Printed Circuit Board Fabrication And Assembly

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Howard Preston
6 years ago
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2 Design Factors for Printed Circuit Boards Familiarization with darkroom procedures Chemicals for PCB fabrication Board selection Shearing and drilling the printed circuit Component identification and variation Component mounting techniques Device mounting techniques

3 Chemicals And The Darkroom M.G. Chemicals Developer Sodium Persulphate (Mercuric Chloride Activator)

4 Board Selection Insulating base material - Grade Fr-4. Glass cloth reinforced with epoxy resin with the addition of flame retardant. Conducting foil material - 1 oz. copper foil with an approximate thickness of in. and a current carrying capacity from 0.50 amps to 6.00 amps depending on the conductor width to in.

5 Shearing And Drilling Layout the dimensions of the board on the clear acetate shielding using a fine point permanent marker. Shear foil side up to avoid tearing the foil, from the insulating base material. Inspect all hole registrations following p.c.b. fabrication and lightly centre punch if required. Typical drill sizes: (drill all holes #70) resistors & IC sockets - #70 machine sockets - #60 diodes & LEDs - #60 FTP sockets - #60 & 1.5mm

6 Component Lead Prep. Remove oxidation from the component leads. Set the bend allowance in all axial lead components. Reposition leads of components (transistors, SCRs, triacs etc.) as required for insertion in the printed circuit board. Reset the rows of pins on D.I.P. packages to allow insertion in printed circuit boards and IC sockets.

7 Component Mounting Component identification and mounting: Axial lead components Radial lead components Foil Correct Radial Mount Service Clinch Incorrect Radial Mount Full Clinch Axial in confined area

8 Component Mounting

9 Component Mounting

10 Component Mounting

11 Component Mounting

Component Mounting Use a third hand to support the p.c.b. All components mount on the insulating base side of the p.c.b. Mount the small axial lead components first Components that do not dissipate more than 1 watt are mounted flush against the p.

12 Component Mounting Use a third hand to support the p.c.b. All components mount on the insulating base side of the p.c.b. Mount the small axial lead components first Components that do not dissipate more than 1 watt are mounted flush against the p.c.b. Large heavy components must be fitted with a clamp to provide support Components that generate more than 1 watt should be mounted half the diameter of the component body above the p.c.b.

13 Component Mounting Device sockets should be used when possible to allow for easy service and minimize component damage. All components should be oriented in the same direction to facilitate reading component identification Special attention must be paid to components with a notch, number, letter, or polarity, used for orientation Heat sinks are used on components which generate heat that normal air convection does not remove Position components with identification turned up on the surface of the p.c.b. to assist with future service

14 Device Mounting TO220 package to a heat sink & chassis 3mm Mica Washer Heat Sink 3mm Shoulder Washer Chassis

15 Device Mounting TO3 package to a chassis or p.c.b. 4mm TO3 Thermal Compound Chassis or P.C.B. Shoulder Washer 4mm

16 Summary MSDS sheets on chemicals for printed circuits. Typical printed circuit board material. P.C. drills must match hole registrations All axial lead components require a bend allowance A service clinch should be used on all components Almost all components mount flush to the board Components with metal bodies require special mounts

17 Where to get more information Electronic Fabrication Second Edition by Gordon Shimizu Electronic Project Design and Fabrication Third Edition by Ronald A. Reis Electronic Techniques Shop Practices and Construction Fifth Edition by Robert S. Villanucci. Electronics Construction Printed Circuit Design

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