Component Layout Considerations – PCB DFM Part 4

4.1 General Component Layout Requirements

  • Through-hole components with polarity or direction requirements should maintain a consistent alignment throughout the layout and should be arranged as neat as possible. For SMD devices, if they cannot be placed in the same direction, they should be consistent in both X and Y directions for example for tantalum capacitors.
  • If the component needs to be glued, ensure the component has at least 3mm space.
  • For PCBs which need have heatsinks, the location and orientation of the heatsink should be considered. There must be sufficient space to ensure that the heatsink does not touch other components. Ensure that a minimum distance of 0.5mm is maintained.
Description: 1. Thermosensitive devices (such as resistive capacitors, crystal, etc.) should be situated far away from heat producing components. 2. Thermosensitive devices should be placed by an air outlet. Tall components devices should be placed behind shorter components to facilitate air flow. large components Figure 14: Placement of thermosensitive components
  • The distance between the devices must satisfy the required space for normal operation, for example, a memory card
pcb Figure 15: Blocked socket.
  • Metal parts with different properties or devices with metallic cases must not touch each other. A minimum distance of 1.0mm should be maintained between components.

4.2 Reflow Soldering

4.2.1 General Requirements for SMD Components

  • It is recommended that fine pitch devices be placed on the same side of the PCB and larger devices (such as inductors) be arranged on the top side.
  • Polarized components should be aligned such that all the positive poles are on one side of the board and negative poles on the other where possible. Avoid positioning taller components next to shorter ones, which may hinder inspection. A viewing angle of no less than 45 degrees must be maintained throughout the layout to aid manual solder joint inspection.
Surface array devices such as CSP, BGA, must have a clearance area of 2mm, but 5mm is ideal. Polarized components Figure 16: Solder joint inspection angle.
  • In general, surface array devices should not be placed on the bottom side of the board. In the event there is, other surface array devices should not be placed on the top side, in a region including the outline of the surface array device extending 8.00mm outwards, see Figure 17; In the event there is, the same region on the top layer, with an additional 8mm boarder about this region, must not contain surface array devices.
surface array Figure 17: Layout requirements for surface array devices

4.2.2 SMD Component Placement Requirements

  • All SMD components shall be smaller than 50mm on at least one side.
  • It is not recommended for two surface-mount gullwing pin devices to overlap, for example, SOP packages as shown in Figure 18.
SOP packages Figure 18: Incompatible layout of two SOP footprints.
  • In the event that solder pads are shared between two SMD components, the packages must be the same, see Figure 19.
shared pad Figure 19: Sharing of solder pads for SMD components.
  • Through-hole devices and SMD components are allowed to overlap when it can be confirmed that the SMD pad and the solder paste printed thereon has no effect on the soldering of the through-hole device. See Figure 20.
Through-hole devices Figure 20: Acceptable through-hole and SMD component layout design.
  • The required distance between SMD components is
Same component: ≥ 0.3mm Different components: ≥ 0.13 × h + 0.3mm (where h is the maximum height difference of the surrounding neighbors) small components Figure 21: Spacing requirements for components.
  • For PCBs requiring solder reflow, SMT device spacing varies according to the table 1.
Description: The quoted value is the largest of the two: either the pad or the body of the device, whichever is most applicable. The values in brackets represent the lowest acceptable value. Table 1: Components spacing requirements.
(Units in mm) 0402 ~ 0805 1206 ~ 1810 STC3528 ~  7343 SOT / SOP SOJ / PLCC QFP BGA
0402 ~ 0805 0.40 0.55 0.70 0.65 0.70 0.45 5.00(3.00)
1206 ~ 1810 0.45 0.65 0.50 0.60 0.45 5.00(3.00)
STC3528 ~ 7343 0.50 0.55 0.60 0.45 5.00(3.00)
SOT / SOP 0.45 0.50 0.45 5.00
SOJ / PLCC 0.30 0.45 5.00
QFP 0.30 5.00
BGA 8.00
  • The distance between fine pitch devices and the board edge must be greater than 10mm so as to not adversely affect printing quality.
Recommendation: Ideally, the distance between the bar code frame and the surface mounted components should meet the requirements shown in Table 2 to preserve the quality of the solder. Table 2: Recommended spacing requirements between printed barcodes and component footprints
Component Type Pitch  1.27mm’s Gullwing Pin (e.g. SOP, QFP) and Surface Array Components 0603 Size and Greater SMD Chip Components and Other Footprints
Minimum Spacing, D 10mm 5mm
bar code Figure 22: Layout requirements of printed barcodes and components 4.2.3 Through-Hole Component Layout Requirements for PCBs Undergoing Reflow Soldering
  • For PCB with non-transmission side larger than 300mm, heavier through-hole components should not be placed in the middle of the PCB. This will reduce the board deformation caused by the weight of the components during soldering.
  • To facilitate plug-in sockets, the socket should be placed where convenient.
  • The distance between through-hole components should be > 10mm.
  • The distance between through-hole components and the transmission edge should be ≥ 10mm, and non-transmission edge should be ≥ 5mm.

4.3 Wave Soldering

4.3.1 SMD Component Layout Requirements for PCBs Undergoing Wave Soldering

  • Wave soldering is suitable for the following SMD components:
  • Chip resistors, capacitors, and inductors that have a package size greater or equal to 0603 and standoff value less than 0.15mm.
  • SOP packages with pitch ≥ 1.27mm and Standoff value ≤15mm.
  • SOT packages with pitch ≥ 27mm and visible pins.
Note: The pins of SMD components undergoing wave soldering must be less or equal to 2mm. Other components must be less than 4mm in height.
  • The long axis of SOP package components should be perpendicular to the direction of travel of the solder wave in the wave soldering process. SOP components also need extra pads at the end of the solder pad rows to act as ‘solder thieves,’ see Figure 23.
solder thief pad Figure 23: Solder thief pad placement for SOP packages undergoing wave soldering.
  • The orientation of SOT-23 package components should be such that the pins point parallel to the direction of travel.
direction of travel Figure 24: Orientation of SOT-23 packages undergoing wave soldering.
  • General component spacing principles: In order to reduce shadow effect problems caused by wave soldering, certain distances must be maintained between components and individual pads.
  • For components of the same type according to Table 3:
components type Figure 25: Layout of components of the same type. Table 3: Distances between components of the same type.
Pad Spacing L (mm/mil) Component Spacing B (mm/mil)
Footprint Minimum Spacing Recommended Spacing Minimum Spacing Recommended Spacing
0603 0.76/30 1.27/50 type0.76/30 1.27/50
0805 0.89/35 1.27/50 0.89/35 1.27/50
≥ 1206 1.02/40 1.27/50 1.02/40 1.27/50
SOT 1.02/40 1.27/50 1.02/40 1.27/50
Tantalum Capacitors 3216 and 3528 1.02/40 1.27/50 1.02/40 1.27/50
Tantalum Capacitors 6032 and 7343 1.27/50 1.52/60 2.03/80 2.54/100
SOP 1.27/50 1.52/60
  • For different component types, the solder pad edge spacing should be ≥ 1.0mm. Distance requirements are shown in Figure 26 and Table 4.
Figure 26: Layout of components of different types and PCB structures. Form 4: Distances between components of different types and PCB structures.
Footprint (mm/mil) 0603 – 1810 SOT SOP Through-Holes Vias ICT Point Solder Thief Pad Edge
0603 – 1810 1.27/50 1.52/60 2.54/100 1.27/50 0.6/24 0.6/24 2.54/100
SOT 1.27/50 2.54/100 1.27/50 0.6/24 0.6/24 2.54/100
SOP 2.54/100 2.54/100 1.27/50 0.6/24 0.6/24 2.54/100
Through- Holes 1.27/50 1.27/50 1.27/50 0.6/24 0.6/24 2.54/100
Vias 0.6/24 0.6/24 0.6/24 0.6/24 0.3/12 0.3/12 0.6/24
ICT Point 0.6/24 0.6/24 0.6/24 0.6/24 0.3/12 0.6/24 0.6/24
Solder Thief Pad Edge 2.54/100 2.54/100 2.54/100 2.54/100 0.6/24 0.6/24 0.6/24

4.3.2 Common Through-Hole Component Layout Requirements

  • In addition to the special requirements relating to the specific structure of the device, through-hole components must be placed on the top layer.
  • The spacing between adjacent components is shown in Figure 27.
spacing Figure 27: Distance between through-hole components.
  • In order to facilitate manual soldering and maintenance/repair the conditions shown in Figure 28 must be satisfied.
Figure 28: Through-hole placement requirements. Through-hole placement

4.3.3 General Requirements For Wave Soldering Through Hole Components

  • The optimum component pitch is ≥ 2.0mm, the distance between the solder pad edges must be at least 1.00mm as shown in Figure 29. In addition, the component bodies must not interfere with each other.
component bodies Figure 29: Through-hole component layout for wave soldering.
  • For a long row of through hole component holes, the components should be positioned such that the row is parallel to the direction of travel of the solder wave. In special circumstances where the row of pads must be aligned perpendicular to the direction of travel, suitable adjustments should be made, such replacing the standard pads with elliptical pads. When the spacing between adjacent pad edges is 0.6mm-1.0mm, the implementation of oval pads or solder thieves is recommended.
board's direction of travel Figure 30: Solder pad alignment relative to board’s direction of travel through the wave soldering equipment Seeed Studio prototype pcb assembly service supports five types of assembly: SMT assembly, BGA assembly, Through-hole assembly, Mixed assembly and Kit assembly. More Chapaters: 1. Brief Introduction 2. Seeed Fusion PCB Specification 3. Panelization and Bridge Design 5. PCB Hole design 6. Solder Mask Design 7. Copper Trace Design 8. Silkscreen Design 9. PCB Lamination Structure 10. PCB Dimensions Specification 11. Fiducial Mark Design 12. Surface Treatment 13. Files for Manufacture Requirements 14. Appendix 

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May 2017