Portland State University JTAG Connector Engineering Design Progress Report March 7, 2011 A Joint Project with the Intel Corporation Hawthorn Farm Campus Group members: David Stepnowski Jeremiah Martin Khaled Alshaiban Long Nguyen Tinh Tran Intel Sponsors: Michelle L. Seale Frank W. Joyce PSU Advisor: Chien Wern Winter 2011
Executive Summary The JTAG project was proposed to the Mechanical Engineering Department at PSU by Intel. Its objective is to design a hand operated device that can reduce the connecting time between a JTAG testing board and a test circuit board. Intel currently employs a process which involves attaching a small testing circuit board, called a surfboard, and block of contact pins using nuts and bolts for attachment to the circuit board to be tested. This process takes three to four minutes. A reduction of the overall connecting time to less than 30 seconds is desired. There is very little space available near the pad contact area of the circuit boards to be tested, which acts as an important constraint on possible designs. The following document outlines the progress of the JTAG project and describes the design process that the JTAG team has followed. The JTAG capstone team has completed the following: a Product Design Specification (PDS) document, internal and external research, and concept evaluation and selection. All milestones were achieved on time. The external research was performed to identify any related products and to quantify their ability to meet the design criteria outlined in the PDS report. None of the products identified in the external research were capable of meeting all the necessary requirements. The internal research phase yielded 17 concepts. Concept screening, evaluation, and selection processes were preformed to reduce the original 17 designs down to one. The Holding Block design received the highest ratings in the selection process and was selected to move onto the detailed design phase. The Holding Block design will incorporate the surfboard and pin block components that are used in the current process. The Holding block design concept is in compliance with the PDS document. The design is expected to: have an application time of less than 30 seconds, exceed the life cycle target of 1000 duty cycles, require no maintenance over the life cycle, and provide a consistent and evenly distributed contact force. A simplified model of the selected design was presented to the customer at Intel and was well received. 1
Contents EXECUTIVE SUMMARY... 1 INTRODUCTION... 3 MISSION STATEMENT... 4 PROJECT PLAN... 4 PDS SUMMARY... 5 EXTERNAL SEARCH... 5 INTERNAL SEARCH... 7 TOP LEVEL DESIGN EVALUATION... 77 DETAILED DESIGN PROGRESS... 11 CONCLUSION... 12 BIBLIOGRAPHY... 14 APPENDICES... 14 2
Introduction JTAG, also known as boundary-scan, is a name given to the IEEE 1149.1 standard (1). The name is taken from Joint Test Action Group, the body that originally created the standard (2). JTAG is used for board level testing. The current method in use at Intel to make board connections for JTAG testing involves the installation of a small circuit board (called a surfboard) to the device under test (DUT). There are two disadvantages of this method that are desirable to improve. Firstly, the surfboard is attached to the DUT with screws and nuts, which require the operator to use a screwdriver to remove the surfboard after every PCB (printed circuit board) is tested. Some DUTs are composed of two printed circuit boards clamped together by nuts and bolts, so they also need to be disassembled before the surfboard can be connected. This installation and removal process is considered overly time consuming. Secondly, the assembly includes many small components that have a possibly of being misplaced. Therefore, the objective of this project is to produce a prototype that will decrease the connection/disconnection time and reduce the probability of lost components while maintaining signal integrity. The JTAG connector must also be capable of attaching to a variety of PCB assemblies as shown in Figure 1. PCB Fasteners DUT Fasteners Surfboard Figure 1: Shows the current method used to fasten a JTAG Surfboard to a bi-level PCB test assembly. 3
Mission statement The JTAG capstone team will design a small hand operated device that will attach testing connectors to a circuit board. The time to apply or remove the testing connectors must be shorter than the currently employed methods. This device should not require operator assembly to function and needs to provide sufficient contact force to maintain signal integrity between the surfboard and the test PCB. This device must not interfere with any existing components on the circuit board and must meet Intel s ergonomic requirements. A working prototype of the connector will be delivered to Intel s representatives no later than June 2011. Project Plan The following project milestones have been completed: PDS report, external/internal research and final concept evaluation. The next phase of the project is detailed design. During this phase of the project the final selected design will be broken down into functional subsystems. Each one of these subsystems will be subject to an individual engineering design optimization process. Models of these detailed designs will be created in a 3D format using the CAD software, SolidWorks. These solid models will be utilized in the fabrication of the first prototypes. System level detailed designs are to be finished and discussed with the sponsors by April 12th. The first prototypes are to be fabricated no later than April 28th, allowing time for design optimization, revision and testing. The final prototype design will be accompanied by a user's guide and other documentation that outlines the functional specifications for the JTAG connector. The deliverables will be handed over to the project team sponsors at Intel during the week of June 13th, 2011. The project schedule may be subject to change, if unforeseen complications arise. Currently, the project is on schedule and within budget. 4
PDS Summary The purpose of the Product Design Specification (PDS) is to focus the efforts of the design team on creating a product that best suits the customer's needs. The PDS report is referenced at every step of the design process to validate design decisions and create design evaluation and selection metrics. Adhering to the process of referencing the PDS, making design decisions, and validating design decisions will ensure a prototype of the JTAG connector that should meet and/or exceed all the customer's needs. A summary of the Product Design Specification table can be found in Appendix 1. External Search The external search presented a wide range of solutions to the JTAG connecter issue. An external search was performed to indentify competing and related products. Currently, there are many products which are designed to provide electrical connections between printed circuit boards. The following are three examples of devices found: 1. Tyco Electronics HXC125 Board-to-Board Connector Mechanism: Using a medium connector between the boards to provide electrical connection, the boards are clamped together by 2 small rods and a spring plate. Comments: This design can provide sufficient contact force and satisfy the space requirement. However, operators would have difficulties in dealing with several small components. Figure 2: Tyco Electronics' HXC125 Board-to-board connector 5
2. Quick Release Fasteners: Mechanism: The printed circuit boards are clamped together by bolts with locks at the ends as shown in Figure 3. The locks are triggered by either turning or pushing the caps at the head of the bolt, depending on the specific model. Comments: This device offers a quick assembly time and is Figure 3: Cam-lock Quick Release Fasteners easy to handle. However, because of the very small size of locking elements, the contact force and clamping force may not be maintained. 3. Bed of Nails: Mechanism: The two boards that need to be connected are held in two separate plates. The operator pulls the lever to bring the plates close together. The pins in the upper plate would provide electrical connections. Comments: Operation on the Bed of Nails is fast and simple with high precision. The main disadvantage of this design is its size. The equipment would not fit within allowable workbench area. Figure 4: X-Test Bed of Nails 6
As can be seen, none of the researched products were capable of meeting all the requirements outlined in the PDS report (Appendix 1). Sub-systems of each design documented in the external search were evaluated and used in later brainstorming sessions. Internal Search The PDS criteria were analyzed, discussed, and resulted in several brainstorming sessions to develop design concepts. These efforts produced 17 designs for the JTAG connector. These designs fell into three basic categories: table mount jigs capable of holding and supporting the test assembly; small hand held connectors that used springs, cams, and levers to apply contact force; and devices that use locking pin assemblies to fasten a surfboard to a test PCB. Top Level Design Evaluation Concept Screening After having produced 17 designs the team began initial screening of the designs. Initially similar designs were combined, giving the combined designs the best features of the similar designs. In the next step of the screening process, each member of the JTAG team was allotted 10 points to grade the feasibility of each design. The five designs that received the highest point ratings were chosen to move on to the concept evaluation phase of the project. Detailed drawing and functional explanations of these five concepts can be found in Appendix 2. Final Design Evaluation The goal of the final design evaluation was to select a single design concept using an unbiased systematic approach. The following steps explain the process that was followed in evaluating and selecting the final design. Seven PDS criteria were selected based on importance and scalability to characterize and evaluate the form and function of the JTAG connector. A 10 point grading system was then used to assign individual weighted values to the seven criteria. Each criterion was then assigned a scalable metric or metric 7
system. The following list shows the seven PDS criteria that were selected, their individual weights, and associated metric or metric system: Application Time; Estimated time spent by an operator to apply the JTAG connector. o Weight: 3.5 o Metric: The PDS criteria target requires an application time of less than 30 seconds. Each team member was timed while acting out a mock assembly process. Their times were normalized with the PDS target value of 30 seconds. This quotient was then subtracted from 1 and then multiplied by 5 to arrive at score. Ease of Use: Educated guess as to the operational ease of use. o Weight: 1.5 o Metric: This criterion was broken down into 3 sub-categories: A.) Required effort to operate the connector (e.g. repetitive motion or application of excessive force). B.) Effort spent on locating the board in the device and aligning the connection holes. C.) Accessibility to the DUT after assembly for further testing. The three sub categories were given individual weighted values based on a 5 point scale. The weighted values of the three subcategories are (A = 2), (B = 3), and (C = 4). o Each design concept was evaluated using the 3 sub-categories and received 0-5 points, 5 being the best grade. The 3 sub-category point values were summed and divided by 15. This normalized point value was then multiplied by 5 to get a total score for the main Ease of use criterion. Life Cycle: The ability of the connector to meet or exceed the PDS target of 1000 cycles before replacement. o Weight: 1.04 o Metric: A thought experiment was done on each of the design concepts to determine likely points of failure. The JTAG team members then assigned 8
each design concept a value of 1-5. A score of 5 indicating that a design would be less likely to fail during the first 1000 operational cycles. Supports the Weight of the Test Assembly: The ability of the design concept to support the weight of the PCB without causing damage to any part of the assembly. o Weight: 0.72 o Metric: A thought experiment was done for each concept design to determine the likelihood of catastrophic failure in the connector. The JTAG team members then assigned each concept design a value of 1-5. A point value of 5 subjectively indicating a design that is impervious to catastrophic failure. Cost: The cost of prototyping, and manufacturing. o Weight: 1.0 o Metric: The total cost is broken down into two categories: prototype cost, and manufacturing cost. Prototype Cost: Assumptions #1: Shop or machining time cost $100 per hour. Each machined feature takes 15 minutes to complete. Refer to Appendix 4 for an example of the cost calculation. Manufacturing Costs: Assumptions #2: Costs are proportional to the prototyping cost depending on the manufacturing processes involved in mass production. The proportionality constant is a value of = 0.1 to 1. A value of = 1 would indicate that each new connector would cost as much as the first to manufacture. The grading of the Cost criteria for each of the design concepts is completed by first calculating the prototyping cost using Assumption #1. The manufacturing cost is then calculated by finding the product of the prototyping cost and the manufacturing cost proportionality constant ( ) using Assumption #2. The sum of the prototyping cost and 9
manufacturing cost is given a relative score of 1 to 5. A score of 5 indicating that the design will be relatively inexpensive compared to the other designs. Refer to Appendix 4 for an example of the total cost calculation. Maintenance: Whether or not the design concept will require maintenance before its life cycle is complete. o Weight: 0.64 o Metric: A thought experiment was done on each of the design concepts to determine the likelihood of required maintenance. Factors taken into consideration during the thought experiment were: number of total parts, number of moving parts, durability of design, and materials used in construction. A subjective value of 1 through 5 was given to each of the designs. A value of 5 indicating that the design concept would not require maintenance. Contact Force: The criterion was subdivided into two separate categories: consistency and distribution. Each sub-category was weighted evenly o Weight: 1.6 o Metric: Consistency and Distribution Consistency refers the ability of the design concept to apply a consistence force over the life cycle of the connector. Distribution refers to the ability of the design concept to apply an evenly distributed load over the contact area. The JTAG team members then assigned each design concept a value of 1-5 for each sub-category. The sum of the two subcategories scores was normalized and multiplied by 5. A high score of 5 for this criterion indicates that the design applies a constant, evenly distributed contact force over the contact area. A low score for this criterion indicates that the design could apply an inconsistent contact force, or one away from the pad area. 10
Design Evaluation Decision Matrix Each design concept was scored on a scale of 1 to 5 using the PDS criteria explained in the Final Design Selection section. The total rating for a design concept was determined by summing the products the PDS criteria weights and design concept ratings. The design evaluation decision matrix is shown below in Table 1. Table 1: Shows the design evaluation decision matrix. Each of the design concepts (A) through (E) were scored on a scale of 1 to 5 according the metrics outlined in section in the Final Design Selection section. A rating of 5 indicates excellent performance and a rating of 1 indicates poor performance. Decision Criteria Weight A) Table Mount Jig (B)Vertical Sliding Clip Design Concepts Rating (5-0) (C) Hook Frame (D) Holding Block (E) Hand Held Clip Application Time 3.50 3 3 3 4 4 Ease of Use 1.50 3 3 2 4 4 Life cycle 1.04 1 3 1 4 2 Supports Weight 0.72 5 5 3 3 2 Cost 1 1 2 4 3 5 Maintenance 0.64 5 5 5 5 5 Contact Force 1.6 2.5 5 4 5 4 Total 10 28 35 30 40 38 Results of Design Evaluation The results of the design evaluation, as shown in Table 1, indicate that the design concept D, the Holding Block, is the highest ranking design. A detailed drawing and a functional explanation for the Holding Block design can be found in Appendix 2 - Concept D. Detailed Design Progress Due to the results of the design evaluation, the group has elected to go with the Holding Block design. This particular design integrates the surfboard and pogo pin block into a 11
holding block. The connecting pins are held in the locked position by springs. The operator pulls up on the control knob thereby creating a gap for the PCB to be inserted. The block is then aligned with the PCB holes and the control knob is released. A pressure plate pushes down on the surfboard and pogo pin assembly with the required contact force. Several issues will be resolved during the detail design process. The block shape will be modified to allow for connection of the test cables to the surfboard. The control knob will be properly sized to meet Intel s ergonomic standards. An external search will be performed to identify any possible off the shelf components which could reduce the total cost of the prototype. Failure points in the design will be analyzed to determine if stress reduction is possible. The materials used in the Holding Block design will be analyzed for mechanical and electrical properties, performance, and cost. To optimize the overall performance of the JTAG holding block, the design will be decomposed into 5 subsystems. Each subsystem will checked against the PDS criteria and analyzed, as necessary, for purposes of material selection and possible form changes. Effects of variations in dimensions and geometry will be considered for purposes of tolerancing. Small scale changes to the parts and subsystems will be considered to better meet PDS requirements. Conclusion The JTAG project is up to date and on schedule up through the Concept Evaluation and Selection phase. A Product Design Specification or PDS report was developed to address design criteria, customer requirements and priorities. This document has been used as a guideline for any decisions made in the steps that follow. In the external search phase of the project, many board to board connectors were studied. The external search verified that an off the shelf, hand-operated connector which meets the requirements defined in the PDS report is not available on the market. Therefore, a design for an original JTAG connector is necessary. 12
In the internal search phase of the project, 17 concepts were generated; five of which were chosen to be evaluated. The seven decision criteria selected from the PDS in order of importance are: application time, contact force, ease of use, life cycle, ability to support board weight, cost (exclude operation time), and maintenance. The seven criteria were used in a decision matrix to select the final design, Holding Block (Appendix 2 Concept D). The Holding Block design concept was checked against the PDS report to assure full compliance. The Holding Block design exceeds many of the PDS target guidelines: reduces application by an estimated 20 seconds, expected to meet and exceed the life cycle target of 1000 duty cycles, require little or no maintenance over the design's life cycle, and provide a consistent and evenly distributed contact force. A simplified model of the final design was presented, explained and well received by the customers at Intel. The next phase of the JTAG project will be detail design, modeling, prototyping, and testing. During the detail design of Holding Block, it will be decomposed into functional subsystems which will be analyzed for optimization. Each subsystem will be subject to an individual design process before a full design is created. Finally, the working prototype will be fabricated for testing and validation. The complete prototype and all documentations are expected to be delivered to the customer by June 2011. 13
Bibliography 1. Corelis Inc. JTAG Tutorial. www.corelis.com. [Online] 2010. www.corelis.com/education/jtag_tutorial.htm. 2. Texas Instruments. IEEE Std 1149.1 (JTAG) Testability. 1997. 3. Intel Inc. Ergonomics Design Guidelines & Assessment Methods. 2009. 14
Appendix 1 Product Design Specification or (PDS) Table Priority Requirements Customer Metric Target Target Basis Verification Performance High Shorter Device Application Time Intel Assembly time (sec) Less than 30 sec Customer Defined Assembly Time Trial High No Electrical Damage Intel Resistivity ~10 8 ohm cm Customer Defined Measurement High Minimal Parts to Assemble During Operation Intel # of Parts to Assemble High Required Contact Force Intel Force (lb) ~0.5 lb Less than 6 Current Process Measurement Customer Defined Estimate Measurement Medium Long Life Cycle Intel # of Testing Cycles Greater than 1000 Customer Defined Cycle Testing Medium Supports the Weight of Test Circuit Boards Without Failure Intel Force (lb) ~ 0.5 lb Customer Defined Measurement Medium Hand Operated Intel N/A No Tools Needed Customer Defined Prototyping Low Connection Versatility Intel N/A Environment Multiple Configurations Customer Defined High Meets Ergonomic Constraints Intel Ergonomics [1] Full Compliance Customer Defined Medium Life in Service Operate in Standard Environment Intel Temperature ( C) & Pressure (atm) Prototyping Measurement / Comparison STP Customer Defined Testing Medium Long Life Cycle Intel # of Duty Cycles Greater than 1000 Customer Defined Analysis 15
Product Design Specification or (PDS) Table, Continued Priority Requirements Customer Metric Target Target Basis Verification Cost Low Low Cost Production Model Intel Cost per Unit in $ $100 per part at lot size of 10 Customer Defined Low Inexpensive Prototype Intel & Team Cost in $ Less than $1500 Customer Defined Quantity Low Mass Production Intel Size and Shape High Weight Medium Materials Fits Within Allowable Footprint Area Must Meet Intel Ergonomics Specifications Manufacturability at Lot Size Lot size of 10 2 5 x 10 mm Pad Intel mm Footprint Intel Force (lb) Full Compliance Customer Defined Team Measurement Customer Documentation Manufacturing Quotes Manufacturing Quotes Manufacturing Quotes Prototyping Measurement High No Electrical Damage Intel Resistivity ~10 8 ohm cm Customer Defined Measurement Low Must Not be Cost Prohibitive Team Cost per Unit in $ Less than $100 per unit Customer Defined Manufacturing Quotes Ergonomics High Must Meet Intel Ergonomics Specifications Intel Company Ergonomic Standards Doc. [1] Full Compliance Customer Documentation Measurement Medium Ease of Use Team Subjective Rating: point scale 1-5 5 Effort, Locating, and Accessibility Observation 16
Product Design Specification or (PDS) Table, Continued Priority Requirements Customer Metric Target Target Basis Verification Quality and Reliability Medium Requires no Maintenance Intel N/A No Maintenance Customer Defined Medium Disposal Low Documentation Supports the Weight of Test Circuit Boards Creates no Disposables Until Life Cycle is Complete Intel Intel Pounds (lb) Life Cycle Greater than or equal to 0.5 lb Greater than 1000 Cycles Customer Defined Customer Defined Manufacturing Quotes Measurement Testing High Final Report PSU Due Date June-2011 PSU Grading High 3D Models and Drawings Intel Project Completion June-2011 PSU Receipt Medium PDS PSU Due Date February 9th PSU Grading Medium Progress Report PSU Due Date March 7th PSU Grading 17
Appendix 2 Internal Search Concepts Concept A, Table Mount Jig Figure 2.1 Conceptual drawing of the Table Mount Jig design. Functional Break Down The figure above shows the Table Mounted Jig with Linear bearings. The entire fixture is located on cammed supports which allow the entire fixture to be raised and lowered. The Base Plate has 3-4 linear bearings fixed vertically to its top surface. There is a fixed stopper plate mounted to the top of the linear bearings. The Surfboard mounting plate is free to move vertically. A hand operated mode of force applies pressure between the Top Stopper Plate and the Surfboard Mount. To operate the device, first the entire fixture is raised or lowered to match the height of the test PCB. The test PCB is then inserted into the fixture such that the edge of the board makes contact with the Edge Locator. The test PCB is then adjusted until the spring loaded locators 18
slide into the PCB's test port holes. Once the test PCB is in place the Surf Board assembly is clamped down into position via a lever mechanism, gravity, spring assembly, or other force applying device. Concept B, Vertical Sliding track Figure 2.2 3-D model of the Vertical Sliding Track design. Functional Breakdown The figure above shows the Vertical Sliding Track Concept. The Surfboard mounting plate can move up and down along the vertical track. The Horizontal PCB Clamp can also travel up and down on the vertical tracks. The pogopin connector and locating pins are fixed to the Surfboard. The surf board clamping mechanism is opened and the test PCB is placed in to the test fixture such that the locating pins and pogo pin connector align with the holes in the PCB. The surfboard clamping mechanism is then aligned and engaged, holding the test PCB in a vertical position. The horizontal PCB clamp is then lowered to make contact with the upper edge of the 19
test PCB. This contact should apply enough force to hold the test PCB in place for the duration of the test. The surfaces that contact the edges of the test PCB will be covered with a non conductive rubber like material. The clamping mechanism could utilize a spring clip, hinge and spring setup, variable height knob, cammed leaver, or other clamping force device. Concept C, Hook Frame Figure 2.3 3-D model of the Hook Frame design. Functional Breakdown The figure above describes the Hooking frame, a device to clamp a PCB, surfboard and pogo pin block together. The pogo pin block and surfboard are placed on the lower plate (with locating edge) and held by two rods with some locking mechanism to ensure they stay in place during assembly operations. The upper plate is held by two straight bars connected to lower plate and can freely move up and down. The PCB will be placed under the upper plate (with a locating edge and locking mechanism). Two hooks are fixed on the lower plate. When the upper plate come to the clamping position, a hook will keep both plates in position and maintain clamping force. 20
Concept D, Holding Block Figure 2.4 3-D model of the Holding Block design. Functional Breakdown The surfboard and pogo pin will be integrated into the holding block. There are springs which force the connecting pins downward. When assembling, the operator uses a finger to pull up the connecting pins, leaving space for the PCB to be inserted. The PCB can be slid back and forth until the two connecting pin are aligned with the holes on PCB. The operator releases the connecting pins when the PCB is properly aligned. When dissembling, the operator pulls the connecting pins up then pulls the PCB out. A model of the concept can be seen in Fig. 2.4. 21
Concept E, Hand Held Clip Figure 2.5 3-D model of the Hand Held Clip design. Functional Breakdown This design uses a subassembly composed of the surfboard, the pogo pin block, and two pins, which after insertion hold the subassembly together. The subassembly is attached to the clamping device/frame. The clamping action is hand operated. The pins that hold the subassembly together also serve to correctly position the pin block over the PCB pad. The operator simply ensures that the pin ends are in the holes found on both sides of the PCB pad. The design can be modified to allow for clamping to be made with one clamp that passes under and over the center of the surfboard and pogo pin block. The arms of the clamping device may need to be modified to meet ergonomic standards. A model of the design can be seen in Fig. 2.5. 22
Appendix 3 Figure 3.1: Shows the Gantt chart for the JTAG project. The table on the left shows the task section, duration, and start and ending dates. The graph on right displays visual representations of the time frame for particular tasks. 23
Appendix 4 Cost Criteria, Calculation Example The following calculation is presented as an example of how the Cost criterion was scored in the Top Level Design Evaluation. The cost of machining a part is based on an estimated labor cost of $100 per hour and that each feature or machined section takes 15 minutes to complete. Using those values, the final cost for each feature is considered to be $25 per feature. It is assumed that the cost of materials used is insignificant relative to machining costs. The figure below represents a fictional part with a number of design features. The total number of features are counted and multiplied by $25 to obtain a cost estimate for the initial machining costs. 2 Holes 3 Sides to Square Up 3 Fillets 1 Ridge 1 Flange Figure 4.1: The figure below represents an example part with a number of design features. All of the machined features are labeled. The label "3 Sides to Square Up" refers to the first operation of making the piece of work square on all sides. 24
Table 4.1: Shows the total number of features and associated cost Features Qty. Cost/Feature Cost $ Holes 2 $25 50 Ridges 1 $25 25 Flanges 1 $25 25 Fillets 3 $25 75 Squared Sides 3 $25 75 Total 10 $25 250 As Table 4.1 indicates the estimated initial fabrication cost for the part shown in Figure 4.1 is $250. The production cost is assumed to be proportional to the initial fabrication cost, materials used and manufacturing process cost. This proportionality is signified by the constant which has a value of 0 to 1. A value of = 1 indicates that the production cost is roughly the same as the initial fabrication cost. For the sake of the costing example it will be assumed that the part will cost roughly = 1/2 of the initial fabrication cost. The following is an example calculation used estimate the total cost of production. Initial fabrication cost = $250 Production Proportionality = 0.5 Lot Size = 10 each Total _ Cost 250 0.5 250 10 $1500 The estimated total cost to fabricate and produce the part shown in Figure 4.1 is $1500. This value is by no means representative of the true cost to fabricate and produce the part. This cost estimating system is only used to compare each of the design cost against each other. 25