Presentation on theme: "IPC-2221 8.1.2 COMPONENT PLACEMENT Whenever possible, through-hole parts and components should be mounted on the side of the printed board opposite that."— Presentation transcript:
IPC COMPONENT PLACEMENT Whenever possible, through-hole parts and components should be mounted on the side of the printed board opposite that which would be in contact with the solder, if the board is machine soldered. Intermixing of through-hole and surface mount parts, or mounting parts on both sides of the board, requires complete understanding of the assembly and attachment processes.
IPC COMPONENT PLACEMENT Whenever possible, if their leads are dressed through the holes, axial and non-axial-leaded components should be mounted on only one side of the printed board assembly. Unless a component or part is specifically designed to accept another part into its configuration, there shall be no stacking (piggybacking) of components or parts. Component leads shall either be surface mounted, mounted in through-holes, or mounted to terminals. Lead and wire terminations shall be soldered or wire bonded.
IPC COMPONENT PLACEMENT The variations in the actual placement of the component's leads into plated-through holes or on the termination area in addition to the tolerances on the component's envelope (body and leads) will cause movement of the component body from the intended nominal mounting location. This misregistration shall be accounted for such that worst case placement of components shall not reduce their spacing to adjacent printed wiring or other conductive elements by more than the minimum required electrical spacing.
IPC COMPONENT PLACEMENT If a component is bonded to the surface of the printed board utilizing an adhesive (structural or thermally conductive), the placement of the component shall consider the area of adhesive coverage such that the adhesive may be applied without flowing onto or obscuring any of the terminal areas. Part attachment processes shall be specified which control the quantity and type of bonding material such that the parts are removable without damage to the printed board assembly. The adhesive used shall be compatible with both the printed board material, the component, and any other parts or materials in contact with the adhesive.
IPC COMPONENT PLACEMENT Thermal concerns, functional partitioning, electrical concerns, packing density, pick-and-place machine limitations, wave soldering holder concerns, vibration concerns, part interference concerns, ease of manufacture and test, etc., all affect the parts placement. Parts should be placed on a 0.5 mm placement grid whenever possible. When a 0.5 mm grid is not adequate, a 0.05 mm placement grid should be used. Certain parts (such as some relays) have leads that are not on standard grids but otherwise the parts should be placed so that the through holes are on grid.
IPC COMPONENT PLACEMENT If equipment or other constraints do not allow for a metric grid, parts may be on in. placement grid. When this is not adequate, a in. grid may be used or even a in. grid. The in. placement grid facilitates not only parts insertion but also standard bed- of-nails testing of the board and of the assembly. If bed-of-nails testing is to be used (including in-circuit printed board assembly testing), the test fixturing becomes much more difficult when components are placed off grid.
IPC COMPONENT PLACEMENT Figure 7-1 illustrates the producibility design allowances for automatic component insertion. Through hole mount printed boards should observe component to edge of the board spacing constraints on two (2) opposite edges to allow direct insertion into wave solder fingers. Other designs will require fixturing. Both component heatsink considerations and board heatsink requirements must be addressed in parts placement.
IPC COMPONENT PLACEMENT If the printed board assembly will not be tested with a bed-of-nails testing then the assembly grid will be limited only by the assembly machinery. If the printed board assembly is testable with a bed-of- nails scheme, a in. grid for plated through hole spacing is preferred. A in. grid allows greater design density and is not a concern with the assembly machinery but is a concern with bare board and completed assembly testing if a bed-of- nails testing approach is utilized. Bare board testing will normally be done at the printed board supplier and there presently is no cost penalty for off grid nor reduced grid printed board testing.
IPC COMPONENT PLACEMENT The designer should allow sufficient component to board edge separation for test and assembly processes. If this is not possible, the designer should consider adding a removable section of board (i.e., breakaway tab). The edge of the component is defined as the physical edge of the component on sides where no leads protrude from the component, and the edge of the surface land pattern for the leaded side of a component. Preferably, components should be a minimum of 1.5 mm from the edge of the board and board guide or mounting hardware to allow for component placement, soldering, and test fixturing.
IPC COMPONENT PLACEMENT Components should not be grouped in such a way that they shadow one another during soldering. Do not align rows of components perpendicular to the direction of travel; stagger them. Component polarities should be oriented consistently (in the same direction) throughout a given design. For wave soldered surface mount chip types, components should be bonded to the printed board prior to automated soldering with an adhesive specially formulated for the purpose.
IPC COMPONENT PLACEMENT Specific requirements for part mounting are functions of the type of component, the mounting technology selected for the printed board assembly, the lead bending requirements for the component, the lead stress relief method selected, and placement of the components (either mounted over surfaces without exposed circuitry, over protected surfaces, or over circuitry). Additional requirements are dependent upon the thermal requirements (the operating temperature environment, maximum junction temperature requirements, and the component's dissipated power), and the mechanical support requirements (based on the weight of the component).
IPC COMPONENT PLACEMENT Mounting methods for components of the printed board assembly shall be selected so that the final assembly meets applicable vibration, mechanical shock, humidity, and other environmental conditions. The components shall be mounted such that the operating temperature of the component does not reduce the component's life below required design limits. The selected component mounting technique shall ensure that the maximum allowable temperature of the board material is not exceeded under operating conditions.