A. Field of the Invention
This invention pertains generally to the field of apparatuses and methods useful in assembly line contexts in which articles, wet from a prior assembly or manufacturing steps, are to be dried. Embodiments of the present invention are particularly pertinent to the drying of such assembly line-manufactured or assembled articles which, because of irregular surfaces or complex assemblages of components (such as on printed circuit boards), tend to trap moisture and severely hinder forced air drying by means of any assemblage of drying equipment, and/or requires considerable space-consuming assembly line drying equipment and attendant expense to achieve acceptable levels and rates of component drying.
B. State-of-the-art
The assembly line production of certain products (printed circuit boards being of most relevance to the present invention) involves one or more stages in which the assembled product becomes wet and must thereafter be completely and thoroughly dried. In the context of printed circuit board manufacturing, thorough rinsing of printed circuit boards is required after the solder re-flow stage of production, after which certain metallic particles and residues must be flushed from the printed circuit boards to insure proper operation of the boards. However, after a printed circuit board is rinsed, it must be thoroughly dried for reasons apparent even to those least familiar with electrical components and systems.
Effective drying of printed circuit boards after the rinse stage is a challenge, even without time constraints. Certainly one could, in theory, simply heat a printed circuit board to a level that water is relatively quickly evaporated from circuit board surfaces and components, but excess heat is notoriously injurious to electronic components. In fact, heat dissipation in fully assembled electronic systems is a critical design consideration. Therefore, merely "baking away" moisture is not a preferred, or even practical approach to drying printed circuit boards.
The only categorical alternative to heat-based drying involves the use of forced air. Relying on forced air drying has significant drawbacks, however. The many "nooks and crannies" of printed circuit boards serve as havens for moisture deposits. Effective drying of printed circuit board assemblies requires moving water from such nooks and crannies, and impelling such removed water off of the circuit board, or spreading the moisture over comparatively larger surface areas to promote rapid evaporation. Effectively dislodging the moisture deposits from printed circuit boards is, however, the most significant challenge in effectively and quickly drying printed circuit boards.
The present state of the art dictates that the printed circuit board assembly industry's approach to the need for greater and greater component drying speed (to prevent backlogs of ever accelerating assembly lines) is to add redundant drying equipment to the assembly line. This is not a desirable solution. Assembly line space, particularly in the high tech arena, is a premium resource. Anything which increases the overall length of an assembly line increases overhead costs. Compounding the problem is the added cost associated with the additional drying equipment (whether in the form of additional, independently operating units, or enlarged unitary systems).
One type of approach currently used in the art of conveyorized drying of printed circuit boards (and other assemblies presenting similar drying challenges) involves delivering one or more air streams from one or more delivery manifolds, each of which is commonly termed an "air knife." Each air knife is mounted in relative close proximity to one or both sides of the assembly line conveyor at appropriate locations along the linear progression of the assembly line where drying is appropriate. The air knife manifolds are typically oriented laterally with respect to the direction of conveyance. The air stream(s) from the manifold(s) are emitted via either relatively narrow slot(s), or a series of relatively small holes, and travel in a direction which is relatively normal to the imaginary conveyance plane approximated by the space traveled by the manufactured articles.
When air emanates from air knives from one or more slots, the air supplied to the air knife manifold is typically turbine-driven. When forced air exits an air knife through small holes (rather than slots), air compressors are typically used to deliver air to the air knife manifolds. In either case, the air knife manifolds are positioned at a sufficient distance from the assembly line conveyor to avoid physical obstruction of other manufacturing equipment or processes.
The present inventor has determined through exhaustive research and experimentation that the physical distance between an air knife manifold and a to-be-dried article drastically affects the efficacy of a drying system. This effect is most pronounced with respect to removing liquid deposits from relatively small spaces and/or irregular spaces on or in the assemblies. Nevertheless, in an assembly line context, this distance can be diminished only to a certain degree because of the overall dimensions of the to-be-dried article as it passes under an air knife, and the irregularity of the surface(s) of the to-be-dried-article.
In an attempt to increase drying efficiency and speed, some drying system designers orient air knife manifolds whereby the emanating air streams impinge the to-be-dried articles at other than normal angular orientations relative to the imaginary conveyance plane of the assembly line. Also, seeking the same objective, additional manifolds are added to enhance drying of assemblies. It has been found, however, that such angling of air knife manifolds and/or adding manifolds, although providing incremental improvement in over-all drying efficacy by the end of the assembly line, provides even less than incremental improvement in such efficacy when measured on and added unit by added unit basis. When analyzing the situation from the perspective of a single drying system, re-orientation of air streams is relatively inadequate in substantially improving the removal of entrapped liquid from printed circuit board assemblies.
In summary, neither increasing the volume of drying air in an air drying system, nor impinging a to-be-dried printed circuit board (or other to-be-dried assembly) with air streams of varying relative orientations materially advance the cause of more effectively and quickly drying printed circuit boards and the like.
Another approach found in the art is that of rotating air knife manifolds about an axis which is substantially perpendicular to the imaginary plane of the assembly line conveyor. This approach, which may be combined with one or more of the schemes described above, can be advantageous in reaching relatively open areas between components mounted on the same article, one or the other (or both) of which may obstruct impingement of air upon the relative open area from certain directions. However, such rotation does relatively little to increase the physical removal of entrapped rinsing liquid, and still requires relatively undesirably long drying times.
Another common approach in the existing art is the pre-heating of the drying air in any of the above-described configurations. Although such pre-heating accelerates the evaporation of residual thin films of surface rinsing liquid on assemblies which are to-be-dried, the relative inefficiency of transferring the beat from said air to the entrapped water in the relatively small confined spaces prevents such pre-heating from substantially improving the overall rate of drying assemblies as desired. Furthermore, as referenced above, there are definite limits to the degree to which drying air temperature may be increased without risking damage to electronic components. Further still, there are obvious energy consumption issues surrounding such an approach as this.
Also in the current art, one or more flat plates which extend adjacent tangentially away from the sides of the air knife slots, or through which a series of holes penetrate in the air knife manifolds, are used to confine the lateral and longitudinal air flow between the plates and the articles to-be-dried. For generally flat articles, and assuming a relatively close proximity between the air knives and the to-be-dried articles, this drying systems approach helps to maintain a relatively high velocity of air (or other drying gas) across the surface of the articles. This high air velocity is also generally tangential to the surface of the articles to-be-dried, and substantially increases both the physical removal of, and the rate of evaporation of the rinsing liquid. However, for the conveyorized drying of articles with complex or irregular topographies, or to which are attached components with substantial thickness, this approach to increasing drying effectiveness is largely offset by the requirements of space between the air knife orifices and the to-be-dried articles, which is required to prevent entanglement between the drying equipment and the conveyorized articles. The resulting gap between the air knife orifices substantially reduces the effectiveness of the plates in the entraining the air flow and improving drying efficiency. Furthermore, the irregularity of components attached to a to-be-dried article (components on a printed circuit board, for example) severely restrict the tangentiality of the air or gas flow which might otherwise be is applied to the typically confined areas where residual rinsing fluid is entrapped, and are therefore less than desirably efficient from an energy usage standpoint.
In view of the above, there exists in the manufacturing industry, the electronics assembly industry in particular, a dramatic need for improved methods and associated equipment which would be useful in the drying a conveyorized articles. Improvements are needed both with respect to overall drying efficacy and with respect to speed at which such articles can be dried in assembly line contexts.