PCB Design for Repair

Summary

This blog presents a range of amendments for schematics or Printed Circuit Boards (PCB) with a focus on Design for Repair (DFR), which may also be referred to as Design for Service (DFS).

Principle
The principle behind Design for Repair (DFR) can be described as the approach or technique of designing for repair or serviceability.

DFR Examples
There is content online which mentions DFR, such as Machine Design, Altium blog and Altium resource. Even though this content mentions DFR, examples showing changes to schematics or PCB design is exiguous. Lack of examples is unsurprising considering the uniqueness of each PCB design. Nonetheless, some examples detailed in this blog should be relevant across a variety of situations.

It should be stipulated that for commercial PCB designs the examples expressed in this blog may not apply for reasons relating to protection of intellectual property.

Schematic Net Naming
A large number of websites state the importance of naming nets and here it is reiterated. The naming of schematic nets should be effected to provide the least ambiguous information, whether the schematic detail is used during development, service or repair.

In the capture below from the Altium Connect Cube project, a common JTAG signal harness was used for soft and hard JTAG programming connections.

The two connections are differentiated in the schematic with the prefix HARD_ or SOFT_.

Extract Harness Connected Cube. Courtesy Altium Ltd

When viewing the net connections on the PCB, the type of the net is clear and understandable.

PCB Extract Connected Cube. Courtesy Altium Ltd

Schematic Component Designators
The naming of schematic components remains a debated topic. This section of the blog does not debate whether a microcontroller designator should be prefixed with "IC", "U" or "DD". Instead, designator naming should aim to use as few alphanumeric characters and be discernible on the PCB.

Certainly, there are exceptions when displaying PCB designators, especially when there are space restrictions or specific commercial equipment. For example, the Samsung S8 motherboard shown in the image below has little to no component designators.

Samsung S8 Motherboard

It is commonplace for PCB design software to offer multi-channel part designator naming or the addition of suffixes to the component designator to identify specific schematic sheets or functions. These features are appropriate when component spacing allows for placement of longer designators.

PCB Extract Spirit Level with Designator Suffix Included. Courtesy Altium Ltd

In space-restricted designs, where the PCB component to board area density increases above forty percent, a compact designator name allows for more valuable use of PCB real estate.

The designators shown in the above image have a suffix 'P' added to identify power supply components. While the addition of the suffix could be advantageous, this must be weighed up against the straightforward designator naming of the original PCB shown below.

PCB Extract Spirit Level with Original Designator. Courtesy Altium Ltd

Component Orientation
In the PCB displayed below, consider the orientation of component U1 with respect to removal through desoldering. It is most likely that the two surrounding relays would require removal to access the component.

Example Component Orientation Preventing Repair

Orientating component U1 as shown in the image below would provide an improved prospect of desoldering.

Improved Component Orientation for Repair

Optimising the orientation of a component for repair at the time of PCB routing will commonly yield a labour-saving result.

Component Placement

Following on from the previous section, placement of components and spacing to adjacent components can cause issues for repair when separation distances are small.

Example Component Placement Limiting Repair

For the example PCB presented above, component U1 is shown fitted between two relays. In the event that U1 requires replacement, twin soldering irons or micro tweezers may be able to remove the component although.

Another placement option could be taken which would result in the component in a more accessible position for service equipment.

Example Component Placement Aiding Repair

PCB Access Points
When designing a PCB to fit inside an enclosure, with a focus on maximising useable space, this can present issues when tolerances are marginal. Consider the small plug-in style PCB pictured below. No facility has been provided to remove or replace the PCB.

Plug-in PCB Without Access Points for Removal

If room on the PCB permits, the addition of access or pull points can significantly improve the ability to remove the PCB.

Plug-in PCB With Access Points for Removal

PCB Current Shunt Option
For designs containing power supplies, the unassuming current shunt can be a benefit to repair personnel.

PCB Current Shunt Option

Designs which use a short-circuited current shunt have no component physically loaded on the PCB. The footprint could be considered a placeholder. If a need arises to isolate a supply rail or measure the current in that rail, the copper shorting the resistor is removed then the appropriate part fitted. This solution should also be considered when developing a prototype PCB.

PCB Plane Thermal Relief
External and internal copper planes often play a large role during PCB layout. During the repair process, the copper plane presents as a large heatsink for soldering and desoldering.

In the capture below, the two leftmost thru-hole pads are directly connected to the copper plane. While a single copper plane directly connected to the thru-hole pads can be soldered, several planes of directly connected copper tend to be a challenge.

PCB Direct Plane Connections

A thermal relief can be applied to the thru-hole pad. Depending on the electrical specifications of the design the characteristics of the thermal relief which include air gap, conductor width and number of conductors should be thoroughly considered.

PCB Relief Plane Connections

PCB Silkscreen Pin Identification
When a new PCB component is drafted, imported from the component manufacturer or taken from an existing design, a review is always sensible.

PCB Extract DT101 PCB. Courtesy Altium Ltd

The picture above, from an Altium example project, shows a four-pin LED with the function of each pin indicated.

PCB Silkscreen Pin Function
Additions to the PCB silkscreen, such as the voltages for the test points in the image below, can deliver technical information without the need to consult project schematics.

PCB Extract DB31 PCB. Courtesy Altium Ltd

Seen below is an extract the Altium Connect Cube with the function or port detailed on the silkscreen for each pin.

PCB Extract Connected Cube. Courtesy Altium Ltd

PCB Silkscreen Assembly Extras
At the beginning and end of a repair commonly involves removal of mounting hardware which can include screws, washers, bolts or standoffs.

PCB Showing No Mechanical Hardware Required or Assembly Information

In the image below the hardware associated with the mounting hole is made evident by the PCB silk screen.

PCB Showing Mechanical Hardware Required

The next image shows the hardware required and the torque setting for the standoff.

PCB Showing Mechanical Hardware Required and Torque Setting

Final Thoughts
The examples in this blog are amendments that can improve the repairability of a PCB. Design for repair is another tool in the hobbyists or Engineers arsenal, which is no different than applying sound judgement to schematic layout and PCB routing, to produce optimal results.

Piezo Buzzer Diode Snubber

Summary

This short post illustrates, using oscilloscope captures, why a requirement exists to include a snubber, such as diode, across a driven (externally modulated) Piezo buzzer element. 

Unsuppressed Circuit
In the schematic below using a Kingstate Piezo buzzer, the snubber component usually in parallel with the Piezo buzzer was omitted.

Unsuppressed Piezo Buzzer

The capture shown in the image below was measured across MOSFET (S1) whilst being driven at a supply rail voltage of 5VDC. Voltages were measured in excess of the supply rails due to the nature of the piezoelectric element.

Component S1 Drain Voltage Unsuppressed

Suppressed Circuit - Diode
Piezo manufacturers usually state that a resistor, diode, Transorb or similar device should be placed in parallel with the buzzer to manage any energy created by the piezoelectric effect.

Component S1 Drain Voltage Suppressed

The capture above shows the reduction in peak voltage, close to the supply rails, when a general purpose diode was fitted across the Piezo buzzer. Results using a 5.1V Zener diode are identical to a general purpose diode.

Diode Suppressed Piezo Buzzer

The circuit shown above implements a diode for snubbing. For a measured peak voltage of almost 30V without suppression, a diode with a reverse breakdown voltage of double the recorded value was chosen.

Suppressed Circuit - Resistor
In some Piezo manufacturer datasheets, suppression is achieved with the use of a resistor. A resistor may be a cheaper solution to snubbing although the value of the resistor should be selected carefully.

Shown below is a capture, performed in the same manner as the previous section, using a 1K resistor in parallel with the Piezo buzzer. The resistor value was selected because it appeared to be a common value shown on posts and forums. Unfortunately the resistance is far too high and voltages in excess of the supply rails are still developed in the circuit.

Resistor 1K Suppressed Piezo Buzzer

Reducing the value of the resistor by a factor of ten yields improved suppression as seen in the capture below. The reduction in peak voltage is almost fifty percent.

Resistor 100R Suppressed Piezo Buzzer

Summary
For any device requiring electronic suppression, being prudent with the type of suppression can provide considerable differences in the result. The captures in this post were applicable to an externally modulated Piezoelectric element with suppression fitted in parallel with the Piezo.

Salvaging Electronic Parts - Part 4 Breville BFP650

Summary

This blog is the next in the series focusing on salvaging electronic parts. For this post, a Breville food processor was broken down with a view to salvage the AC motor.

Breville BFP650

Image courtesy www.productreview.com.au 

Salvaging
There were several plastic screws and individual plastic pieces which required removal from inside the food processor which allowed the microswitches, circuit board and motor to be removed.

Microswitches
Four micro-switches from Merchant Corporation were removed from the food processor. Three of the switches were interfaced to the push buttons seen of the outside of the Breville unit and the last switch was associated with the bowl interlock switch.

Breville BFP650 Micro-switches

Shown in the image above are the Merchant Corporation switches SM-51 and SM-31F from the Breville. The SM-31F switch features a short throw mechanical operation with a domed actuator top.

Circuit Board
The circuit board for interfacing the switches and controlling the motor is a single sided board. The switching relay manufacturer is from Tianbo Relay. There are associated passives for speed control and driving the power LED.

Breville BFP650 Control Circuit Board

The relay is sealed at the base with a bead of clear silicone; this part could be salvaged. The two power resistors and diode may be handy for spares however parts such as the M205 fuse may not be worth salvaging.

On the solder solder side of the circuit board there is a straight forward routing layout. A noteworthy item is the use of slots in the board to address cable management. Items to avoid are the undersized pads for cables and the excess use of solder on many of the joints.

Breville BFP650 Control Circuit Board Solder Side

AC Motor
The weight of the Breville unit appears mostly due to the copper windings of the 1000W motor. Manufactured by Chasekin Limited (HK), part number YXQ-250A33, there is no website for the company so details regarding the motor are scarce. Curiously, Breville list a 5 year warranty for the motor and all the other items have a 2 year warranty.

Breville BFP650 Motor

To take a closer look at the motor internals the plastic cover, water catcher and mounting plate required removal.

Breville BFP650 Motor Top View

With the mounting plate removed, the nuts holding the motor frame were visible.

Breville BFP650 Motor Mounting Plate Removed

The motor was further disassembled by removing the cover (end bell).

Breville BFP650 Motor Assembly

Visible in the image above is the rotor to the left, stator towards the bottom and end bell towards the top. Not pictured, but mounted on the side of the motor windings, was a thermal fuse.

The rotor was packed at the ends with a set of steel and fiber washers however no bearings were visible in the end bell. The fixture was removed and located inside was a steel bush. No bearings meant that this motor could not be used for extended periods of time.

Breville BFP650 Motor Bush

Salvaging parts from the Breville BFP650 was a misadventure and could be considered misspent time; for this particular product the author concurs.

Breville BFP650 Hardware

Some parts were salvaged from the Breville and the motor could be taken for metal recycling. The design and quality of the parts however, mostly prevent reuse for other purposes.