAST™ and MPC® based cleaning technologies, respectively. ACKNOWLEDGEMENTS The authors would like to thank StenTech and Photostencil for their support during this study. In addition, ZESTRON would like to thank Speedline Technologies for providing the necessary tools and support. A special appreciation goes to ERSA/Kurtz North America for providing a 10 stage reflow oven and an ERSA Scope to ZESTRON’s Engineering Team. REFERENCES i Preventing Contamination-Induced Assembly Failure. SMT Magazine, 2002. Schweigart Ph.D.; Muelhbauer Ph.D. ii An ErsaScope (Imagedoc,, V1.2.111) was used to measure the stand off height. For 0603 the stand off height was 1.0 MIL, while for 0805 the stand off was found to be 0.8 MIL iii With over 100 formulations and over 10 variables, 10010 experiments can be performed.
iv The authors started to screen eutectic formulations in the early 1990, and lead-free products in 2000. Since then over 600 currently available products have been successfully tested. v The Aquastorm 200 and an HotFlow 10 stage reflow oven situated at Zestron’s Application Technology Center was used to conduct all experiments. vi For cleaning application without low stand off components, belt speeds of 2-2.5 fpm are typical.