Beta Layout (V2) Reflow Controller Double Sided PCB (Second Reflow)

Summary

This blog explores the suitability of the Beta Layout (V2) Reflow Controller for second reflow PCB's (boards). The concept behind second reflow is that the surface tension of solder retains a component during the solder reflow process. This retention of the component is due to the ratio of component weight to the total component pad surface area (aka component land pattern).

Literature
There is no shortage of literature released on the subject of double sided PCB's and the reflow process. Reading on the subject can be found in journals such as Component Candidacy of Second Side Reflow with Lead-Free Solder, in documents Weight Limits For Double Sided Reflow Of QFNS or on websites such as Surface Mount Process.

The engineer that was cited in the document "Weight Limits For Double Sided Reflow Of QFNS" is Phil Zarrow of ITM Consulting, who also authored another document Reflow Soldering of Through-hole Components. This document originally started me on the double sided reflow process and only recently did i consider using this with my home reflow oven.

Weight to Area Rule
Described in the literature by Phil Zarrow is a ratio for weight of a component to the total pad mating area. Originally listed in imperial it states that to hold the component:

Grams per square inch must be ≤ 30 (grams)

Changing to metric measurements:

Grams per 645.16 mm squared must be ≤ 30 (grams)

Reflow Testing with Small Components
Using a sample board to test solder reflow with small components, the amount of solder paste was varied. 

In the image below the resistors circled in RED had an over application of solder paste, resistors circled in yellow had a moderate to normal application of solder paste and the diode circled in blue had a normal application of solder paste.

Sample PCB With Various Solder Paste Amounts Applied

The solder paste on the board was allowed to sit for an hour, then the board was reflowed in the oven with components facing downwards.

Sample PCB After Reflow

After reflow, as can be seen in the image below, all components were located and most reflowed correctly except for the additional solder paste. The normal application of solder paste was sufficient hold both the resistors circled in yellow and small diode circled in blue.

Sample PCB Reflow Closeup

For the resistors used on the sample PCB, 0805, the mass per resistor is listed as 0.45 grams per 100 pcs. The mass of the resistor was taken from the Yageo document. 

Yageo Resistor Mass

For the resistor pads and dimensions the details were taken from another Yageo document.

Yageo Pad Size

In that same document the parameters W, width, and I2, pad depth, were used.

Yageo Pad Dimensions

For the 0805 package there are two pads of 1.25mm x 0.35mm, giving a total of 0.875mm squared. Using this value to determine the maximum mass from the metric ratio "645.16 mm squared ≤ 30 grams" yields 0.0407 grams.

For the Yageo 0805 resistor with a mass of 0.045 grams, using just the mating area of the Yaego resistor pad, the rule recommends that 0.0407 grams should be the limit. This rule appears to have some tolerance which may be due to the larger land pattern on the PCB possibly involving solder surface tension between the physical sides of the resistor pad to the land pattern.

Reflow Testing with Medium Components
Next sample board was prepared with Stannol flux for a small Panasonic inductor, ELL-6RH series.

Sample PCB with Solder Paste on Inductor

The footprint on the sample board was larger than the size recommended by Panasonic however the solder paste was added and the part was placed in position.

Panasonic Recommended Footprint

Immediately the board was placed component side down inside the reflow oven and the soldering cycle started.

Sample PCB with Failed Reflow

The inductor held its position against the sample board until the reflow cycle where it promptly fell off the board.

Summary

Although the inductor tested on the sample board in this blog was not suitable for reflow under the circumstances, this same result is not true for larger devices such as the TO-263-5 part shown below. The thermal pad beneath the body of the device (TO-263-5) is usually large in area which facilitates better adhesion and suitability for second reflow boards.

TO-263-5 Package

With a number of factors influencing double sided PCB reflow for the home hobbyist, each factor should be taken at its own merit and how it is applicable to a specific board, reflow oven and components to be populated.

Beta Layout V2 Controller RS232 to USB Upgrade

Summary

As mentioned in a prior blog involving the Beta Layout V2 Reflow Controller, the on-board RS232 can be removed and USB added using an adaptor such as the Adafruit - CP2104.

Beta Layout V2 with USB

Hardware Required
Beta Layout V2 Controller
Adafruit USB to TTL Adaptor #CP2104

1 x 1k 0805 resistor
2 x M2.5 plastic screws
2 x M3 x 15mm bolts
2 x M3 star washers
2 x M3 flat washers
2 x M3 nuts
1 x plastic L bracket (home made)
3 x small lengths of hookup wire

Opening the Controller

As detailed on a previous blog the Beta Layout case does not use fixing to hold the two part enclosure together. Instead there are a pair of opposing plastic clips on the lid and base, concealed by the ventilation holes, which were released using a flat blade screwdriver to gain access to the controller.

Beta Layout V2 Controller

There are only a handful of components associated with the existing TTL to RS232 driver (MAX232) and these components are all surface mount capacitors for the charge pump or supply decoupling. For more details on the RS232 driver see the Maxim datasheet - MAX232.

In the image below the components to be removed are circled in RED.

Logic Board RS232 Driver

To unsolder the MAX232 driver it was easier to unscrew the logic board so that the board could be worked on directly. While the 10 way header for the serial port can be unplugged, the Thermocouple wires are soldered directly to the logic board so the connector must be removed from the plastic panel.

To remove the Thermocouple connector from the panel of the unit, the bolt connected to the metal plate is first unscrewed, then the bolt holding the Thermocouple connector together is removed. This allows the connector to come apart allowing the logic board to be removed from the unit.

Thermocouple Connector Disassembly

Removing the RS232 Driver

One of the easiest methods to remove the MAX232 driver is to use two soldering irons, unless you have a tool for your soldering iron that can remove SOIC packages! Alternatively a set of side cutters to chop the legs off the driver and solder wick to clean up the mess works wonders if there is no interest in saving the driver.

Logic Board RS232 Driver Removed

In the image above the driver and four charge pump capacitors were removed. The capacitor to the lower left of the MAX232, for the power supply decoupling, was left on the PCB. For people choosing to use another flavour of USB to TTL adaptor they may find that external 5V power is required. This capacitor, even though small, may be able to provide some minimal power supply decoupling for an alternative adaptor board.

The Adafruit adaptor board which was used in this upgrade, is bus powered, so this decoupling capacitor is not used and could be removed. Below is an image of the changes to the logic board. These are explained below.

Logic Board with Adafruit USB Adaptor

Logic Board Changes

For the image above the changes listed below were made.

1. Bridge the transmit lines on the MAX232 footprint.
   
   As shown in the image below there are two pins which need to be linked 'shorted' together. These pins are actually pin numbers 7 and 10 on the actual MAX232 transceiver.
   
   

   

   Bridge MAX232 Transmit Lines

   
   
2. Connect a 1k resistor between the receiver lines on the MAX232 footprint.
   
   Since the receiver line from the USB to TTL converter can provide enough power to supply the ATMEGA a current limiting resistor was added between the boards. The rail to rail steering diodes inside the ATMEGA cause this phenomenon. See this EEVBlog #831 Episode for a detailed description on YouTube.
   
   

   

   Add 1K Resistor Receive Line

   The resistor is fitted between pin 12 of the MAX232 and the via directly adjacent to the right between the transceiver pads. As the board via's are untented the resistor can be soldered directly to the via.
   
   
3. Connect TTL transmit, receive and 0VDC to the Adafruit adaptor.
   
   The original transmit and receive lines on the PCB header were reused for connection to the USB to TTL converter. As shown in the image below the three connections are all made to the inside pins on the header.
   
   

   

   USB to TTL Connections

   The transmit connection is the TXD pin, receive the RXD pin and 0VDC is the GND pin on the Adafruit USB to TTL adaptor.

Mounting the Adafruit Board

To mount the Adafruit board a custom L bracket was made from plastic and fashioned to cover the old 9 pin serial connector cut out.

Adafruit USB to TTL L-Adaptor

Since there are only two mounting holes (ID 2.5mm) on the Adafruit board a pair of 2.5mm self-tappers were used to hold the board in position.

Mounted Adafruit Board

To hold the L-bracket in position against the panel of the Beta Layout unit, a pair of 3 x 15mm bolts were used with the usual star washers and nuts on the rear and M3 flats against the outside of the units panel.

Mounted Adafruit in Case

As a dry run the unit was assembled without fixing the power board down to check the clearances.

Final Mounting of USB to TTL Adaptor

Testing the Adafruit Board
To be prudent the USB was connected to a PC running TeraTerm. The unit was powered with the Thermocouple fitted into the connector.

USB to TTL Converter in Device Manager

Running on Windows 7 the drivers installed automatically for the Silicon Labs USB controller. Below is a capture of the USB connection running at 9600 baud.

TeraTerm - USB to Beta Layout V2

The USB to TTL adaptor was working as expected and so the Beta Layout unit was reassembled by fixing down the power board.

Power Board Fixings Under Connectors

Final Mounting 
Note that one of the plastic screws to hold down the power board was located underneath the mounted Thermocouple connector. The second screw was located on the opposite end of the power board underneath the Adafruit PCB. 

Should repairs need to be performed on the power board the Thermocouple connector and USB board would need to be removed to gain access to the unit.

Comair Rotron Fan Program Input - Control and monitoring of 4 Wire CD24R7X

Summary

This blog investigates control and speed monitoring of a 4 wire Comair Rotron fan using the Program input and Tacho output. The fan speed was controlled using a resistor then separately tested with an optocoupler driven by an external PWM signal.

Comair Fan Control Article

Tim Shafer, of Comair Roton, wrote a brief but helpful article relating to methods of fan speed control. This article is available on the Comair Rotron website or from Digi-Key as a PDF Document. The details from Tim were a start although did not clearly define methods of interfacing with the 'Program' input connection used by Comair Rotron fans. Even after reviewing the data sheet for the fan (CD24R7X) neither a suggested resistor range or recommended PWM frequency was suggested for the fan. Time for some bench testing and fan characterisation.

Comair Rotron CD24R7X

A test unit was purchased from Digi-Key and arrived in the Comair Rotron packaging. There was plenty of cardboard to secure the fan inside the box for transport which was a pleasant find in these times of bubble wrap packaging. The test unit was manufactured Dec 30th, 2015 with a serial number of 362.

Comair Fan Testing

Since the fan was earmarked as a test device to regulate temperature and air quality of a room, the range of fan speeds against changes in the Program input needed to be known. With the article from Tim Schafer in mind, the Program input was tested with an optocoupler driven from a signal generator then separately with a variable resistor. Additionally the effect of fan speed against variations in power supply voltage was also captured.

Comair Fan Test Setup

To perform the required tests, the fan was connected to a Rigol DP832 PSU and Function Generator, Agilent oscilloscope and vanilla multi-meter for current measurement verification.

The Comair fan tacho output connection (open collector) was pulled up to the 24VDC supply via a 220K resistor and then monitored by the oscilloscope.

The fan program input was connected to the optocoupler collector and emitter to 0VDC. To change the speed of the fan the Program input must be connected to 0VDC. The input LED of the opto was driven directly by the function generator.

Test Setup: Duty Cycle Measurements

Test 1: PWM Control

With a PWM frequency of 1kHz, the duty cycle was changed in fixed steps and plotted against the average fan current.

Duty Cycle against Fan DC Current

The duty cycle against the derived fan RPM was measured next. To derive the fan RPM, the frequency of the Tacho output was divided by two, since the fan produces 2 PPR - Pulses Per Revolution, then multiplied by sixty to convert into Pulses Per Minute aka RPM.

Duty Cycle against Fan RPM (Derived)

From tests performed on this fan, the speed changed from 1488 RPM at full speed down to 702 RPM at the lowest speed. This range of change in RPM equated to a change in duty cycle from 99% to 1%. 

One possible method to check the rotational speed of the fan, based on the duty cycle provided to the Program input, is the formula below which serves as a guide with a + 10% variation in the Tacho reading. A line of best fit was applied on test data from a single fan.

Estimated Fan Speed (RPM) = (7.9 x Duty Cycle) + 720

Where the duty cycle is the ON duration of the period as a percentage of the total period.

Test 2: Resistive Control

With a 100k variable resistor connected between the fan Program input and 0VDC, change in the fan tech output was recorded against changes in the resistance.

Test Setup: Resistance Measurements

Again the fan speed in RPM was derived from the 2 PPR Tacho output.

Program Resistance against Fan RPM (Derived)

The fan RPM varied from 1485 RPM down to 714 RPM which equated to approximately a resistance in the range of 12k to 0R. Tests were conducted with resistances over 20k with no change in the fan speed noted.

One possible method to check the rotational speed of the fan, based on a resistance between the Program input and 0VDC, is the formula below which serves as a guide with a + 6% variation in the Tacho reading. 
A line of best fit was applied on the measured data between 0 and 12k for the test fan giving the estimation below.

Estimated Fan Speed (RPM) = (0.068 x Resistance in Ohms) + 720 

Test 3: Power Supply Fluctuations

Lastly the DC supply to the fan was varied with PWM to determine the stall speed of the fan for a 1% duty cycle. It should be noted that when the supply was raised above 29VDC the fan speed became erratic. The recommended 28VDC limit listed on the unit by the manufacturer should not be exceeded.

DC supply Voltage against Fan DC Current

For the tests conducted with the fan the operational voltage range was found to be 16V to 28VDC with a stall voltage of approximately 6VDC.

Additional Measurements
The PWM frequency was varied in the range from 100Hz to 10kHz with some minor difference in the current drawn by the fan. A difference of 2% was noted between some of the duty cycle readings which may be attributed to instrumentation and or user measurement error.

As a matter of completeness the Tacho output was captured when the Program input was shorted to 0VDC, as shown below. This equated to approximately 717 RPM.

Tacho Output: Program Input Shorted to 0V

The Tacho output shown below was captured when the Program input was connected to a 100k potentiometer to 0VDC. The pot setting was at the highest resistance. This setting equated to 1428 RPM.

Tach Output: Program Input connected to 100k Potentiometer

Operating Specifications for the Comair Fan (Suggested) 
For the fan under test in this blog, the Comair Rotron CD24R7X, the suggested operating parameters are as detailed by the manufacturer with some additional information below.

Supply Voltage: 16 - 28VDC, Nominal 24VDC

Supply Current: Max 1.22A at 24VDC (Full Speed)

Fan Speed: 700-1490RPM + 5%

Program Input Resistance: 0R to 12k equating to slowest to fastest fan rotation. Estimated Fan RPM = (0.068 x Resistance in Ohms) + 720 

Program Input Frequency: PWM of 1kHz, duty cycle 1% to 100%. Estimated Fan Speed (RPM) = (7.9 x Duty Cycle) + 720

Tach Output: 2 PPR (Non-Isolated Open Collector) 28VDC at 20mA

Connections: Red           - 24VDC
                      Black         - 0VDC
                      Blue/White - Tacho Output
                      Yellow        - Program Input

Beta Layout Reflow Controller (V2) Teardown

Summary

Even though the Beta Layout Reflow Controller (V2) has become obsolete, I wanted to perform a teardown of the unit for those planning on building a similar reflow oven controller looking for design inspiration.

There are numerous Reflow Oven controllers based on Arduino, Raspberry Pi and similar based hardware platforms. Most of these designs are Open Source hardware and software allowing for improvements not available on a commercially manufactured product.

Beta Layout Block Diagram
As a basic representation the block diagram below shows the Beta Layout V2 controller.

Beta Layout V2 Controller Block Diagram

Opening the Controller

The Beta Layout plastic case does not use fixing to hold the two part enclosure together. Instead there are a pair of opposing plastic clips on the lid and base, hidden by the ventilation holes, which were released using a screwdriver to gain access to the controller.

Beta Layout V2 Controller

Inside the Case (Power Board)

Upon opening the case the power and logic boards are exposed. These are board names which I have assigned for the ease of this blog, not the manufacturer. Both boards are solder masked, immersion gold plated, untented vias and without component overlay possibly making servicing difficult.
The power board, shown in the lower part of the image above, features two IEC connections, AC transformer for logic board power, Solid State Relay (SSR), female 9 pin D connector and Thermocouple connector.

Beta Layout Power Board

The IEC connection (oven side) and SSR are protected with an M205 250V 8A ESKA sand filled fuse. Power for the logic board is provided from a 230V to 6V transformer with 300mA capability. The AC supply feeds a full wave bridge on the logic board.

Performing the control of the oven heating is the now obsolete Sharp SSR S216S02. This AC SSR was rather solidly rated at 240VAC 16A featuring zero crossing. For those in need of repairing a failed SSR, there were no direct replacement from Sharp at the time of writing this blog.

Connection for the RS232 was made with a panel mount D socket connector. The Thermocouple connector uses a two part design with what appeared to be standard hook-up wire, type TR-64, not dedicated Thermocouple wire. For reference regarding the proper cable see the IEC/ANSI details on this Omega sensor site.

Inside the Case (Logic Board)
The logic board is of a double sided design with the buttons, LEDs and crystal oscillator populated on the side facing the enclosure lid.

Beta Layout Logic Board

6VAC from the power board transformer is rectified and DC regulation is achieved with a 78L05. There appears to be an option on the connector with the AC power to drive the logic board from 5VDC directly. A single diode on one of the unused connector pins supplies the 5V rail on the board directly. At the brains of the operations is an Atmel 32A microcontroller. The thermocouple interface is made with an SPI K type thermocouple converter from Maxim - MAX6675. Lastly the TTL to RS232 is handled with a jellybean level translator from Maxim - MAX232.

Temperature Measurements
Out of curiosity the Beta Layout controller case was left open and a reflow cycle was performed. Close to maximum temperatures of some of the devices were captured. Firstly is the SSR which peaked a little over 65C however was captured using the Flir Camera when marginally cooler.

Beta Layout V2 Controller SSR Temperature

The second device measured was the AC transformer which hovered at a steady 37C.

Beta Layout V2 Controller AC Transformer

Lastly and unexpectedly the bridge rectifier rounded off the three warmest devices between the logic and power boards.

Beta Layout V2 Controller Rectifier Temperature

The bridge like the SSR had temperature peaks depending on what part of the reflow cycle was active at the time.

Summary
As was shown in a previous blog, the Beta Layout V2 Controller does the business for reflow soldering with off the shelf ovens. Some tuning of the reflow profile was required. From a hardware perspective the design would be considered outdated now, possibly the V3 controller with LCD interface addresses issues with controller connectivity and upgradeability.

The RS232 interface was possibly the only oversight at design time. USB 2.0 had been released in 2000 and this board was purportedly designed in 2008. Regardless with new technology there is an easy way to upgrade the Beta Layout V2 RS232 to USB using an Adafruit - CP2104, more in a coming blog.