Section 1 Design Considerations

1. Section 1 Design Considerations IPC Designer Certification Study Guide

2. Section 1.1Interrelated Considerations for Design Design Considerations

3. The end product requirements are the characteristics of an individual part or assembly in its final completed state. To ensure the part or assembly will work as intended, the environment in which it will operate must be known at the time of the design. Interrelated Considerations for Design - 1.1

4. Equipment environmental conditions such as ambient temperature, heat generated by components, ventilation, shock, vibration, etc., necessitate different materials, tolerances, and final product configurations. 2221 3.2.1 SM782 3.5.2 3.5.3 Table 3-6 Interrelated Considerations for Design - 1.1

5. To facilitate communication between the designer and manufacturers, different performance classes have been developed to reflect progressive increases insophistication, functionalperformance, and frequency and/or intensity of inspection or stresstesting. Interrelated Considerations for Design - 1.1

6. These are identified by a class designation where: • Class 1 is defined as General Electronic Products • Class 2 is defined as Dedicated Service Electronic Products • Class 3 is defined as High Reliability Electronic Products Interrelated Considerations for Design - 1.1

7. Each has its definition which can be related to end use environments such as computers, telecommunications, aerospace, or automotive applications. The user has the responsibility to determine the class to which their product belongs. 2221 1.6.2 2222 1.5.1 Extra 1.1 Interrelated Considerations for Design - 1.1

8. Producibility levels have also been established to help communicate the design complexity to the manufacturer. These levels reflect progressive increases in sophistication of tooling, materials or processing and, therefore progressive increases in fabrication costs. Interrelated Considerations for Design - 1.1

9. There are three levels of producibility: Level A: General Design Complexity -- Preferred Level B: Moderate Design Complexity -- Standard Level C: High Design Complexity -- Reduced Producibility Interrelated Considerations for Design - 1.1

10. Interrelated Considerations for Design - 1.1 2221 1.6.3 Most printed board manufacturers can produce Level A products at a very high yield and, therefore, at a reasonable cost. The number of available manufacturers that have the precision capability to manufacture product at the C level drops to approximately 20%.

11. Interrelated Considerations for Design - 1.1 The number of manufacturers able to accommodate designs that are state-of-the-art and need even greater sophistication of tooling, materials, and processing is around 1%. State-of-the-art technology cannot be standardized due to the fact that every few years, the levels shift causing that which was moderate to become general, and so on.

12. Interrelated Considerations for Design - 1.1 Since the design is intended to meet all the requirements of the product including performance, cost, reliability, etc., all of the issues must be discussed at the beginning of the designprocess. These discussions should include manufacturing engineering for both the board assembly and test.

13. If a company wishes to stay competitive with the products they sell to the customer, the days of tossing the design “over the wall” are gone. The designer must be aware if “backward compatibility” is required. This means the design must be able to be used in any past installation without modifications. Extra 1.2 Interrelated Considerations for Design - 1.1

14. The physical constraints of the installation interface are the primary consideration when redesigning a board for a product already in the field. Thus, design for excellence is design for producibility and involves all the disciplines needed to manufacture and maintain the product. 2221 3.2 3.5 SM782 3.4 3.4.4 Interrelated Considerations for Design - 1.1

15. Section 1.2 Copper Clad Laminates Design Considerations

16. Copper clad laminates used to produce printed boards consist of three parts: • the resin - which is a natural or synthetic resinous material • the reinforcement - such as different forms of paper, matte glass, or woven glass • the copper foil 2221 4.1 Copper Clad Laminates - 1.2

17. The resin and reinforcement make up the base material which is the insulating material upon which a conductive pattern may be formed. Base material may be rigid or flexible or both. It may be a dielectric or insulated metal sheet. Copper Clad Laminates - 1.2

18. Product safety may require that the base materials withstand tests performed by a product safety agency, such as Underwriters Laboratories. Tests include flame retardance (UL 94), printed board construction (UL 796), and base material (UL746). Copper Clad Laminates - 1.2

19. Copper forms the cladding and is available in two types; rolled annealed, or electrodeposited (ED). Unclad base material can also be used when producing printed boards using additive technology, where copper is deposited only where required. 2221 4.1.1 4.2.1.2 4.3 2222 4.3 Table 4-1 Copper Clad Laminates - 1.2

20. The most popular laminate resin system isepoxy. The most common thicknesses for laminate for rigid base material are 0.75mm (.030 inches), 1.5mm (.060 inches), and 02.4mm (.090 inches); however, minimum thicknesses for rigid basematerial is 0.05mm (.002 inches). Copper Clad Laminates - 1.2

21. There are various improvements that have been made to epoxy resins over time. These include difunctional epoxy, multifunctional epoxy, BT epoxy and others. All of the improvements are intended to provide better and more consistent dimensional stability and minimal thermal expansion characteristics. 2222 4.3 Table 4-1 Copper Clad Laminates - 1.2

22. The most popular reinforcement is woven glass. It provides structural strength to the resin and comes in various thicknesses to accommodate the various thicknesses of the sheets used to produce the laminate. A sheet of woven glass that has been coated with resin is referred to as prepreg or preimpregnated reinforcement. Copper Clad Laminates - 1.2

23. This material is also known as “B” stage since it is at a partial stage of cure of the resin. B stage material can be handled, combined with other sheets, and then laminated under heat and pressure to form the base material. 2221 4.2.1.2 4.3 Extra 1.3 Copper Clad Laminates - 1.2

24. Copper foil thickness is defined in ounces. The origin of the practice comes from the days when a copper foil was used to cover roofs. Therefore, half ounce copper is defined as the weight of a square foot of copper foil that is 17 micrometers thick (0.0007 inches). Copper Clad Laminates - 1.2

25. Subtractiveboards can be made by the process that starts with copper foil which is then plated and etched where the unwanted copper is removed. Printed boards can also be additively produced. This is where the copper is patterned on a bare laminate in an electroless process (no electricity involved in moving the atoms of copper to the surface of the board). 2221 4.4.9.1 Table 4-5 Copper Clad Laminates - 1.2

26. New resins are appearing on the scene. These include such products as cyanate ester, polyimide, or PTFE (teflon); however, epoxy resin is still the mostpopular resin used in the United States. Copper Clad Laminates - 1.2

27. Another polymer material used with the printed board is an epoxy permanent polymer coating known assolder mask. The fact that the polymers are very similar to the laminate permits good adhesion of the two systems to one another. Extra 1.4 2222 4.3.1 2222 Fig 4.1 2222 4.3.2 Copper Clad Laminates - 1.2

28. Copper Clad Laminates - 1.2

29. Section 1.3 Thermal Management Techniques for Printed Boards Design Considerations

30. Material selection for the printed board is an essential element of the design to accomplish the structural strength properties needed to: • support the electronic components • handle any vibrational requirements • dissipate the heat from the conductors and the components Thermal Management Techniques for Printed Boards - 1.3

31. 2221 4.1 The characteristics that must be understood by the designer are the safe continuous operating temperature of the materials and the coefficients of thermal expansion (CTE). Some of these properties are provided by the reinforcement. 4.1.1 7.3.1 Extra 1.5 -1,1, -1,1, Table 7-4 2222 Table 4-1 Fig 7-2 Thermal Management Techniques for Printed Boards - 1.3

32. Copper is a relatively good conductor of heat as well as being the main material used to conduct electricity. This capability permits the use of large planes of copper to perform the heat sinking function necessary to keep the board and the board assembly cool. Thermal Management Techniques for Printed Boards - 1.3

33. Depending on the amount of current being drawn through the circuit conductors, they can also become a heat generator. Therefore, proper conductor width and thickness are characteristics that mustbe checked to ensure that the wattage being passed through the conductor does not raise the temperature of the copper above a safe temperature, which may increase the failure rate of a printed board. Thermal Management Techniques for Printed Boards - 1.3

34. Charts are usually used to determine the original heat management goals for the conductors. These are later verified when the whole assembly comes together and the total heat generated by the components and the conductors are assessed. 2221 Fig 6.4 Thermal Management Techniques for Printed Boards - 1.3

35. The resin system responds to heat and temperature cycling by expanding. The higher the resin content, the greater the expansion rate. This expansion causes a strain on the barrels of the platedthrough-holes. Insufficient copper in the hole, or having copper that is not ductile are factors that influence the ability to withstand the thermal strain. Thermal Management Techniques for Printed Boards - 1.3

36. The hole size also is a contributor to the equation since the circumference of the barrel provides more copper volume in the larger holes. The combination of reinforcement and resintogether provide the expansion model for the thickness or Z axis of the board. The industry defines the expansion rate in parts permillion (ppm/ C or a percentage of the thickness. Thermal Management Techniques for Printed Boards - 1.3

37. 2222 Table 4-1 All the laminate constructions have a very uniform rate of Z axis expansion until they reach a particular point. At that temperature the expansion rate increases dramatically and the most damage can occur. This point is known as the glass transition temperature, or Tg. Many laminate structures are sold by their Tg capability. 2221 7.3.1 Fig 7-2 Table 7-4 Thermal Management Techniques for Printed Boards - 1.3

38. The reinforcement accounts for the dimensional characteristics of expansion in the X and Y axis. Some designers look for product that has high dimensional stability characteristics. In this regard, they are trying to achieve a low CTE of the material in the X and Yaxis to reduce the thermal mismatch between the board and the components. Thermal Management Techniques for Printed Boards - 1.3

39. Another way to accomplish the same properties is to include a special plane in the board. These constraining core planes are also very effective in removing the heat from hot spots or hot components. Thermal Management Techniques for Printed Boards - 1.3

40. 2221 5.2.6 Heat is transferred to the plane through plated through-holes that are filled with a conductive material. Known as thermal vias they conduct heat away from the component, or other hot areas, and move the heat to the cooler planes. Planes are then connected to the frame of the housing containing the board. Thus heat is moved by conduction to the cooler surface. 7.3.1 Fig 5-2 Fig 5-3ab Fig 7-2 Extra 1.6 Thermal Management Techniques for Printed Boards - 1.3

41. Section 1.4 Thermal Management Techniques for Assemblies Design Considerations

42. Temperature management is one of the most important aspects of printed board assembly design. As electrons pass through components and board circuitry, heat is generated. The component manufacturer usually specifies the amount of heat that each component generates based on the way it is used in the circuit. Thermal Management Techniques for Assemblies - 1.4

43. Heat is generated mostly by complex integrated circuits, power transistors, transformers, and any component that draws a great amount of current; however, even resistors or capacitors can create heat if they are not compatible with the rated current that they must manage. Thermal Management Techniques for Assemblies - 1.4

44. Most of these conditions relate to the number of watts that a component needs tomanage. Thus passive components for the same value will be manufactured to handle different current capabilities. They are then specified as 1/2 watt, 1 watt, 2 watt, etc. 2221 7.0 Thermal Management Techniques for Assemblies - 1.4

45. Heat and thermal cycling is the enemy of board performance and long term reliability. Thus, the components should be distributed as evenly as possible across the board and oriented in a position which allows the best airflow over the components. Thermal Management Techniques for Assemblies - 1.4

46. Components that run too hot affect their neighbors; change value or even fail. They can also damage the printed board mounting substrate, and cause problems in the solder joints that attach the components to the circuitry. Part of this problem is created by the difference in thecoefficient of thermal expansion (CTE). Thermal Management Techniques for Assemblies - 1.4

47. The definition of CTE is that it represents the linear dimensional change of a material per unit change in temperature. The variation can create a thermal expansion mismatch between the component and the printed board, which places a mechanical stress on the solder joint. The problem is not too severe with through-hole components, however, it can be very detrimental with surface mounted parts. 2221 7.3.2 7.3.3 Fig 7-2 Thermal Management Techniques for Assemblies - 1.4

48. Solder in sufficient volume provides a certain amount of relief. Solder, when heated, becomes plastic in nature, thus the mechanical stress is taken up by the solder providing a certain amount of compliancy. Now the enemy becomes thermal cycling. If the assembly is continually turned on and off, the components become hot and then cool. Thermal Management Techniques for Assemblies - 1.4

49. These conditions cause the solder to change states many times and eventually can cause a crack in the solder due to the solder joint fatiguing under the strain of the continual change from plastic to brittle. The greater the difference between the high and low ends ofthe temperature excursion and the greater the number ofcycles, the sooner the problem can become noticeable. The first indication is an intermittent signal; the final is an open joint. Extra 1.5a Thermal Management Techniques for Assemblies - 1.4

50. For all of the obvious reasons it is important to keep the assembly as cool as possible. Components can be individually provided with a heat sink to transfer or dissipate the excess temperature to the air or to another solid member that may be cooler. Thermal Management Techniques for Assemblies - 1.4