|New Mega Ion cleans and tests for cleanliness. |
The U.S. space agency NASA has some of the most stringent electronics requirements in existence, and for good reason. NASA people still shudder over the most tragic example ever of a suspected circuit board cleanliness failure — January 27, 1967, when a circuit assembly in an Apollo command module developed a short circuit in an on-board control system. The resulting fire consumed the entire module and all three astronauts who were on board.
There are doubtlessly many other examples of failed brake systems, pacemakers, weapons systems, and navigation satellites and other electronic systems that we depend on for our everyday lives and well-being. Many a company has learned the hard way that product reliability is directly related to the ionic cleanliness of a circuit board.
Since this is a well-established fact, much time and capital is spent every day on assembly lines worldwide to assure ionic residues are controlled. The biggest question is, which methodology works the best in a given manufacturing environment?
It would be nice to think that an Apollo-type failure could be avoided today by following the current IPC Electronic Industry Standards. In fact just following the current standards can leave electronic systems vulnerable to a host of failure modes related to circuit board residues. Visual inspection often misses residues under capacitors, resistors, and SMT array components. These "spot" residues can be removed by steam or heated IPA/DI water extraction and measured by ion chromatography or electro-migration methods both of which require a trained technician or chemist and a considerable amount of time and effort. Even though these newer analytical tests have the ability to detect these hidden residues, a simpler, easier-to-perform test has held the top position as the most used cleanliness test performed to assess the cleanliness of electronic circuits for many years.
The "ROSE" (resistivity of solvent extract) test has been the industry standard production line test for measuring the cleanliness of circuit assemblies for 45 years. Originally established as a test for military electronic hardware soldered with RMA (rosin mildly activated) fluxes, the test was performed by flowing a 75/25 percent mixture of isopropyl alcohol and de-ionized water across the surface of the circuit and measuring the drop in resistivity of the mixture. The "ROSE" test was incorporated into the military specifications (MIL-P-22809) in the 1970s as a requirement for building military hardware. In the late 1980s the Military adopted industry standards developed jointly through the IPC. This kept the "ROSE" test as the daily standard for class 3 (high reliability electronics). Practicality demanded an automated method of running this test on the production line. Various suppliers quickly responded and developed automated testers where the IPA water mixture was reused by incorporating a de-ionizing mixed bed filter. This remains the standard for conducting daily production tests.
This all worked quite well for some time. Then things changed and there were new fluxes and the new designs. With the introduction of surface mount, spaces became tighter; 1206s became 0201s. Then there were the new fluxes; the water-soluble fluxes in the 1970s, the no-cleans of the 1990s and today's lead-free fluxes. These fluxes contain many new resins that are not necessarily soluble in the 75/25 IPA water mixture. Some have tried heating the mixture to improve solubility with limited success. Exacerbating the problem, the on-going miniaturization of electronic packages is creating further problems with trying to dissolve residues in smaller and smaller spaces.
There are problems with the ROSE test. The test method relies on dissolving the residue to measure the effect on the resistivity. If the residue is not dissolved, then the remaining residue is not detected by the rose method. This limitation leaves companies producing high reliability military and medical hardware with significant exposure. The main problem with the ROSE test today is that it is limited to isopropyl alcohol and water and these solvents do not adequately dissolve most of today's fluxes trapped under SMT components.
Solving ROSE Problems
Many inline and bath cleaners use the resistivity of the rinse water as a "check of board cleanliness". Although rinse resistivity is a good indication of good DI water quality and proper rinsing of cleaning agents and dissolved flux, it is not a quantitative measure of board cleanliness as defined in the ROSE test. To know how much flux residue remains on the circuit board after cleaning one must dissolve the remaining residue.
So why are we not monitoring the resistivity of the cleaning process until all flux is dissolved? Many cleaning chemistries today incorporate reactive alkaline components which are so ionic that they would overwhelm the resistivity measurement during the cleaning cycle. A non-reactive solvent-based cleaning chemistry with solubility parameters selected to match today's flux residues would allow a true cleaner and tester in one operation.
New Solvents Needed
This new approach to combine board cleaning with a quantitative cleanliness assessment requires new solvents with improved solubility to extract these residues. Several new solvents are now available and others are currently being developed.
However, today's testers are limited due to compatibility problems with the ion exchange resins currently used; they are designed to be used with water and very few organic solvents. In addition, they are usually constructed of plastics like acrylics and PVC, which are not generally compatible with stronger organic solvents. New solvents, new resins, in combination with stainless steel construction have led to a new generation of cleaner that can clean all residues and verify true cleanliness levels.
The new Austin American Mega Ion cleaner/tester addresses all of the aforementioned problems. This cleaner is made with stainless steel and is compatible with virtually every non-ionic organic solvent or blend. The ion exchange resins are special resins selected for organic solvent compatibility and patented under US patent #5,733,378.
The Mega Ion process utilizes a highly energetic "spray-under-immersion" which can be combined with heat to further optimize flux removal. In production, a single board is placed in the process chamber. The process chamber is a stainless steel tank with a sealed lid. In step one of the process, the heated solvent blend is pumped from a sealed storage tank to fill the process chamber. Once the tank is filled, the solvent is turbulently circulated by strong spray-under-immersion jets for a fixed time. Starting and ending resistivity are noted. This allows a calculation of total contaminate removed in NaCl equivalence.
In the second step, the cleaning solvent is cleansed of dissolved flux residues and other ionic contamination by circulation through the ion exchange resins. Once the resistivity of the solvent is at original starting levels, a traditional ROSE test can be run and results reported as NaCl equivalents. These data are displayed or stored to a database. If a programmable level of cleanliness is not met, a re-clean can be initiated.
The Mega Ion is designed to serve as a single board cleaner for rework and repair, prototype, ECOs and add-ons. It can also serve as a multi-board cleaner for pre-conformal-coating applications. Solvents designed for use with Mega Ion are specifically formulated for dissolving the residues. Even newer solvent blends are being developed for each flux type or to meet spacing requirements. In addition, the Mega Ion incorporates a closed-tank process which meets the tightest new VOC requirements. The system's explosion-resistant design meets a higher standard than most testers on the market.
Finally, the unit dries the electronics at the end of the cycle — something no other tester does. This feature, along with the closed-tank operation eliminates operator exposure to the testing solvents which are flammable (IPA) or combustible. The Mega Ion is patent pending and is available now.
Contact: Austin American Technology Corp., 401 Industrial Boulevard, Burnet, TX 78611 512-756-4150 E-mail: firstname.lastname@example.org Web: http://www.aat-corp.com