Design for Repair Part Two

Summary
This blog continues a previous Design for Repair post with a few further examples which may be applied when designing a Printed Circuit Board (PCB) for repair or serviceability.

Debugging Interfaces
For projects containing devices with firmware or configurable logic such as a microprocessor or FPGA (Field Programmable Gate Array), the capability to debug the design during the design phase is considered necessitous. Customarily programming or debug connections are not available for the end-user or customer, however, this interface can be an effective tool for repair.

Examples of programming interfaces are JTAG, SWD and UART

JTAG 10-Pin. Courtesy Keil

Cypress MiniProg SWD. Courtesy Cypress

Debug Interface PCB Connector
In place of physical connectors such as a pin header, the PCB can incorporate connections using tinned copper or gold fingers when cost saving on components is critical. The image below is an example of a standard UART interface implemented with PCB fingers.

PCB Connector for UART Debug

Surface Mount Component Footprint
Utilising the smallest component footprint in a product's design can, in some instances, lead to the repair of a Printed Circuit Assemblies (PCA) transitioning from an in-house repair process to an external repairer.

SO, TSSOP and DFN Footprints

For the surface-mount packages shown in the capture above, removal of the component could be achieved utilising desoldering tweezers or a hot air rework station. These leaded surface mount components remain the practical choice for a myriad of designs although semiconductor manufacturers are migrating to smaller leadless packages such as UFBGA (Ultra Fine BGA) and DSBGA (Die Sized BGA).

BGA PCB Component

For passive components, footprints such as 0402 and 0603 are a regular package size seen on a PCB design. Smaller package sizes such as 0201 and 01005 normally require additional equipment for board level component repair.

Component Solder Mask
For components with a pad to pad pitch 0.5 mm and smaller, including a solder mask between pads can be a challenging prospect.

The presence of a solder mask 'island' between component pads can be beneficial for service personnel performing rework and during visual inspection. 

PCB Extract Spirit Level U1 Top Layer. Courtesy Altium Ltd

In the capture above the components pad to pad spacing of 0.5 mm was used with the default solder mask expansion of 0.1 mm which resulted in no solder mask between pads.

In the following capture, the default solder mask expansion layer was modified from 0.1 mm to 0.05 mm for the five pads on the right of the image. 

PCB Extract Spirit Level U1 Solder Mask - Modified. Courtesy Altium Ltd

The components pads viewed in 3D below emphasises the change to the solder mask expansion.

PCB Extract Spirit Level U1 in 3D with Solder Mask Modified. Courtesy Altium Ltd

Connector Numbering Using Silk Screen
Numbering connector pins using a PCB overlay removes uncertainty and may well be a labour-saving for service personnel. Consider the capture of the connector below where pin numbering has not been provided on the silkscreen.

PCB Extract Spirit Level with Numbering Removed. Courtesy Altium Ltd

The second capture, shown below with the original silkscreen, conveys the pins numbering legibly. For a densely populated board, free space on the silkscreen may not allow for numbering of individual pins. In these instances, a constant incrementing pin numbering pattern may still be beneficial for service personnel.

PCB Extract Spirit Level with Original Numbering. Courtesy Altium Ltd

Danger or Hazard Identification Using Silkscreen 
A PCB silkscreen can be used to convey functional information relating to the PCB, product operation and any dangers service personnel may be presented with. 

High Voltage Identification Using PCB Silk Screen

In the example design above, the silkscreen on the PCB provides details relating to the relay contact operating voltage.

PCB Test Point, Pad and Via
Adding the function of a test point to a components silkscreen was mentioned in the previous Design for Repair (DFR) post as a benefit for service personnel.

Other aspects of the test point which may need to be considered during PCB design include, fitting of a physical component, test point hole size, test point annular ring size or single-sided compared to a through-hole pad.

Test Point, Via and Pad

In the capture above, PCB mount Test Point TP2 is fitted to the board.

TP18 is the same component as TP2 and is not fitted to the board. With a sizeable diameter of 3.7 mm, this test point may not be suited for boards with a high component or routing density.
 
TP19 is a more common test point size. This features a 0.5 mm hole with a 0.8 mm annular ring. A hole of approximately 0.3 to 0.5 mm, without solder mask tenting, allows reliable connections with an oscilloscope or multimeter probes. Should there be a requirement, wires can be soldered to the thru-hole pad.
 
A major drawback with TP2, TP18 and TP19 is the area used by thru-hole parts which apply to all layers on the PCB.
 
One of the most common test point types shown in the capture is TP20. This test point is provided as a pad on a single side of the PCB. Pad diameters may range from 0.2 to 0.8 mm. Usually, this test point is used for In-Circuit Testing or programming boards. An important detail is the test pad wastes no real-estate under the pad compared to a thru-hole via. Wires can be soldered to the pad however it lacks the robustness of a thru-hole pad.

Surface Mount Nuts, Spacers and Standoffs
Disassembly and assembly of a product may constitute a significant portion of a products repair time if individual nuts, bolts, washers and stand-offs are used. Another option to minimise individual hardware items may be surface mount nuts or spacers.

M3 Self Clinching Nut (PEM)

Companies such as Pemnet, Würth Electronik and Keyelco manufacture hardware suitable for most designs.

SMT Right Angled Surface Mount Block. Courtesy Würth

Final Thoughts
As each product design is unique, the examples in this blog should be drawn from a designers cache when applicable.

Salvaging Parts BiPAC 7800VDOX

Summary

This post details which circuit board components could be salvaged from a Billion BiPAC 7800VDOX Wireless Router.

BiPAC 7800VDOX Wireless Router

Router Disassembly

To separate the Wireless Router's grey lid from its base, four rubber feet located on the base of the router enclosure were removed. The removed feet expose the screws retaining the lid.

BiPAC 7800VDOX Wireless Router PCB

The above image shows the Wireless Router internal Printed Circuit Board (PCB) with the light pipe and rear panel removed.

Parts to Salvage

Cabling

There are two antennas in the lid with RF connectors (RSP / AMC style) which could be salvaged.

Heat Spreaders

On the PCB are several ceramic heat spreaders (heatsinks) attached to various Broadcom components. These are held down with double-sided tape.

Mounted Heat Spreaders

Removing the heat spreaders can be achieved using a tool such as a wide flat blade screwdriver to lever the heat spreaders off carefully.

7800VDOX Wireless Router PCB with No Heat Spreaders

Removing the heat spreaders exposes the Broadcom chipsets comprising of the processor, RF and communication hardware. The Broadcom hardware is not suitable for salvage, possibly repair to a similar router.

Inductors

There are open core leaded and PCB mount inductors which could be recycled for other designs.

Wireless Router PCB Inductors

Diodes

Identified in the image below are single and dual colour surface-mount LEDs.

Wireless Router PCB LEDs

Oscillators

A 25 MHz thru-hole Crystal is located next to the Broadcom chip responsible for the Ethernet clock and a second surface mount Crystal oscillator can be found by the Broadcom processor.

Wireless Router PCB Oscillators

Relays

The two Fujitsu FTR-C1CA003G miniature relays have a coil operating voltage of 3 V DC with DPDT 1A contacts. These thru-hole relays are worth removing.

Wireless Router PCB Relays

 

Resistors

Distributed across the top and bottom side of the PCB are resistors with case sizes larger than 0805 which can be repurposed. Most are standard resistor values.

Wireless Router PCB Resistors

DC-DC Regulators

A mixture of Fitipower DC-DC Step Down Switch mode chips (FR98860 and FR9888) could be removed.

Wireless Router PCB DC-DC Controllers

Connectors

The vertical USB connector and DC jack could be removed and reused in another design. Removing and reusing the right-angle Ethernet or RJ connectors should be achievable.

Capacitors

Any of the radial Electrolytic capacitors could be reused. It should be noted that the Su'scon capacitors (SD and SK series) although rated with a useful operating temperature of 105°C (max), have a 2000 hour operating life at the maximum rated temperature. Consider that the Wireless Router may have been operating for 10 years continuously which may have significantly reduced the lifetime of the capacitor.

The other surface-mount Electrolytic capacitors (22uF) appear to be the Panasonic FK series which are rated to 105°C with a 2000 hour operating life at the maximum rated temperature.

Wireless Router PCB Capacitors

Unlisted Components

Several additional types of components could be removed from the Wireless Router PCB although some are extremely small (0402 footprints), obsolete or specific to the product. The components listed in this post should have equivalents from different component manufacturers allowing salvaged parts to be used in a prototype PCB design or used for repair.