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 Reflow Controller with USB (FTDI FT311) connection to Android Phone

Summary
As a follow-on from the original Beta Layout post, this information shows one method of connecting the Beta Layout reflow controller serial interface, to an Android compatible phone with some off the shelf hardware.

Reflow Controller
The Beta Layout Reflow Controller (V2) provides a connection to it's inner workings through a serial port (RS232), 9 pin D type connector. While moving a laptop to the reflow controller every time the controller requires and adjustment, there are devices such as the FTDI USB specific hardware to suit interfaces with Android phones.

USB Development Module
One of these devices, FT311, is a plug and play USB Host chip for Android devices. There is an associated development module, UMFT311EV, that provides a number of interfaces, one being RS232. The header pinouts on the module suits off the shelf adaptor boards and some shields.

FTDI FT311 Dev Module

RS232 Adaptor
The FTDI board is TTL so an RS232 shield such as the model from DFRobot can be used to make the required conversion.

DFRobot RS232 Shield

While the 5V and 0V power header is pin compatible between the boards, the communications header with TX, RX, CTS and RTS requires a few jumpers.

Linking Interboard TTL
From Section 4.1.2 of the UMFT311EV datasheet the hardware connections are identified.

UART Hardware Pinouts

Since the hardware handshaking is not used these two pins 5 and 6, can be joined together for now.

DF Robot J1 Pinouts

The corresponding Tx and Rx connections as shown on the DF Robot shield schematic, follow the Arduino shield mapping and are available on pins 1 and 2.
To make the modifications, pins 1 and 2 on the Robot shield are snipped off or pulled to the side as not to mate with the FT311 development module.

DF Robot Jumper Connections

Using wire links or a pin header inserted into J1 on the Robot shield, pin 1 RXD is linked to pin 4 or RXD for the USB. Then pin 2 TXD is linked to pin 3 or TXD for the USB. Lastly pins 5 and 6, CTS and RTS are linked.

The FT311 module and Robot shield can be fitted together.

RS232 Cable
Since both the communications devices sport a 9 pin female D connector a null modem cable is required between them.

Hardware Assembly
The other two pieces of hardware required are a power supply or plug pack to suit the development module and the USB charging cable used with the Android phone.

Assembled FT311 and RS232 Shield Hardware

Android Terminal Program
To communicate with the Development board FTDI provide AOA HyperTerm, which is a basic terminal interface for the Android. Available on the Google Play Store and passes the Android MyPermissions and 360 Security checks.

With the HyperTerm application installed and the hardware setup powered then phone can be connected to the charging cable. In doing so the HyperTerm application is automatically launched.

AOA HyperTerm Application from FDTI

To configure the communications select the Settings button then choose the interface required. For the Beta Layout reflow controller, 9600, 8, N, 1 are the communications settings and since the hardware handshaking is looped at the DF Robot shield, the default settings in HyperTerm can be used.

AOA HyperTerm Communications Settings

After selecting Configure commands can be exchanged with the reflow controller.

AOA HyperTerm Communications Settings Confirmation

Starting with sending help and a CR the list of available commands is returned.

Beta Layout commands on AOA HyperTerm

In the screenshot above the help screen and status data is displayed in the HyperTerm window. The status information is configured for bursts at five second intervals which is used to track the starting and operational temperatures.

The final setup of the hardware as used on the bench is shown below. For similar and compatible Android USB hosts, shields or development kits the same process should be possible!

Beta Layout, USB Dev Kit, RS232 Shield with Android and AOA HyperTerm

Beta Layout Reflow Controller with Sunbeam BT2600 Mini Bake and Grill for Reflow Soldering

Summary
While designing a prototype circuit for an LED torch it became apparent that many of the white LED controllers were surface mount leadless devices. Many of these devices have exposed pads meaning that standard soldering with an iron can be difficult.

It is possible to solder smaller devices with exposed pads by directly applying the heat beneath the device through thermal vias in the PCB, or use the oven in the household kitchen to reflow, however neither of these are ideal solutions.

Reflow Controller
Beta Layout have for some time sold a Reflow Controller. This controller (Version2) has been reviewed with the Severin oven which Beta Layout provides however there are no reviews for ovens available locally in Australia. 

Beta Layout V2 Reflow Controller

At the time of writing the Beta Layout controller sold for €119 plus shipping. There are a number of kits and Arduino style solutions on the web although for an off the shelf solution this was worth trying the controller with a locally available oven to determine suitability for home reflow soldering.

Reflow Oven
The only factor determining the oven to choose was the maximum load of the reflow controller which, for the version two model, is 1500W.

Searching the local market results in a grill from Sunbeam BT2600 with elements top and bottom. The oven retails from the GoodGuys for just under $60 AUD.

Sunbeam BT2600 Oven

Noteworthy Items
It should be noted that as the controller is European made the associated mains plug and socket suit the European market. No option to order an Australian plug to female IEC plug or male IEC to Australian socket.

Oven and Reflow Controller Learn Cycle
Beta Layout's controller is for the most part plug and go. On the hardware side there is the main AC power, oven AC power and the thermocouple. 

A learning cycle must be completed before the controller can be used. The thermocouple can readily be lashed to a dummy PCB (spare leaded components or enamelled copper wire) and placed in the most central position inside the oven.

Sunbeam BT2600 Oven Inside

With the door down the upper and lower heating elements are easily visible. On the front control panel the oven is switched to 240C and the timer set to OFF.

The dummy PCB is placed in the middle of the oven, door closed, power ON and the learning process is ready to start.

Test PCB for Learning Cycle

Once the learning button is pressed the oven comes to the required temperature and then the oven switches off. Subsequently it should be noted that the oven was left running standalone, in the garage, for an additional 10 min to reduce some of the VOCs. Time for a test board.

Test Board and Oven Temperature Profile

For the reflow tests the same PCB is used. A few resistor pad pairs are cleaned with flux, solder paste added and surface mount resistors applied. To apply the solder paste without a stencil, a syringe with a 0.5mm needle allows a relatively clean and controlled application of the paste.

With the PCB placed in the middle of the oven, the door is closed and the solder button is pressed on the controller. The thermocouple was left attached to the test PCB.

Beta Layout Controller - Preheat Stage

Several minutes later the reflow process has completed and the oven has cooled down sufficiently that the PCB can be checked.

Test PCB after Reflow

Inspecting the PCB, all joints to the resistors are clean and the component alignment is good - not faultless. Changes to the profile may help eliminate some bubbling on the joints.

Before operating the oven again the RS232 port on the controller is connected to a laptop and a standard terminal program, TeraTerm. Using the controllers interface the automatic temperature measurements were configured to be output at 5 second intervals. A number of cycles were run across the day as the ambient temperature increased.

Below are the graphed profiles of the oven with some minor variation in the starting ambient temperature.

Reflow Oven Temperature Profile

Several subsequent tests using the oven yielded repeatable reflow results with similar sized PCB's. Reducing the oven thermostat from 240C to 210C made little difference to the results however reducing the thermostat to 190C caused issues with proper wetting and should be avoided.

Shortly after the reflow cycle was complete the door of the oven was opened and a thermal image taken. Some of the heat has already escaped from the opening of the door.

BT2600 thermal image shortly after reflow

The temperature profile used by the controller can be manually configured through the serial interface using a terminal program. 

In summary this combination of controller and oven is a valuable addition to the prototyping hardware setup which can yield excellent results for the home hobbyist.