Abstract:
The present invention provides a solder plating system with automatic monitoring of wash fluid pressure. The system automatically activates an alarm and/or initiates shutdown of a solder plating machine when the pressure reading indicates a failure of the wash fluid supply. The system thereby reduces the number of parts that are affected by failures in the wash fluid supply system. In some cases, problems with the wash fluid supply are detected before any parts are affected.

Description:
TECHNICAL FIELD 
     The present invention generally relates to semiconductor packaging, and in particular to a system and method for solder plating lead frames. 
     BACKGROUND ART 
     A common integrated circuit (“IC”) package type has the IC die encapsulated in plastic with external metal leads. The packaging process begins by placing the IC die on an island of a metal lead frame. FIG. 2 illustrates a typical lead frame  100  with islands  110 . The IC die is connected to leads  120 , with soldered wires for example. At this stage of the process, leads  120  are connected by dambars  130 . To encapsulate the die, two halves of a plastic mold are placed around the IC die and the mold is filled with liquid plastic. As illustrated in FIG. 3, dambars  130  retain liquid plastic  140  within the mold  150 . The plastic  160  that forms between the leads  120  next to the dambars  130  is referred to as “flash.” After the plastic solidifies, the mold is taken away and the dambars and flash are removed. 
     To facilitate soldering of the leads, the metal lead frames are solder plated, usually prior to forming the IC package. After solder plating, lead frames are rinsed with hot de-ionized water. Poorly rinsed lead frames have discoloring due to acid contamination and other impurities, which are potential causes of corrosion and failure to pass solderability testing. 
     Solder plating machines commonly employed include a de-ionized water rinse to remove acid contamination and other impurities. However, parts such as lead frames are sometimes poorly rinsed due to problems in the de-ionized water supply, which causes a loss of material or expensive reworking. Problems with the de-ionized water supply are manifest by fluctuations, decrease, or loss of de-ionized water supply pressure and produce interruptions or variations in the flow of de-ionized water against the parts being rinsed. 
     It is known to provide a solder plating machine with a de-ionized water pressure gauge and to institute periodic inspection of that gauge. The gauge and inspection process sometimes leads to the discovery of problems in the de-ionized water supply. Nonetheless, there remains an unsatisfied need for a solder plating machine and method that reduces the occurrence of poorly rinsed lead frames. 
     SUMMARY OF THE INVENTION 
     The present invention provides a solder plating system with automatic monitoring of wash fluid pressure. The system automatically activates an alarm and/or initiates shutdown of a solder plating machine when the pressure reading indicates a failure of the wash fluid supply. The system thereby reduces the number of parts that are affected by failures in the wash fluid supply system. In some cases, problems with the wash fluid supply are detected before any parts are affected. 
     One aspect of the invention relates to a solder plating system comprising a solder plating machine, a wash fluid supply system for supplying wash fluid to the solder plating machine, a pressure sensor for monitoring the pressure of wash fluid supplied by the wash fluid supply system, and a switch control system for automatically switching an alarm based on one or more readings supplied by the pressure sensor. 
     Another aspect of the invention relates to a solder plating system comprising a solder plating machine, a wash fluid supply system for supplying wash fluid to the solder plating machine, means for monitoring a pressure of the wash fluid supplied by the wash fluid supply system, and means for triggering an alarm when the wash fluid pressure varies outside preset parameters. 
     A further aspect of the invention relates to a method of operating a solder plating system comprising supplying wash fluid for a solder plating machine, obtaining pressure readings for the wash fluid supplied to the solder plating machine, and automatically activating an alarm based on one or more of the readings. 
     A further aspect of the invention relates to a method of operating a solder plating system comprising supplying wash fluid for the solder plating machine, obtaining readings relating to a pressure for the wash fluid supplied to the solder plating machine, and automatically shutting down the solder plating machine based on one or more of the readings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a high level schematic of a solder plating system according to one aspect of the present invention. 
     FIG. 2 is an illustration of a lead frame strip. 
     FIG. 3 is a cross-sectional illustration of the lead frame of FIG. 2 along line  3 , with an IC die mounted on the lead frame and the lead frame and die placed within a mold. 
     FIG. 4 is a high level schematic of a circuit according to another aspect of the present invention. 
     FIG. 5 is a schematic of an exemplary circuit according to a further aspect of the present invention. 
     FIG. 6 is a flow diagram of a operating a solder plating system according to a further aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a high level schematic of a solder plating system  10  according to one aspect of the present invention. Solder plating system  10  comprises alarm  12 , switch control system  14 , wash fluid supply system  16 , pressure sensor  18 , and solder plating machine  20 . Pressure sensor  18  measures the pressure of wash fluid supplied by wash fluid supply system  16 . Pressure sensor  18  produces a signal that communicates the pressure measurement to switch control system  14 . If the signals received from pressure sensor  18  indicate a problem or possible problem with wash fluid supply system  16 , switch control system  14  automatically activates alarm  12  and/or initiates a shutdown of solder plating machine  20 . 
     Solder plating machine  20  can be of any type of solder plating machine, particularly one of those suitable for solder plating lead frames. Such machines are commercially available from companies including AEM_Tech Engineerin Pte Ltd., EBARA Corporation, and PAL Sales, Inc. The lead frames can be of any suitable material, copper being the most common. Solder plating machine  20  can perform such pre-soldering functions as washing and etching of the lead frames. 
     Solder plating machine  20  deposits solder by any suitable means, including electrolysis. The solder is deposited in a layer, typically from about 1 to about 20 microns thick. The solder can have any suitable composition, including, for example, compositions comprising one or more of tin, indium, bismuth, zinc, nickel, copper, silver, gold, and palladium. The solder can include lead or be lead-free. The purpose of the solder coating is generally to facilitate soldering of leads formed from the lead frame and/or to prevent corrosion of the leads. 
     After solder plating, solder plating machine  20  washes the lead frames with wash fluid supplied by wash fluid supply system  16 . Washing removes residues and/or impurities that can cause corrosion and/or failure of a solderability test. The wash fluid can be organic or inorganic, heated or unheated. Generally, the wash fluid is hot de-ionized water optionally containing a surfactant. The wash fluid is generally sprayed against the plated part with some force. To ensure effective washing, a minimum pressure for the wash fluid may be specified. For example, the wash fluid pressure can be specified to be at least about 20 psi, at least about 25 psi, or at least about 30 psi. 
     Wash fluid supply system  16  can be part of solder plating machine  20  or can be a separate system and may supply wash fluid for other units. Wash fluid supply system  16  can comprise one or more of a reservoir, a pump, a filter, and a heater. Where the wash fluid is de-ionized water, wash fluid supply system  16  generally comprises an ion exchange resin and one or more filters. 
     Pressure sensor  18  measures the pressure of the wash fluid supplied by wash fluid supply system  16  and supplies switch control system  14  with a signal indicative of the wash fluid pressure. The signal is generally electrical and can be, for example, a voltage that varies in proportion, or in inverse proportion, to the pressure. The voltage can be stepped up, stepped down, or otherwise scaled to provide a voltage that varies with a desired range. 
     Switch control system  14  receives the signals from pressure sensor  18 . Switch control system  14  can be any device that trips alarm  12  and/or initiates a shutdown of solder plating machine  20  based on signals from pressure sensor  18 . For example, switch control system  14  can be a relay that trips alarm  12  when a voltage from pressure sensor  18  drops below a predetermined value, indicating the wash fluid pressure has dropped below a critical value. On the other hand, where the logic of the switch control system is more complicated, switch control system  14  can comprise a logic circuit. If the decision to switch is based on past readings as well as current readings from pressure sensor  18 , switch control system  14  can be provided with a memory to record past readings. 
     Switch control system  14  can be configured to initiate a shutdown of solder plating machine  20  when readings from pressure sensor  18  fall outside of acceptable parameters, thus suggesting a failure or impending failure of wash fluid supply system  16 . One way to enable switch control system  14  to initiate a shutdown of solder plating machine  20  is to connect switch control system  14  to a shutoff switch provided with solder plating machine  20 . 
     Whether or not switch control system  14  is configured to initiate a shutdown of solder plating machine  20 , switch control system  14  is generally configured to trigger alarm  12 . Alarm  12  can be, for example, an audio alarm, a visual alarm, or both. Where switch control system  14  is configured to initiate a shutdown of solder plating machine  20 , switch control system  14  can be configured to trigger alarm  12  under the same criteria used to initiate a shutdown of solder plating machine  20  or under different criteria. In one aspect of the invention, switch control system  14  triggers alarm  12  based on signals that suggest a failure of wash fluid supply system  16 . In another aspect of the invention, switch control system  14  triggers alarm  12  based on signals that suggest a possible fault in wash fluid supply system  16 . In a further aspect of the invention, switch control system  14  triggers alarm  12  based on signals suggesting an approaching failure of wash fluid supply system  16 . 
     A complete loss of wash fluid pressure, even if rather brief, generally indicates a failure of wash fluid supply system  16 . A failure can also be indicated by a sustained drop of the pressure below a critical value, for example, about 23 psi or about 25 psi. On the other hand, a brief drop in pressure slightly below a critical value can be deemed acceptable or can be treated as suggesting a possible fault in, or an approaching failure of, wash fluid supply system. A gradual steady decrease in pressure while the pressure remains above a critical value can suggest an approaching failure of wash fluid supply system  16 . 
     The interpretation of pressure readings depends on the nature of the wash fluid supply system  16  and solder plating machine  20  and is the type of information that is generally gather through experiences with particular equipment. In this regard, it can be advantageous to provide switch control system  14  with a series of rules that are used to decide when to activate alarm  12  and/or initiate the shutdown of solder plating machine  20 . A rule based switch control system can comprise an expert system, for example. 
     To accommodate the case in which pressure readings are indicative of wash fluid supply failure or predictive of impending wash fluid supply failure, but the relationships are not readily ascertainable, switch control system  14  can be provided with an artificial intelligence system, such as a neural network, Such a system uses historical data on pressure readings and system failures to develop a predictive capability. Optionally, the artificial intelligence system can improve its predictive capability over time by gathering additional data. Once the artificial intelligence system is trained, it is capable of distinguishing those pressure readings that are indicative of wash fluid supply failure or predictive of impending wash fluid supply failure. 
     FIG. 4 is a high level schematic of a circuit  500  according to another aspect of the present invention. Circuit  500  can be used in a solder plating system  10 . Circuit  500  comprises pressure sensor  502 , power supply  504 , switch control system  506 , alarm switch  508 , and alarm system  510 . Power supply  504  supplies power to pressure sensor  502 , alarm  500 , solder plating machine  522 , and control system  506 , where control system  506  requires a power supply. Switch control system  506  receives signals from pressure sensor  502  and controls alarm switch  508  and cutoff switch  520 . Alarm switch  508  switches alarm system  510 . Cutoff switch  520  shuts down solder plating machine  522 . 
     While power supply  504  is illustrated as supplying power to each of pressure sensor  502 , alarm  500 , and solder plating machine  522 , these component can have separate power supplies. In adapting a commercially available solder plating machine into a system according to the present, it is convenient to power the various circuit components using one power source, which can be the solder plating machine&#39;s power supply. Such a power supply provides, for example, about 24 VDC. 
     Likewise, cutoff switch  520  is conveniently a cutoff switch supplied as part of solder plating machine  522 . Such a cutoff can provide an orderly shutdown of solder plating machine  522 . However, cutoff switch  520  can cut off the power to solder plating machine  522  without following any particular procedure. 
     Switch control system  506  can comprise a logic control circuit. A logic control circuit permits the implementation of complex decisions to activate alarm switch  508  and/or cutoff switch  520 . However, a logic circuit is not required and switch control system  506  can comprise one or more switches or relays that control alarm switch  508  and/or cutoff switch  520 , 
     One aspect of the invention relates to solder plating systems in which the switch control system controls only one of an alarm system and a solder plating machine. In this regard, either the alarm switching components or the solder plating machine controlling components are optional. In addition, alarm switch  508  and/or cutoff switch  520  are optional to the extent their functions can be carried out by switch control system  506 . 
     FIG. 5 is a schematic illustration of an exemplary circuit  600  according to one aspect of the present invention. Circuit  600  comprises pressure sensor  602 , circuit board  604 , solder plating machine  606 , audile alarm  608 , and visual alarm  610 . Pressure sensor  602  and solder plating machine  606  plug into circuit board  604  at connection points  612  and  614  respectively. Circuit board  604  is also connected to stop button  607  on solder plating machine  606 , audible alarm  608 , and visual alarm  610 . 
     Circuit board  604  comprises relays  620  and  622 , switches  624 ,  626 ,  628 ,  630 , and  623 , voltage regulator  634 , and timer  636 . Relay  620  is triggered when the output of pressure sensor  602  exceeds a critical value. Relay  620  in turn controls switches  624  and  626 . Switch  624  activates stop bottom  607 . Switch  626  activates timer  636  which, through switch  632 , causes relay  622  to alternately open and close. Relay  622  controls switches  628  and  630 . Power from voltage regulator  634  is provided to alarms  608  and  610  through switchers  628  and  630 . When relays  622  alternately opens and closes, alarms  608  and  610  cycle on and off. Thus, when the voltage from pressure sensor  602  exceeds a critical value, circuit board  604  shuts down solder plating machine  606  and initiates alarm signals, which cycle or pulsate. 
     FIG. 6 is a flow diagram of a method  700  of operating a solder plating system. In step  710 , a pressure measurement is taken from a wash fluid supply. In step  720 , the determination is made whether the pressure has varied outside pre-set parameters. This determination can involve historical pressure measurements or only the most recent measurements. The present parameter can be simply a minimum pressure limit or can be more broadly defined, for example, to permit temporary fluctuations in the pressure below a pre-set limit or to detect a gradual decrease in pressure that may forecast an imminent failure. 
     Monitoring of the pressure continues while the pressure remains within the preset parameters. If the pressure goes outside the preset parameters, a shutdown of the solder plating machine is implemented in step  730  and an alarm is activated in step  740 . In a variation of this method, the step of shutting down the solder machine is skipped or initiated based on different criteria from those employed in deciding whether to trigger an alarm. 
     What has been described above is the present invention and several of its specific aspects. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.