Abstract:
A test vehicle for an SIR testing system is described. A circuit board having a plurality of interleaved circuit patterns is provided with replica components mounted thereon. The interleaved circuit patterns have different conductor spacings to permit a qualitative evaluation of electronic assembly manufacturing processes. The replica components have connection leads which are soldered to circuit pads on the circuit board. The circuit pads are connected into two groups which are used along with the interleaved circuit patterns to evaluate the manufacturing process using SIR measurements.

Description:
This application is a divisional application of U.S. application Ser. No. 08/965,484, which was allowed on Nov. 6, 1997 now U.S. Pat. No. 6,054,720. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a technique for measuring the quality of an electronic assembly soldering process. Specifically, a test vehicle is provided which may be used in a surface insulation resistance (SIR) test to evaluate the quality of an electronic assembly soldering process. 
     The manufacture of electronic circuits includes mounting integrated circuit components onto a single printed circuit board of a module which may be used in a larger electronic system. The circuit board includes conductor patterns which terminate in a connection to either a component, or a connector for making external connections to the circuit board. The process of preparing the board is well known. which can involve a variety of soldering and cleaning techniques. The solder process typically involves the application of flux, solder, and heat to the solderable areas on the circuit board in order to form a soldered connection to components, followed by cleaning to remove unwanted soldering byproducts. The solderable areas include not only connection pads for components, but plated through via holes which extend to the surface of the printed circuit board. The via holes provide a connection between conductors on opposite sides of the printed circuit board. 
     The soldering techniques typically employed to make external connections to the circuit board include solder paste reflow, wave, fountain, thermode, laser, and hot gas soldering. Subsequent removal of post-soldering byproducts is usually accomplished using aqueous or solvent-based chemistries in a cleaning machine, or without cleaning if no-clean soldering materials are used. 
     The process of removing the soldering byproducts becomes more difficult as the density of circuit conductors on the circuit board increases, and as the profile or dimensions of the components on the board and the relevant spacing between the components and the board decreases. An incomplete removal of the soldering byproducts has been known to produce latent electrochemical migration or corrosion effects between conductive elements on the assembly. This is particularly acute when a mildly activated resin or organic acid based flux systems are used. The electrochemical migration or corrosion effects may result in latent product reliability problems, such as open circuits caused by corrosion or electrical shorts resulting from electrochemical migration. 
     One of the techniques which has been developed for evaluating the ability of an electronic assembly process to adequately remove soldering byproducts is known as the surface insulation resistance (SIR) testing technique. The SIR testing methodology has been developed with industry standard specifications controlled by the Electronic Industry Association (EIA) and the Institute for Interconnecting and Packaging Electronic Circuits (IPC). SIR testing employs the use of a special printed circuit board test vehicle which is processed through an electronic assembly manufacturing process. The test vehicle includes four wiring patterns of interleaved conductors which are not normally connected together. The wiring patterns comprise two electrical conductors having a width of 0.016 inches and a spacing of 0.020 inches between the conductors. The test vehicle is subjected to the soldering and cleaning process without any external components or a solder mask which is used in the majority of useable electronic assemblies. Thus, in some ways the standard SIR testing techniques are deficient since the process of removing soldering byproducts from a useable circuit board is effected by the component placement on the printed circuit board. 
     As new component technology reduces the profile of the various circuit packages which are attached to a printed circuit board, the spacing between the printed circuit board and the components gets smaller, presenting a challenge for removing soldering byproducts which may reside between components and circuit board. Whereas previous component bodies are spaced 0.01 inches from the board surface, facilitating the removal of soldering byproducts which enter the space, the newer components provide a spacing of 0.002 inches which increases the difficulty in removing soldering byproducts which may be trapped beneath the component. This in turn increases the risk for latent electrochemical migration or corrosion which cannot be tested using a test vehicle which does not account for the reduced spacing between the circuit board and components. 
     The additional improvements in reducing the size of electrical conductors also presents a greater difficulty in evaluating the potential for electrochemical migration or corrosion. Whereas the previous test vehicles used a conductor width of approximately 0.016 inches and conductor spacings of 0.020 inches, newer printed circuit designs allow for a conductor spacing of 0.006 inches versus the previous 0.020 inch conductor spacing. 
     Given the foregoing improvements in electronic assembly density, a better test vehicle is needed for evaluating the soldering and cleaning processes in this new high density environment. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a SIR test vehicle for evaluating printed circuit manufacturing processes for electronic assemblies. 
     This and other objects of the invention are provided for by a printed circuit board which supports a plurality of interleaved circuit patterns. The interleaved circuit patterns have a pair of spaced apart conductors which terminate on a via hole on the circuit board. The via holes on the circuit board are connected by a conductor within the hole to a conductor on the underside of the circuit board. A replica component having the dimensions and lead configurations of an actual component is supported on the circuit board over at least one of the interleaved patterns. The test vehicle with the replica component is subjected to the manufacturing process through which actual circuit board assemblies are to be fabricated. The interleaved circuit patterns provide an ability to measure the effects of electrochemical migration or corrosion which occurs as a result of unwanted soldering byproducts being incompletely removed from the circuit board surface. The replica components are supported above the circuit board at distances which are contemplated for actual components which are to be used on circuit boards. The effects of soldering byproducts which remain on the electronic assembly, and particularly soldering byproducts which reside between the replica component and the circuit board, may be determined by measuring the resistance between the interleaved conductors of the circuit pattern. 
     In a preferred embodiment of the invention, not only is the resistance between the interleaved circuit patterns measured as an indicator of the manufacturing cleaning process effectiveness, but the replica circuit component connections to the circuit board are divided into two groups, and resistance measurements are made between groups of connections as an indication of the potential for electrochemical migration or corrosion effects. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a test vehicle for making SIR measurements before component placement; 
     FIG. 2 is a side view of FIG. 1 showing component placement on the circuit board; 
     FIG. 3 is a top view of one quadrant of the circuit board constituting the test vehicle; and 
     FIG. 4 is a bottom view of the circuit board quadrant of FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a plan view of a test vehicle circuit board  10  is illustrated. The circuit board  10  is divided into four quadrants  6 ,  7 ,  8  and  9  having identical interleaved circuit conductor patterns and connector configurations. The circuit board of FIG. 1 is intended to include four surface mounted replica components  11  (shown only in FIG. 2) which approximate the actual component configuration which will be on printed circuit boards being assembled in a tested soldering and cleaning process. It is contemplated that the test vehicle  10  of FIGS. 1 and 2 will support replicas of state of the art integrated circuit packages, such as a TQFP package having a height of 0.055 inches. These packages include between  32  and  256  connection leads about the circumference of the package. The minimum spacing of the leads is approximately 0.005 inches. The footprint offered by the foregoing TQFP package is approximately 0.079 inches, and when mounted on a quadrant  6 ,  7 ,  8  and  9  of the circuit board  10 , substantially covers four interleaved circuit patterns  14 ,  15 ,  16  and  17 . The leads  19  around the circumference of the package connect to four sets of circuit board pads  21 ,  22 ,  23  and  24  placed around the circumference of circuit patterns  14  through  17 . In the embodiments shown,  128  connection leads are connected to circuit board solder pads of sets  21  through  24 . 
     Each of the quadrants  6  through  9  of the printed circuit board  10  supports an identical replica component  11  having the same number of leads supported on the same number of circuit pads. Each of the individual quadrants  6  through  9  provide for separate tests of surface insulation resistance once the test vehicle comprising circuit board  10  and mounted replica components  11  have been put through an electronic assembly manufacturing process. 
     The individual quadrants  6  through  9  of the circuit board  10  have a plurality of via holes  32  which form terminations for a pair of conductors of each of the interleaved circuit patterns  14  through  17 . Each of the ends of the conductors for the circuit patterns  14  through  17  are connected to a via hole  36  through  43 , where they are connected to conductors running on the other side of the circuit board  10  to terminals of connector  26  as will be evident from the discussion of FIG.  4 . The significance of the location of the set of via holes  32  is to simulate a circuit board hole pattern which results in flux used in the soldering process entering the space  35  between the replica components  11  and circuit board  10 . Space  35  containing the circuit patterns  14  through  17  represent a challenge for the manufacturing process to effectively remove flux residue or soldering byproducts which may have migrated into the space  35  through the via holes  32  as well as laterally around the component leads  19 . The space  35  may be as small as 0.002 inches using the TQFP circuit packages. 
     FIG. 3 illustrates on a larger scale an individual quadrant  6  of the test vehicle of FIG.  1 . Each of the circuit patterns  14  through  17  is an interleaved pattern of two conductors, and the spacing between the interleaved patterns is different for each pattern. By having differently spaced interleaved patterns on the circuit board, it is possible to quantify the manufacturing process performance as it relates to electrochemical migration or corrosion. 
     The conductors  14   a ,  14   b ,  15   a ,  15   b ,  16   a ,  16   b ,  17   a  and  17   b  simulate the circuit conductor of the printed circuit boards used in state of the art manufacturing processes. The spacing, between conductors of each interleaved circuit pattern may be, for example, 0.006 inches, 0.012 inches, 0.019 inches, and 0.032 inches. 
     Each of the circuit pads sets  21  through  24  comprise individual circuit pads which are grouped into two groups  21 ( a ),  21 ( b ),  22 ( a ),  22 ( b ),  23 ( a ),  23 ( b ) and  24 ( a ) and  24 ( b ). Group (a) and (b) are connected to alternate circuit pads, and to individual terminals of the connector  26 . For instance group  21 ( a ) connects to terminal  50  and group  21 ( b ) connects to terminal  51  through the via hole  49  and conductor  44 . Groups  22 ( a ) and  22 ( b ) are connected to terminals  52  and  53 , groups  23 ( a ) and  23 ( b ) are connected to terminals  54 ,  55  and groups  24 ( a ) and  24 ( b ) are connected to terminals  57 ,  58 . 
     Similar connections are provided for circuit pad sets  22 ,  23  and  24 , with each of these sets of circuit pads divided into two groups, and each group connected to a respective terminal of the connector  26 . Any type of connector may be substituted for the connectors  26  shown. For example, a series of printed conductive pads at the edge of the board may be used so the board can fit into a slot. 
     FIG. 4 shows how the via holes  36  through  42  and  48  and  49  are connected to respective terminals of connector  26  at the edge of the board. A central via hole  56  is left unconnected. This via hole presents a worst case flux or soldering byproduct residue removal scenario since it resides under the center of replica component  11 . 
     The test vehicle in accordance with the preferred embodiment is run through an actual manufacturing process for soldering the components  11  to the boards, including a fluxing operation, a soldering operation, and a subsequent cleaning operation to remove soldering byproducts from the electronic assembly. The quality of each of these processes is reflected in the measured resistance between each of the conductors of circuit patterns  14  through  17 , as well as the resistance between two groups of each of the sets of component pads  21 ,  22  and  23 . Following the manufacturing processes, these resistance values are measured at the connector  26 , and an evaluation of the manufacturing process is made. For instance, soldering byproducts which have migrated underneath the replica components  11  which were not removed during cleaning, may produce corrosion and/or electrochemical migration, affecting the resistance measurements of circuit patterns  14  through  17 . The four quadrants  6  through  9  of the test vehicle provide the capability of independent SIR measurements. The combination of replica components  11 , circuit board conductor patterns  14  through  17 , and their relative position with respect to the via holes provide a rigorous evaluation of the soldering and cleaning process for increased circuit densities. 
     The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only the preferred embodiments of the invention, but as aforementioned, it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings, and/or the skill or knowledge of the relevant art. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.