Material requirements planning system and procedures for use in process industries

Abstract

A system for controlling the flow of semiconductor products and their components through a production facility including assembly and final testing of a large number of different products with multiple product grades. The system stores information on the demand and inventory of all product grades together with grade distribution data giving the yield of all co-products of a product family from testing the common component of that family. The system includes means for calculating the number of common components to be tested to cover the requirements for all co-products of a family. The distribution data also gives the yield of all by-products of a test for particular products, and the system operates to adjust the inventory status for other products corresponding to such by-products, to avoid excess production of such other products in meeting projected demand. For cases where the data indicate for any product family a surplus of a higher grade and a shortage of a lower grade, the system includes means to decide whether to downgrade a portion of the higher grade to the lower grade; the projected inventory status of each grade is adjusted accordingly.

Claims

What is claimed is:

1. In combination in a system for controlling the flow of a plurality of products and components thereof through the assembly and testing procedures of a manufacturing operation comprising plural sequential process steps, including the development of an assembly and test schedule for the successive periods of a forward planning time frame, and wherein the products are arranged in families comprising different grades meeting respective performance specifications corresponding to different quality ratings, said system including data file means for storing information representing the inventory status of each product grade and the expected demand for each product grade during each of said periods, said system being of the type which is arranged to make calculations with respect to such stored information so as to determine the net requirements for each product grade during each of said successive time periods;

that improvement to such system wherein said data file means stores grade distribution information giving the co-product yield for each grade of each product family from testing the common component of the product family; and

said system further comprising means to calculate, based on said grade distribution information, the number of such common components to be tested for each of said periods in order to meet the net requirements projected for the co-product grades for those periods.

2. A system as in claim 1, wherein said data file means further includes stored by-product information giving for at least certain of the products the identities and percentages of items which will be produced as by-products when testing for any particular one of said certain products selected;

said system further including means to calculate and to store for a projected test of such particular product the number of each such by-products to be produced; and

means responsive to such stored by-product number data for updating the effective inventory of a corresponding item when the net requirements for that item are being developed subsequently.

3. A system as in claim 1, including means operable in making said calculations with respect to said stored information, for determining the net requirements for at least two grades of the same product family, to downgrade a surplus portion of the higher of said grades to the lower grade so as to minimize the need for additional assembly or testing to produce the lower grade.

4. A system as in claim 3, wherein said downgrading decision is made by taking into account the relative demands and yields of the two grades.

5. A system as in claim 1, including means operable, in calculating said number of common components, to decide whether to downgrade a portion of a higher grade to cover the requirements for a lower grade of the product family.

6. A system as in claim 5, wherein the downgrading desicion is based on the relative demands and yields of the two grades.

7. In combination in a system for controlling the flow of a plurality of products and components thereof through testing procedures of a manufacturing operation comprising plural sequential process steps, said system including means for developing an assembly and test schedule setting forth the planning goals for each of the products for the successive periods of a forward planning time frame, each of the products being required to meet respective performance specifications to be designated as such product, said system including data file means for storing information representing the inventory status of each product and the expected demand for each product during each of said periods, said system being of the type which is arranged to make calculations with respect to such stored information so as to determine the net requirements for each product;

that improvement to such system wherein said data file means stores information giving for at least certain of the products the identities and yields of items which are expected to be produced as by-products when testing for any particular one of said certain products, said items corresponding to other products being manufactured in said manufacturing operation;

means to calculate, based on said by-product information, the number of such by-product items which will result from testing for such particular product; and

means for adjusting the effective inventory data for said other products in accordance with said by-product number data.

8. In combination in a system for controlling the flow of a plurality of products and components thereof through the assembly and testing procedures of a manufacturing operation comprising plural sequential process steps, including the development of an assembly and test schedule for the successive periods of a forward planning time frame, and wherein the products are organized in families comprising different grades meeting respective performance specifications corresponding to different quality ratings, said system including data file means for storing information representing the inventory status of each product grade and the expected demand for each product grade during each of said periods, said system being of the type which is arranged to make calculations with respect to such stored information so as to determine the net requirements for each product grade during each of said successive time periods;

that improvement to such system comprising means operable in making said calculations with respect to said stored information, for determining the net future requirements for at least two grades of the same product family, to downgrade a surplus portion of the higher grade to the lower grade and to adjust the effective inventory of both grades.

9. A system as in claim 8, including means responsive to the relative demands and yields of the two grades for making the decision as to whether to downgrade.

10. A system as in claim 9, wherein said responsive means comprising means to calculate, for the two products, the following: ##EQU2## where I is a counter beginning with a current week W

N is a total number of periods (weeks) in a plan

D.sub.I is a demand for the grade in the period

B.sub.I is a by-product supply in the period

Y is a yield to the grade

(N-I+1) is a factor which gives greater weight to near term demand than future demand.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a computerized system for planning the testing of products as part of a manufacturing process. The invention is illustrated for use in planning the testing of semiconductors in the form of integrated circuit (IC) chips.

Semiconductor chips require testing and grading against specifications before they can be sold. The testing of any lot of product will also sort the functional units into separate grades depending on their electrical characteristics. More rigorous or selective testing can then be performed on these grades to produce products which are guaranteed to perform in the more strenuous environment demanded by military or aerospace applications. In addition, certain customers may require special tests to be performed on the product for their specific use.

Production of semiconductors begins in wafer fabrication during which the electronic circuits are grown on thin wafers of silicon in a series of alternating photographic and deposition processes which create hundreds or thousands of identical circuits on each wafer. This process can take from two to upwards to ten weeks depending on the complexity of the process and the number of "layers" involved.

The effectiveness of the wafer fabrication process is dependent on many environmental factors over which the manufacturer has varying degrees of control but which have a definite effect on the functionality and electrical characteristics of the resulting product. In order to eliminate further expensive processing of non-functional circuits, the wafers are probed at this point--i.e. each circuit is powered up, tested and the failures are marked with ink to be later discarded.

After probing, the individual dice are separated, inspected for visual defects and mounted in the individual packages which allow them to communicate with the real world. These packages come in many shapes and sizes from two-lead metal cans to upwards of sixty-four lead dual in-line pin ceramic or plastic packages. Wires finer than a human hair connect the terminals on the dice to the leads on the packages which are then sealed to protect the chip from environmental and mechanical damage.

After this assembly process is complete, the units are tested and graded according to their electrical performance against the range of specifications. The simplest grades are measured at room temperature and are known as commercial. The more difficult grades are those for military and aerospace applications. These must pass strict government standards after being subjected to extreme environmental stress, including continual operation for a week under high temperatures. In addition, the critical characteristics of the higher grade products must not drift out of specification over the whole operating temperature range.

The testing process can be considered as a series of hurdles which the product must pass. Product which fails can be saved to retest against a different set of hurdles, sold as the highest grade for which it has qualified or conceivably sold as a lower grade if no other demand for the product exists.

The process is made especially complex by the large number of different types and grades of product. For example, in one commercial plant, 80 wafer types are assembled into 160 different package configurations which yield over 2500 different end products after testing. The simplest product has 2 grades while the most complex had 88 different grades of standard catalog items and customer specials.

The task for production control is to identify and schedule the correct sequence of tests and an adequate supply of untested material coming out of assembly to support the demand for all of the items. The goal is to accomplish this with minimum inventories and optimal utilization of scarce and expensive computerized test equipment whil maintaining timely and accurate deliveries.

2. Description of the Prior Art

The standard prior art technique for planning the type of production used for semiconductor chips--that is, the technique for specifically determining the requirements for production of the source materials needed in a higher level product--is called Material Requirements Planning (MRP). Such planning is carried out by a computer operating under software control. The computer computations typically are based on a Bill of Materials which defines the source components of any product, the sub-components of such components, and so forth down to the lowest level materials--usually raw materials.

Briefly, Material Requirements Planning performs its planning function by considering the demand for an item and subtracting out any inventories to determine additional production required. This net requirement is then "exploded", that is, multipIied by the appropriate quantities of each component and advanced by the time needed to produce the item. The result is the demand for each component. The process is repeated for each component in turn until the lowest level is reached.

SUMMARY OF THE INVENTION

In the testing process described herein, all untested products are given a first pass test to eliminate non-functional units and produce a preliminary grading. The output from the test becomes input to the military or second pass test. Optionally, the output from either the first pass or second pass test may become input to a test for so-called customer specials. Any product which passes any of the tests can be branded with its identifying grade and sold as a finished product.

The procedure is controlled by a software program operating with a data base defining the proper components for finished goods, customer specials (if used), second pass grades and first pass grades. The procedure resolves three issues in the testing environment which standard MRP does not address. These are: by-product planning, co-product planning, and downgrading, which may be described briefly as follows:

BY-PRODUCT PLANNING

When testing for military grades, some of the product will pass the test. The product which fails is still good product, however, and is called a by-product. Normally this by-product will be another item already in the Bill of Materials and most often the component item itself. Standard MRP does not account for the production of these by-products nor does it consider them as available inventory to be netted out of additional production requirements.

CO-PRODUCT PLANNTING

A special case occurs for first pass grades where two or more grades are simultaneously produced from the same test, and are referred to as co-products. In effect these grades are by-products of each other. Standard MRP which calculates items one at a time sequentially cannot simultaneously plan the component quantity which must be tested to meet the demand for all of these grades together.

DOWNGRADING

Because grading is a natural phenomenon independent of demand, its distribution does not necessarily match the distribution of demand. This may result in surplus inventories of higher grades and shortages of lower grades. There are circumstances where it makes economic sense to downgrade these higher grades to cover the shortages rather than to produce more of the lower grades. Standard MRP always covers shortages by additional production. In the system described herein, additional processing is employed to determine which products, when and in what quantities should be downgraded.

A general purpose computer system operating under software control as described herein resolves the above three issues through logic procedures inserted into standard MRP software.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagrammatic representation of hierarchical levels to which items can be assigned for testing;

FIG. 2 is a flow diagram giving an overview of the entire MRP system as modified to incorporate the present invention;

FIG. 3 is a flow diagram showing the logic of finished goods scheduling represented by functional block 3 of FIG. 2. (Note: the logic is standard prior art MRP and is shown herein only for completeness);

FIG. 4 is a flow diagram showing the logic of customer specials represented by the functional block 4 of FIG. 2;

FIG. 5 is a flow diagram showing the logic of second pass test planning represented by functional block 5 of FIG. 2;

FIG. 6 is a flow diagram showing the logic of first pass test planning represented by functional block 6 of FIG. 2; and

FIG. 7 is a flow diagram showing the logic for planning untested material represented by the functional block 7 of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is based on the concept that in some manufacturing industries there are relationships between the products being processed through testing which make it important and advantageous to plan the testing of certain product families together. To that end, the system to be described categorizes products into different levels so that each level can be processed using the distinctive procedures which apply to that level. A multi-level product structure used in the system to be described is as follows:

Level 0--Finished Goods

Level 1--Customer Specials

Level 2--Second Pass Tested Standard Items

Level 3--First Pass Tested Standard Items

Level 4--Untested Product

Defining these in reverse order (i.e. in the direction in which product actually moves), Level 4 contains untested material as it comes from assembly, and Level 3 contains the grade distribution resulting from the first ambient test of newly assembled product. Typically in the semiconductor industry there are three grades--J, K and L, with J the lowest grade and L the highest. These grades may be branded and shipped as their level 0 branded standard commercial counterparts or issued for higher grade testing.

Level 2 contains the military versions of the commercial grades. The product is stressed, burned in and retested for its drift over a temperature range. An L so tested which retains its electrical characteristics might be called a U. A K might become T and a J and S. The L which fails as a U might still be an L or perhaps have degraded to a T or S, K or J. The exact distribution would. be spelled out on a "lot traveller" accompanying the group of items to be tested and specifying characteristics derived from the Bill of Materials stored in the data base of the computer which controls the flow of product testing.

Level 1 contains customer specials which are produced by additional selective testing of Level 2 or 3 standard products for some customer specific parameters. This level may also contain specials which are made from other specials and thus can be divided into several sublevels. Level 1 can be considered optional, and the processing in Level 1 will not be discussed in detail herein.

Level 0 contains finished goods which are the final branded versions of the products manufactured at one of the lower levels. No additional testing is required to move a product to this level. Due to branding variations, however, several finished products can be made from the same unbranded component.

While this concept of levelling serves to group together products which interact in the planning process and which are planned using the same logic, it also retains the original concept of low level standard MRP code, namely that products are always at a higher level than their components. Level 0 items are always made from Level 1 2 or 3 items. Level 1 items are always made from other Level 1 items or from Level 2 or 3 items. Level 2 is always made from Level 3 which is always made from Level 4.

FIG. 1 presents a simplified exemplary illustration of the type of product-level hierarchy described above (omitting any showing of the "specials" level). It will be seen that the initial group of untested items referred to as B are assembled from other components (not identified in the diagram but listed in the Bill of Materials). A planned first pass test will be expected to produce products graded as J, K, L (identifying prescribed electrical characteristics), in pre-established ratios in accordance with experience data stored in the data base. Rejects will be discarded.

If it is desired to produce a number of items of military grade U, a group of grade L products will be tested. That is, military grade U requires the superior electrical characteristics of commercial grade L, but must be capable of such performance under military environmental conditions. Upon such testing, some of grade L will pass, to be designated grade U. Others will pass the military, requirements but with lower electrical characteristics, and be designated grades S or T (as indicated by the dotted lines). Still others will fail the military requirements, but still pass the electrical tests to be accepted back as grade L or as grades J or K. The final graded products can be branded and transferred to "finished goods".

In standardizing the product flow through test in such a level-oriented fashion, it is desirable to close out all test lots to a stock room after each level of testing is completed. This improves inventory control and lot traceability, and makes it possible to capture the actual grade distribution of all intermediate tests as inventory transactions which later become the basis for up-dating the grade distribution data base.

The grade distribution data base (which is not used in standard MRP planning) contains the planned yield distribution to all grades resulting from testing a component. If it is desired to plan a test for item SC from component JC, the yield must be known. For example, if 100 units of SC are needed and the yield is 50%, the demand for component JC will be 200. In addition, however, the present system takes into account the yield and identity of the by-products of this test. In this case there might be a 45% yield to JC (components which failed the SC test) while the other 5% are scrap, destroyed in the testing process.

A typical grade distribution data base entry might include the following:

    ______________________________________
    PRODUCT       COMPONENT   BY-PRODUCTS
    ______________________________________
    Item  TC          KC          SC    KC   JC
    Yield .40                     .10   .30  .10
    ______________________________________

    ______________________________________
                  Period 1
                         Period 2
    ______________________________________
    Demand          500      500
    Inventory        50
    By-product      100      100
    Net demand      350      400
    ______________________________________

    ______________________________________
    For example:
    Period (I)    3      4      5    6    7    8 (N)
    ______________________________________
    L     Beginning  200
          Inventory
    Y = .3
          Demand      50    150  100  300  500  100
          By-Product 100    100  100  100  100  100
          Supply
    K     Beginning   0
          Inventory
    Y = .4
          Demand     600    500  800  300  900  700
          By-Product  50    100   50   40  200  300
          Supply
          (N - I + 1)
                      6      5    4    3    2    1
    ______________________________________
     1. K has a shortage of 550 in week 3 (600 - 50)
     2. L has a surplus of 250 in week 3 (200 + 100 - 50)
     3. (a) Donor formula = (6 .times. -50) + (5 .times. 50) + (4 .times. 0) +
     (3 .times. 200) + (2 .times. 400) + (1 .times. 0)/.3 4500
     (b) Receiver formula = (6 .times. -550) + (5 .times. 400) + (4 .times.
     750) + (3 .times. 260) + (2 .times. 700) + (1 .times. 300)/.4 27200
     (c) Since (b) is larger than (a) the surplus of 250 L should be downgrade
     to K leaving 300 more K to make from new testing.

    ______________________________________
    Item    Demand   Yield       Test Qty.
                                        Output
    ______________________________________
    L       100      .20         500    200 L
    K       300      .30         1000*  300 K
    J       450      .50         900    500 J
    ______________________________________

    ______________________________________
    Item Demand     Yield   Test Qty.
                                    Output
                                          Downgrading
    ______________________________________
    L    100        .20     500     170 L -70
    K    400        .50     800     255 K +45
    J    850        1.00     850*   425 J +25
    ______________________________________

• Inventors
  Carlson, Jr., William H.; Shafer, Paul H.;
• Assignee
  Analog Devices, Inc. (Norwood, MA)
• Published
  Feb-24-1987
• Current US Classes:
  700/106
  700/95
  705/28
  705/8
• Application #
  665036
• International Classes
  G06F 015/00
• Field of Search
  364/403 364/468
• Examiner
  Smith; Jerry
• Agent
  Parmelee, Bollinger & Bramblett
• US Patent References:
  4459663