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Section 3 Component and Assembly Issues

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  1. Section 3Component and Assembly Issues IPC Designer Certification Study Guide

  2. Section 3.1 Considerations for Component Mounting Component and Assembly Issues

  3. Component mounting and attachment are fast becoming the most important element of printed board design. The issues have always been important because of component density and conductor routing considerations. Considerations for Component Mounting - 3.1

  4. However, an increase in the complexity of the assembly process occurs due to: • The drive for more functions per assembly. • Combining surface mount and through-hole components on one printed board. • Using both sides of the printed board to attach the parts (impacts assembly, solder joint integrity, reliability and testing). The tradeoffs to be made regarding component mounting must be considered early in design. 2221 8.0 8.2.3 Considerations for Component Mounting - 3.1

  5. Through-hole components are mostly mounted on the side opposite to that which comes into contact with the solder. Automatic insertion techniques are preferred, so rules for these conditions should be taken into account when arranging through-hole parts. Considerations for Component Mounting - 3.1


  6. These rules include appropriate clearances for the insertion heads of the automatic equipment, and having sufficient clearance between the lead diameter and the component hole used for attachment and electrical connection. The orientation of the components is also important. Considerations for Component Mounting - 3.1

  7. This includes the direction in which the components are lined up electrically with respect to the polarity of polarized components to one another and usually with respect to the board edges. In addition, uniform component orientation, i.e. all pin #1s located at the lower left, reduces machine cycle time, thus controlling cost during the assembly operation. 2221 8.1.2 8.1.11 8.2.3 Fig 7-1 Considerations for Component Mounting - 3.1

  8. The edge of the board becomes the design envelope. Except for connectors, components should not extend over the edge of the board or interfere with board mounting. Design for Assembly (DFA) principles dictate that the designer also know how the assembly will be manufactured. Considerations for Component Mounting - 3.1

  9. Automated techniques require that standard assembly panels be used to maximize the use and the efficiency of the equipment. Special fixtures can accommodate any shape, however, these fixtures add unnecessary cost to the assembly process. Considerations for Component Mounting - 3.1

  10. Thus the board perimeter at LMC (least material condition) should be the boundary that no component, at MMC (maximum material condition), extends beyond. Assembly equipment limitations must be recognized early in the design process. Mounting rails for the automatic machines may require additional clearance. All requirements should be documented on the assembly drawing. 2221 8.1.5 Considerations for Component Mounting - 3.1

  11. There are many other parameters that must be considered for component mounting; component body centering, mounting over conductive areas, clearance between components, and physical support are just a few. When designing mixed assemblies that include standard SMT parts along with through-hole parts the designer must have close contact with the assembly manufacturing representative to ensure an assembly doesn’t require work-arounds of the process being used by the manufacturer. Considerations for Component Mounting - 3.1

  12. Since many services are provided by the infrastructure of the Electronic Manufacturing Services Industry (EMSI), it is preferred to select the company, or companies, you will work with and tailor the design to their specific process. It should be evident that the design is done once, whereas the board and board assembly are produced many times. Even design changes are easier to implement working with a single or small group of assemblers. 2221 8.1.7 8.1.10 Fig 8-8 Considerations for Component Mounting - 3.1

  13. Another element of component mounting that must be considered is the lead clinching requirement. Is it allowed? Is it required? Sometimes it is left to the discretion of the assembly manufacturer. If the requirements are restrictive they should be indicated on the assembly drawing. Considerations for Component Mounting - 3.1

  14. Some designs require that the dual-inline packages (DIP) have the four corner leads partially bent to a 30 degree angle. This requirement (and any others) for lead clinch should be specified. Considerations for Component Mounting - 3.1

  15. 2221 8.3.1 Another element that must be considered is the electrical test considerations. Test point lands must be identified when completing the component placement. In fact, the test strategy should be established before the design starts. 8.3.1.2 8.3.1.3 8.3.1.4 8.3.1.5 Fig 8-19 Fig 8-20 Considerations for Component Mounting - 3.1

  16. Section 3.2 Axial and Radial Lead Mounting Differences Component and Assembly Issues

  17. Axial leaded components are through-hole components that have the lead wire extending from the component body, or module body, along its longitudinal axis. The leads normally come out in a straight line and must be bent. They enter the holes in the printed board perpendicular to the body of the component. Axial and Radial Lead Mounting Differences - 3.2

  18. The component is horizontally mounted with the component body parallel to the board surface. When bending the leads care must be taken to avoid damaging the seal where the lead connects to the component body. Axial and Radial Lead Mounting Differences - 3.2

  19. When mounting axial leaded parts, the designer should space the holes at a sufficient distance to avoid bending the lead too close to the body of the component. The goal being that the body of the component should be approximately centered between the two mounting holes. 2221 8.3.1.6 8.1.6 Fig 8-21 Axial and Radial Lead Mounting Differences - 3.2

  20. Thelead extension serves as a form of stress relief and therefore, the tighter the lead is bent to the body of the component the greater stress that can be transferred to the component as the board expands when it heats up in service. The lead diameter helps to determine where the bend begins. This is usually specified as a minimum of one lead diameter from the component body before the bend radius starts. Axial and Radial Lead Mounting Differences - 3.2

  21. If the component has a weld, or other lead configuration as the lead exits the component, the amount of straight lead distance should be considered after the weld. Although the lead diameter determines the distance prior to bending, that dimension should never be less than 0.75mm [.030"]. 2221 8.1.11 Fig 8-9 Fig 8-10 Axial and Radial Lead Mounting Differences - 3.2

  22. Radial leaded components may have two or more leads exiting from the body. The leads usually come from the same surface and bending may not be necessary as the leads can be inserted directly into the holes of the board. The lead spacing of radial leaded components varies greatly, so the hole spacing is usually predicated by the exit of the leads from the body of the component. Axial and Radial Lead Mounting Differences - 3.2

  23. The DIP is an excellent example of a multiple radial leaded part. The leads of the DIP need not be bent to enter the holes because the part was specifically designed to avoid having to perform the pre-bend operation. However, some small capacitors must have their leads spread slightly in order to have sufficient clearance between the holes. 2221 8.3.1.7 Fig 8-23 Fig 8-24 Fig 8-25 Axial and Radial Lead Mounting Differences - 3.2

  24. Axial or radial leaded parts are intended for through-hole mounting. The axial leaded components are intended to mount horizontally, while the radial leaded parts mount in a vertical axis which is perpendicular to the board. Axial and Radial Lead Mounting Differences - 3.2

  25. For some designs that have a very dense packaging requirement, axial leaded parts may be vertically mounted provided that they are not too heavy (less than 14 grams), and that they do not extend too high above the surface of the board (approximately 15mm). This characteristic is only appropriate for through-hole components. Their surface mount equivalents are always mounted horizontally, parallel to the board surface. 2221 8.2.3 8.3.1.8 Fig 8-26 Axial and Radial Lead Mounting Differences - 3.2

  26. Metal Electrical Face (MELF) components are cylindrical parts that have no leads and are always mounted so that the solder joint is between the land on the surface and the metal face of the part. Thus vertical mounting has never been considered for this part, however, on some very dense designs the MELF has been installed in an unsupported (unplated) hole and then soldered to the lands on the external layers. This practice requires good thermal management to avoid solder cracking as the board expands. Axial and Radial Lead Mounting Differences - 3.2

  27. Axial and Radial Lead Mounting Differences - 3.2 As intermixing components continue to prevail, innovations in the design process become necessary in order to package all the components and circuitry within the design envelope. Through-hole components are modified for surface mounting; surface mount leaded parts are modified for through-hole mounting. 2221 8.2.3

  28. Section 3.3 Design Differences for SMT vs. Through-Hole Component and Assembly Issues

  29. Design Differences for SMT vs. Through-Hole - 3.3 Surface mounting is the term given to the method of electrical connection of components to the surface of the conductive pattern. Surface Mount Technology (SMT) does not utilize component holes.

  30. Design Differences for SMT vs. Through-Hole - 3.3 The technique is not new. In the 1960s it was called planar mounting. It came into vogue when ceramic flat pack components were introduced which were hot-bar soldered to the surface of the printed board at a time when most designs used through-hole leaded components. 2221 8.1.1.3 8.3.1.9 Fig 8-27 Fig 8-28

  31. The industry quickly learned to deal with intermixing of components that mounted to the surface, and those that had to be inserted in plated-though, or unsupported holes. Because the goal has always been to reduce the complexity of the manufacturing and assembly processes, many designers took components that were intended for one or the other technique and modified the leads to accommodate the majority of the components on the board. Design Differences for SMT vs. Through-Hole - 3.3

  32. Through-hole components had their leads flattened, coined, or bent, so that they could be surface mounted. And surface mount parts had leads configured so that they could be inserted into holes. Lead temper and lead length being the major consideration for that approach. 2221 8.4.4 Fig 8-33 Design Differences for SMT vs. Through-Hole - 3.3

  33. The industry, then and today, still mounts through-hole axial, radial, and multiple leaded components. When determining the spacing of the lead bends, several considerations are taken into account. These include the distance from the body of the part the leads can be bent, the lead stiffness, the lead diameter (used to determine the bend radius) and the grid system used for the board to locate as many holes as possible on the selected grid. 2221 8.1.1.2 8.1.11 8.2.1 8.3.1.9 Fig 8-27 Fig 8-28 Design Differences for SMT vs. Through-Hole - 3.3

  34. Intermixing SMT and THT components will be used within the industry for many years. The design for manufacturing issues encourage the designer to work closely with the assembly process engineers in order to achieve the best through-put of the assembly operation. The boards are normally assembled in a panel format, and require careful consideration as to how the parts are positioned, oriented, and arranged, in order to speed-up the assembly operation. 2221 8.1.1.2 8.1.11 8.2.1 8.3.1.9 Fig 8-27 Fig 8-28 Design Differences for SMT vs. Through-Hole - 3.3

  35. The clearances around the parts are determined by the maintenance required. The heads of the insertion or pick and place equipment play a major role in that the designer normally leaves room to provide sufficient clearance for the clinching mechanisms; however, the assembly sequence varies from manufacturer to manufacturer, so the emphasis is usually placed on how difficult it is to replace a faulty component. Design Differences for SMT vs. Through-Hole - 3.3

  36. With that parameter taken into consideration there is usually sufficient clearance for the component placement equipment. Some companies provide this clearance as a standard around the body and land pattern of each component, however, there is no general consensus on what the clearance should be. Much depends on the density of the design and whether the component is repairable or disposable. 2221 8.2.1 Design Differences for SMT vs. Through-Hole - 3.3

  37. Section 3.4 Differences Between Automatic & Manual Insertion Component and Assembly Issues

  38. There are many things that must be taken into account when determining the method of through-hole insertion. Manual insertion techniques, although they have greater flexibility for placing components very densely or close together, can be error prone. To reduce the possibility of placing a part in the wrong location has necessitated a great variety of manufacturing assembly aids. Differences Between Automatic & Manual Insertion - 3.4

  39. These aids or systems help to conveyorize the process so that each operator on an assembly line has only a few functions to manage. Components are kitted into groups which, when kept to a minimum number of different or similar parts, are easy to manage. Differences Between Automatic & Manual Insertion - 3.4

  40. Automatic component insertion is the act or operation of assembling discrete components to the printed board by means of electronically controlled equipment. The orientation of the components, the clearance between components, the sequence in which the parts are to be assembled, plus many more factors all become issues that the designer must address. Differences Between Automatic & Manual Insertion - 3.4

  41. The size of the board is also important since many assembly companies want to treat the boards in a panel format to ease board handling. The relationship between the board fabrication panel and the assembly panel, plus the amount of room needed for tooling, or conveyor guides, are issues that impact the design methodology and approval of the final layout. 2221 8.1.1 8.1.1.3 Differences Between Automatic & Manual Insertion - 3.4

  42. The days when printed boards consisted only of through-hole parts are past. Today’s designs have intermixed assemblies that mount both through-hole and surface mount parts. The parts that were only on one side of the board are now on both sides, thus the design process establishing parameters for component positioning must take into account the placement, insertion, and attachment processes used to develop the final assembly. Differences Between Automatic & Manual Insertion - 3.4

  43. Some design facilities pause after initial component placement in the CAD system to send a preliminary arrangement to the assembly company, thus creating a true concurrent engineering environment. Differences Between Automatic & Manual Insertion - 3.4

  44. Lead clinching is another important consideration to be taken into account when deciding if a part is to be manually or automatically inserted. Most automatic equipment heads have the feature capability to clinch leads. They are trimmed to size and then swaged, clinched, or partially clinched to retain the parts during the solder operation. Differences Between Automatic & Manual Insertion - 3.4

  45. 2221 8.2.1 It becomes an important parameter for the designer to understand the exact methods for part retention in that the land size or electrical clearance of adjacent conductors can be affected. Manual assembly is less precise and requirements of the design should not exceed the physical dexterity of the operator. 8.2.2 8.3.1.2 8.3.1.4 Fig 7-1 Fig 8-19 Differences Between Automatic & Manual Insertion - 3.4

  46. Last, but not least, is the relationship between the hole size and the lead diameter. In general, automated assembly requires a slightly larger hole than manual techniques. The larger hole is intended to account for the differences in machine accuracy versus printed board hole location accuracy. Differences Between Automatic & Manual Insertion - 3.4

  47. Some companies provide additional targets called fiducials to compensate for hole to machine location mismatch. Other systems require that the hole is oversize from what it could be with manual insertion. But, it is important that the hole diameter should not exceed the lead diameter too much. If this occurs the solder might not stay in the plated-through, or unsupported hole. The maximum for automated attachment is usually 0.7mm [.028"] larger than the lead. 2221 8.3.1 2222 9.2.2 Fig 9-3 Differences Between Automatic & Manual Insertion - 3.4

  48. Section 3.5 Manual vs. Pick-and-Place SMT Placement Component and Assembly Issues

  49. Surface mounting (SMT) is the process of electrically connecting components to the surface of a conductive pattern that does not utilize component holes. The process requires placing the components on the pattern and attaching them using solder. Manual vs. Pick-and-Place SMT Placement - 3.5

  50. The attachment process can take a variety of forms but fall into two distinct groups; those where solder is added to the joint and those where existing solder (tin and lead) is reflowed. The differences between manual and automatic placement depends a great deal on the method of attachment. 2221 8.4 Manual vs. Pick-and-Place SMT Placement - 3.5