ADVANCE VAPOR DEGREASING OF HIGH RELIABILITY PRINTED CIRCUIT BOARD ASSEMBLIES BY Richard D. Jackson

Transcription

1 ADVANCE VAPOR DEGREASING OF HIGH RELIABILITY PRINTED CIRCUIT BOARD ASSEMBLIES BY Richard D. Jackson I. Purpose of Paper The purpose of this paper is to provide information concerning the cleaning of ionic contamination of printed circuit boards using the Advance Vapor Degreaser (AVD) technology. 11. Company Specifics Collins Avionics Operations Division of Rockwell International is a manufacturer of high quality Commercial Avionics systems. Collins primary market consists of Business and Regional Airline Manufacturers to which it delivers over 4200 products per month. Our production facility manufactures over 10,000 circuit board assemblies each month. Collins places over 3.5 million SMD devices per year and 13.5 million through hole components per year. Board sizes range between 2.0 and 216 sq. inches. While we are not a military contractor, we do follow mil standards including MIL-STD-454 for soldering and MIL-P for cleaning Selection Criteria When defining a cleaning process for our facility, component and assembly characteristics must be considered. Examples include peizo electric crystals, which cannot withstand high drying temperatures required of an aqueous or semi-aqueous system. Complex geometries, ceramic impatt diodes, and RF shields cannot be allowed to trap solvents or water which can promote corrosion. These type of characteristics are common in a high technology/high reliability- electronics assembly facility. Therefore, we set as a criteria that the cleaning system be able to satisfactorily clean critical applications of rosin flux without the use of water. IV. Candidates a.) Aqueous with saponifier or Semi-Aqueous: Because of the concerns surrounding water entrapment these candidates were not considered viabie. b.) HydroChlorofluorcarbon (HCFC): Due to the nature of HCFC's toxicity and the fact that it is an ozone depleting substance, this alternative was not considered viable. c.) AVD: This is the only process that we felt met our criteria of cleaning complex geometries of RMA flux without the use of water or ozone depleting chemistries. 679

2 V. Process Description The AVD process is made up of a cleaning tank and a rinsing tank and two distinct types of chemistries. The cleaning tank contains a combination of solvating agent and rinsing agent. The solvating agent and rinsing agent are physically mixed in the boiling sump. The ratio is approximately 60% solvating agent to 40% rinsing agent by volume. Rinsing agent vapors are condensed and returned to the clean (rinse) sump, from where rinsing agent condensate overflows back into the boil sump. Printed circuit boards are immersed into the boiling sump containing the 60/40 solvating agent/rinsing agent mixture. Soils are dissolved by the solvating agent. Following the cleaning in the boil sump, the PC boards are immersed into the rinse sump, where the solvating agent containing dissolved soil is removed. The solvating agent and dissolved soil float to the top of the rinse sump and are swept over the weir and back to the boil sump. Following immersion in the rinse sump, the PC boards are moved into the vapor zone for a final rinse with rinse agent that condenses from the vapor. Finally, the parts are slowly raised into the freeboard zone where the rinsing agent vaporizes. The cleaning cycle is now completed and the PC board is clean and dry. VI. Solvating Agent The solvating agent is an aliphatic ester blend that has a low vapor pressure and a'high boiling point, along with excellent solvency properties. Material compatibility is excellent except for selective plastics (See Appendix 1). The solvating agent is a non-ozone depleting substance, non-flammable, and has low toxicity. VII. Rinsing agents The rinsing agents are a perfluorinert material. These rinsing agents have excellent rinsing and drying properties, but are poor solvents. We have experimented with two different perfluorinert materials. Both materials have similar properties; high vapor pressure, low toxicity, non-flammable and non-ozone depleting. The main difference between the two rinsing agents is their boiling points. 680

3 ITvDical Properties I RA-6000 Basic Formula Boilins Point 1Liquid Density Vapor Pressure Thermal Conductivity Heat of Vaporization C6F14 56 deq.c 1.68 gm/ml 4.49 PSIA 0.57 w/(cm) (C)x 10*3 b.p. cal/gm Coefficient of 1.6 ml/ (ml)(c) x Thermal Expansion 10*3 1 7 I 11 Surface Tension I12 dynes/cm PF-50/ 70 C7F14 80 deq.c II 1.72 gm/ml 1.53 PSIA b.p. cal/gm 13 dynes/cm I1 I The main draw back of Perflorinert materials is their long atmospheric lifetimes and their potential for global warming. Based on these concerns we have evaluated alternative cleaning techniques. Due to the complex geometries, stringent cleanliness requirements and the need for corrosion free assemblies, perflorinert materials were the only alternative. We planned to control the emissions of Perflorinated chemicals (PFC's) into the atmosphere through the use of efficient machine design. The new type of vapor degreasers required to contain and recover PFC emissfons are much more complex than the traditional vapor degreaser. Sub zero temperature condensing coils, mechanical delivery systems and temperature control mechanisms are all used to contain the PFC's in the cleaning equipment, thus reducing emissions. We had a global warming impact study performed by an independent company. This impact study compared the potential global warming impact if we were to continue to use our existing CFC-113 batch degreasers, versus replacement with the Advance Vapor Degreasing units. Below are the results of this study. : * Present monthly vapor degreaser solvent usage: (Based on 5 batch units and one In-Line unit) CFC-113 = 1320 lbs. 111 trichloroethane = 550 ibs. * AVD rinsing agent required: (Based on 3 AVD 50 hours/ week) PFC (C6F14) = 210 lbs. 681

4 ~ This first table shows the impact that each chemical contributes to the production of equivalent C02 for each pound of chemical. Notice that the CFC-113 and 111 trichloroethane diminish in C02 production from the 20 year span to the 100 year span,'whereas the PFC increases in equivalent C02 from the 20 year to 100 year span. The reason for this is the long atmospheric life span of the PFC chemicals. The C02 generation of the CFC-113 and the 111 trichloroethane begin to diminish because the atmospheric life spans have begun to deteriorate. However, the PFC's continue to generate C02 because of their unique ability to resist decomposition. - MATERIAL 20 YEARS 100 YEARS CFC TRICHLOROETHANE PFC MATERIAL 20 YEARS CFC TRICHLOROETHANE 0.2 PFC YEARS YEAR TIME FRAME 87% 100 YEAR TIME FRAME 82% The use of the AVD units has substantially reduced the amount of C02 that is being released into the atmosphere. This is due to two factors. One is that the centralization of AVD units and the operating philosophy has enabled us to reduce the number of machines required to effectively meet production's needs. Secondly, the machine design minimizes the emission of Perf lorinert material. 682

5 VI11 Ultrasonics Ultrasonics is used in the rinsing sump to aid in the removal of the solvating agent from the PC board. This is necessary on densely populated PC boards and boards which incorporate electrical connectors. We have found that solvating agent will become trapped underneight electrical connectors and other electrical devices that have extremely low clearance from the board surface. The agitation of the rinsing agent alone did not provide enough pressure to drive the solvating agent from underneight these components. Therefore, we incorporated ultrasonics to aid in the displacment of the solvating agent. 1. Frequency: The frequency range of the ultra sonics sweeps between 38 and 42 KHz. This is altered intentionally to eliminate the possibility of setting up a resonate frequency which could potentially cause damage to 2. electronic components. Wire Bond Issue: A particular issue with the use of ultra sonics was that ultrasonics damaged or degraded wire bond strength on active electronic components. I approached Collins Component Engineering department with the specifications of our ultrasonic process, including the frequency and exposure time. We contacted each of our active electronic component device manufacturers and explained what our intentions were and provided them with the facts concerning the ultrasonics. Each of the component manufacturers responded back to us that there was no reason the ultrasonic units in question would have any detrimental affects on their devices. In fact, many of the component manufacturers stated that ultrasonics were used in their facility for cleaning purposes. This correspondence along with a number of published articles (See Reference Material) provided the assurance needed to implement an ultrasonic process. As a control measure, we calibrate our ultrasonic units once per month for frequency range settings. 3. Problems: We have had one component fail in ultrasonics. This is a hermetically sealed germanium crystal diode. The diodes were opening up after the cleaning operation. To prevent further component production failures we moved the insertion of this divice to a post cleaning operation and hand cleaned the part with alcohol. 1x0 Cleaning Results To monitor the cleaning ability of the AVD units we measured ionic contamination using an Omega Meter SMD 600. This device uses a wash solution of 75% by volume of ACS reagent grade isopropyl alcohol and 25% by volume of distilled water. The wash solution is measured through a set of deionization columns to achieve a resistivity exceeding 20 megohm centimeter. A sample is submerged into the clean test solution and the solution is agitated and heated to promote removal and solubilization of all ionic residues from the test sample. The 683

6 change in resistivity is continuously monitored. Knowing the area of the sample in inches, the volume of solution in milliliters and the resistivity in megohms, the system automatically calculates the contamination that has been added to the solution in Ifmicrograms of equivalent Sodium Chloride per square inch". This measurement is compared to standards set by MIL-P "Military Specifications Printed Wiring Assemblies.ff The standard for printed wiring board assemblies is 14 micro grams of NaCl per square inch. 1. Control Board: The board used for cleanliness testing purposes was a double sided surface mount board that has the following physical characteristics. Dimensions: 11.5" x 3.0" Components: IC's = 88 Diodes = 48 Resistors = 54 Capacitors = This board was screen printed with RMA flux paste, auto placed and IR reflowed. The board was then placed in the Omega Meter 600 SMD and the following Ionic contamination was recorded. Xbar = 6.6 micro grams NaCl/sq.in. A sample of twenty boards were then screen printed, auto placed, and IR reflowed using the same equipment. They were then cleaned in an upright degreaser using CFC-113 as its solvent. The cycle time for this operation was: Lower basket into vapor sump = 2ft. /min Vapor soak boards - 2 minutes Wand boards with clean solvent 30 seconds Vapor soak boards - 2 minutes Raise boards into flash off = 30 seconds They were then tested for ionic contamination in the Omega Meter 600 SMD. The following results were observed. Xbar: = 3.98 micro grams NaC1jsq.h. Std. Dev. = The next sample of twenty boards were screen printed, auto placed and IR reflow the same as above. They were then cleaned in an AVD unit that used Solvating Agent 24 and Rinsing Agent The cycle time for this process was. Wash time 2.00 minutes Wash Dwell - 30 seconds Rinse Time 14 minutes Rinse Dwell - 30 seconds After cleaning, the boards were tested in the Omega Meter 600 SMD. The following results were recorded. Xbar = 4.3 micro grams NaCl/sq.in. Std. Dev. =

7 With the above process, solvating agent residue is noticeable. This is especially evident around connectors and coils. 4. The next sample of twenty boards were screen printed, auto placed and IR reflowed using the same equipment. They were then cleaned in an AVD unit that used Solvating Agent 24 and Rinsing Agent 6000 and ultrasonics in the rinse sump for the duration of the rinse cycle. The cycle time for the process was: Wash time minutes Wash Dwell seconds Rinse Time - 14 minutes Rinse Dwell - 30 seconds After cleaning the boards were tested in the Omega Meter 600 SMD. The following results were recorded. Xbar = micro grams NaCl/sq. inch Std. Dev. = The use of the ultrasonics decreased the occurrence of solvating agent residue. However, solvating agent residue was still evident on densely populated PC boards. 5. The next twenty boards were processed using the same equipment as the other boards. The cleaning process was the same except we changed the rinsing chemistry to a Performance Fluid 50/70. This is a higher boiling point Rinsing Agent. The cycle time for this process is as follows: Wash Time - Wash Dwell - Rinse Time - Rinse Dwell minutes 30 seconds 4.5 minutes 30 seconds After cleaning, the boards were tested in the Omega Meter 600 SMD. The following results were recorded. Xbar * micro grams NaCl/sq. inch Std. Dev. = 0.00 With the above process, cycle time reduced dramatically and the need to remove excess Solvating Agent residue is nearly eliminated. 6. The last set of twenty boards were processed using the same equipment as the other boards with the exception that we used ultrasonics in the rinse sump of the AVD unit during the duration of the rinse cycle. The boards were cleaned and the following results were recorded. Xbar - '.136 micro grams NaCl/sq. in. Std. Dev. =.452 micro grams NaCl/sq. in. 685

8 x. Economics The combination of the Performance Fluid 50/70 and the ultrasonics eliminated the residue solvating agent found on earlier PC boards. We have also eliminated the practice of touching up PC boards after the cleaning process. A. Material Cost: Prior to implementation of the AVD units, Collins operated five batch style vapor degreasers using CFC-113 as their solvent. We replaced the five batch style degreasers with three mid-size AVD units based on our capacity study. Machine design is very important in containing the expensive Performance Fluid 50/70. The AVD machines have three sets of condensing coils. The first set, or primary condensing coils, are chilled water (45 deg.f) fed. These coils are approximately 18 inches above the boiling sump and condense the majority of the Performance Fluid. The second set of condensing coils are located directly above the primary coils. These coils operate at below 0 deg.f and insure drag out emissions are minimized. The third set of condensing coils are at the entrance of the machine and are sub zero temperature condensing coils. These coils minimize turbulence inside the machine and further reduce drag out losses. To further reduce chemistry losses, a mechanical arm is utilized to lower and raise the basket in and out of the machine in a controlled fashion. This helps minimize vapor blanket disturbance and drag out that occur due to incorrect ascent and descent rates. The following table demonstrates the material cost of the two processes. The CFC-113 chemistry usage is based on five machines and the PF 50/70 and Solvating Agent 24 is based on three machines. Chemistry Bath Life Usage/Day Matl. Cost CFC week 3.6 GAL/DAY $113.69/gal PF 50/70 infinite.60 GAL/DAY $173.61/gal Solvating 5 mos..45 GAL/DAY $44.81/gal Agent 24 Cost/Yr. $149,388 $38,020 $7,

9 B. Utility Cost: The AVD machines cost more to operate, from a utility standpoint, than the conventional batch style degreasers. This is because of the use of chilled water, heating elements and agitation pumps. The following table compares the utility cost of an AVD machine with that of a batch type degreaser. MACHINE KILOWATTS/DAY $ PER KILOWATT YEARLY COST AVD 120 $. 07/KWH $2,016 CFC $. 07/KWH $302 D. MISCELLANEOUS COST: Indeed, the implementation of an AVD unit into production is much more taxing of resources than implementing a simple single blanket batch degreaser. The installation alone requires a plumber, electrician and process engineer to be successful. These resources must be available to implement and maintain the efficiency of the machine to prevent excessive di scharge of perfluorinert material. The following are estimates of resource requirements needed to maintain the machine efficiency. 1 Skilled Labor Operat ion I Electrician 1. Check.heating elements. 2. Check microprocessor operation General Maintenance 1. Change filters 2. Empty water separator 3. Maintain fluid levels and proportions Refrigeration 1. Clean traps 2. Check operation of refrigeration unit Time Requirement 10 minutes/wk. 1 hour/wk. 30 minutes/wk. 687

10 XI. CONCLUSIONS: 1. Advance Vapor Degreasers use the basic vapor degreaser physics except it is a two chemical process. One chemistry is for cleaning and the second is for rinsing. 2. Cycle times are approximately the same for AVD units in comparison with traditional CFC-113 vapor degreasers. 3. AVD units clean as well if not better than traditional CFC-113 vapor degreasers. 4. The operating cost of an AVD unit is lower than the traditional CFC-113 vapor degreaser. 5. Implementation cost of an AVD unit is higher than a traditional CFC-113 vapor degreaser. 6. The AVD chemistries are non-ozone depleting and non-toxic. 7. The use of machines that are designed for chemical efficiency reduces the effect of global warming when compared to traditional CFC-113 vapor degreasers. 8. The selection of the correct rinsing agent is contingent on the product being processed. 688