Soldering Machines

This post is written on the request of one of the reader of our blog. thanks for the mail, it encourages us to write more articles when readers request for more.

Soldering is a process which is used to join two materials using a joint. The joint usually called a soldering material which has a lower melting point. Soldering machines are the tools used to do the soldering process. One more material used during the soldering process is the flux. Flux facilitates the soldering process. Soldering machines come in various shapes and sizes. It can be in the form of a gun or it can be in the form of a handle. A removable heat element at the tip of the machine facilitates soldering process. 

Temperature controlled or not?

The soldering machines are either temperature controlled or may be of fixed temperature. The temperature controlled soldering machines are a big set up and are commonly named soldering station. the soldering station is a combination of heat element, handle and a power supply regulation circuit. Soldering stations do have a display to read the tip temperature. A thermistor on the tip enables to read the temperature which is displayed on the screen.

Soldering iron power rating.

           Soldering machines are available in various power ratings. A machine with high power rating should not be used for minute joint soldering. That may damage the electronic components. Where as low power machines should be used for minute joints. the power ratings of the soldering machines range from 10-50W.

Soldering iron tip

Generally called a bit which can be detachable and comes in various sizes. The size is determined by the size of the joint that must be soldered. There are various tip sizes available. A slanted tip at the edge may have a more heat holding capability where as straight bits have less heat holding capability.

Various vendors who provide soldering machines:

METCAL

SOLDRON

ADVANCETECH

MAX TECHNOLOGY

METROQ

Wetting current (Sealing current)

Switches/Relays require some initial current to break the oxidation layer that is formed across the contacts. Supplying this amount of current helps the contact form reliably. The current supplied heats the oxide layer. This amount of current is obviously an additional burden on your power circuit but is the minimum current required to form a contact. Oxidation generally happens in humid conditions where the oxide layer is formed across the joints. The oxidation can generally be treated as resistance when you speak as an electrical engineer. The scenario of wetting current can be seen in the case of switches where you provide the stimulus but still doesn't turn up, where little more current would have been required there. Generally, in the humidity tests, these kind of issues come up on the board. The amount of whetting current depends on the material that is used for the contacts. For example, a gold plated contact may form a reliable contact with less wetting current. the two generic circuit parameters you may across in this scenario are punch through and stand-off voltage. Stand-off voltage is the minimum voltage that need to be provided for the oxide layer to break through. 

Dithering in circuits

Are you designing a board which requires agency certifications? Do you want to control any EMI/EMC concerns that are arising out of power supply section. One way to address your concerns is dithering of power supply. You might have taken proper concerns in layout but sometimes the power supply may be the source of noise in spite of extreme care taken in design. Dithering is nothing but a small electronic circuit to control the switching frequency of the power supply. The frequency spectrum of the oscillator can be spread to increase the band of frequency. Frequency spread meant distributing the total energy among the multiples of fundamental frequency. The dithering circuit is a simple op-amp circuit controlling the voltage into one of the frequency determining pin of the switching power supply. The voltage on this pin of switcher determines the frequency of operation of the switcher. Have you ever used a spectrum analyzer to probe a frequency generator in the circuit? If you try to probe the signal, you see a peak at the fundamental frequency of the switcher but if the signal is measured with dithering applied we can see the signal distributed over a band of frequencies. This technique can also be said as spread spectrum where the frequency is spread over a band.

PCB Routing guidelines - Part 1 (Power Supply)

Power supply design is one of the critical factor in circuit design. Designer may design a flawless design but if they don't give sufficient guidelines to the layout engineer or if the layout engineer doesn't follow the appropriate guidelines the whole effort to put in during the design phase may be wasted. Whether you are working with AC or DC, or whether you are working with linear supply or switching regulator, following the recommended guidelines is a must. The thermal performance, functional qualification, certification (EMI/EMC) depends on the proper layout of power supply also. Finally, a good stable power supply is a result of good layout design practices. We always have to 

remember that switching power supplies are the main sources of EMI in the circuits as they contain the switching elements. Based on the switching that is happening in the switching mode power supply the noise is either radiated or coupled to other circuits. 

The collection of layout guidelines that need to be followed by the layout engineer are listed below.

1. Have a separate plane for power and signals. 

2. Isolate the power and signal planes with continuous ground plane.

Note: For the above two points we are talking about the multi-layer boards.

3. The ground path loop should be as small as possible. 

4. Have a complete copper pour underneath the switcher/linear regulator and connect this to thermal pad. This helps to sink the heat to the PCB.

5. Placement is very critical in regulators. Placement must be in such a way that all the components should not be distributed randomly.

Note: For the above point reason is that the return current should not change direction.

6. Never try to connect the grounds in a daisy chaned way. The noise on one circuit may propogate to other easily. Try to use a star type routing for ground. 

7. Isolate the signal and power grounds and have a common node for connecting all together. 

8. Place the capacitors at the input and output of regulator as close as possible. 

Reason: The additional trace acts as inductor affecting the transient response of the switcher.

9. The grounding of the capacitor should be as close as possible to the supply regulator. Avoid longer ground traces. Having a lesser ground trace lengths help to eliminate the unnecessary ground loops.

10. The most important point to remember is that layout engineer should always know the amount of current that flows on the power traces. Correspondingly, the width of the must be layout. 

11. When there are via to connect different points of the circuit, have sufficient stitching via to support desired current.

12. The power trace connecting the various points should not be free flown. The length should be maintained as short as possible.

13. The board will have signal traces and power traces as well. Power traces should be maintained thicker to allow sufficient current to flow. Irrespective of lowest current limit, the power trace should be > 12mils always and further based on current flow, the width will change.

14. Switching regulators have a inductor at their output, take care that you don't route the low power traces near the inductor. 

15. The placement of digital circuit must be such a way that they should be placed away from the inductor.

Reason: The reason for the above two points is that inductors are the source of EMI on the boards.

16. The feedback path in the regulator is very critical to determine the output of regulator. Try to keep the resistive dividers in the feedback path as close as possible to the feedback pin. If not possible, route the feedback trace thicker. 

17. The feedback trace must be away from all the noisy paths. If required provide ground isolation to the feedback paths from other circuits/traces.

18. The components at the output of regulator (inductor, capacitor, schottky) should be placed as much closer as possible. 

19. Have a perfect ground plane around and underneath the regulator. This helps any EMI from the power circuit to ground directly.

What are the effects of bad layout? The effects of layout are more seen as the load current is more.

1. Desired regulation may not happen.It meant the output may vary from the desired output.

2. Too much noise at the output of the regulator.

3. Increases design iterations and hence increases project cost.

4. Has to have a good mechanical design to compensate for bad layout practices.

5. Ground bounce leading to improper ground reference.

6. Simultaneous switching noise affecting signal integrity.

"Free Synch" in displays

Refresh rate is one of the most important specification of monitors. Monitors often refresh their buffers at a standard rate. This for the displays is constant at 60Hz. But these days displays are coming up with different refresh rates which is going up to 120Hz. Also, that the refresh rate can be varied dynamically based on activities on the display. These days the graphics processors are becoming performance intensive with extreme stress on display technologies. the display should sync w.r.t graphics capability of processor else you may see monitor breaking down in extreme cases (like for example take the case of high end gaming). To eliminate this sync issues, AMD came with a new technology called free sync where the refresh rate of display can be varied as per the performance requirement. Let us assume a case where there is no activity on the display, in such a case refresh rate can be reduced to lowest, thus, helping in lower power consumption. Screen-tear is one of the problems seen in high end gaming where the screen is overlapped or occupied by the previous screen. Free sync is enabled on display port with capability of performing in extremely performance intensive applications. Most of the PC games demand change in frame rates and free sync helps to compensate as per demand. The older v-sync process can't be used here as the frame expectation is different from graphical processor and display technology.

        Free sync from AMD is similar to the G-sync from Nvidia. There is a compability issue here. To take advantage of G-sync both the display and GPU should have that technology. This doesn't mean that a G-sync can't be used with normal processors. R9 series from AMD is one example which has the Free sync technology. 

Latch Up

Have you ever faced a situation in which a component fails immediately after power up? or did you observe your circuit recovering after power cycle? Then you might have observed the latch up condition unknowingly. Latch up occurs when unintentional low impedance path is formed in the digital circuits of a integrated circuit. The integrated circuit is generally built with mosfet structures. So, most of the latch issues are seen in CMOS logic. The parasitic paths formed within the mosfet structure leads to high current and hence damage the mosfet.

To explain it more from the mosfet structure, latch is the generation of a low-impedance path in integrated chip between the power supply and the ground rails due to interaction of parasitic pnp and npn bipolar transistors. One has to remember that the circuit designer using CMOS logic integrated circuit can't avoid latch up as this can be eliminated only when integrated circuit physical design is done.

Have you tried power sequencing in a circuit, or tried power states in the board? During these conditions, we will try to power on/power off various circuits on the board. In that case, when signals are applied at the input of unpowered CMOS circuit, latch up occurs, where the unintentional current may be transferred. Latch up may also occur when the signals applied to a CMOS logic cross the power thresholds. 

Inrush current limit - Part 1

Are you expecting inrush current in your circuit? or have you heard you heard about the surge current? Have you ever dealt with surge current? Are you a beginner and don't know what component/circuit to use? Then after reading this article you must be able to decide after need to be done. Also, if we see our SMPS designs, inrush current limiter is commonly used.

             To start this discussion, let us take a scenario when we power on the circuit. Initially, there will be a surge current that flows. This is because of the capacitor bank that is present in the circuit which draw a significant current instantaneously to charge themselves. Also, we can put it the other way, when the current drawn by the load is greater than steady state current of the circuit, it is called surge current. Surge current may not be significant in some cases. This depends on the underlying circuitry used. If the surge current is of small duration such that the circuitry doesn't get affected by it, then we need not have any protection. Also, the affect of surge current may not be straight forward, it may reduce the life time of a component. The graph here shows the surge current drawn in a circuit over a time.

                 The tradition way of limiting the current is to use a fuse to limit the current. But if the current exceeds the fuse limit, then fuse blows off and only way to recover the circuit functionality is to replace the fuse with a new one. But this is a tedious job. One way to avoid fuse blow is to use some other circuit which handles the inrush current. Thermistor is one such component. we always use resistors in circuits to limit the current, for example, if we take the case of LED, it is always preferable to use a series resistor. But series resistor will not suffice if the inrush current is in amperes range and in fact the resistor may blow away. Also, resistors doesn't have the property to change their resistance unlike thermistor. Thermistors are better suited for these applications.

Thermistors are of type positive temperature co-efficient (PTC) and negative temperature co-efficient (NTC). NTC type are used as inrush current limiters. NTC thermistors are designed to be used in inrush limiter circuits. The principle of NTC is that resistance decreases with increase in temperature. so, initially, thermistor has very high resistance. As the current starts flowing, the resistance decreases as drop/temperature increases. 

     Using thermistor is also a cheaper option rather than having a complex circuitry (Active circuit) to limit the current. NTC is the most commonly used in the industry. 

General specifications to be checked while selecting a NTC/PTC as current limiter:

1. Resistance offered vs temperature 

2. Hold current (current at which thermistor provides low resistance)

3. Trip current (the current at which the thermistor offers high resistance)

4. Recovery time (for thermistor to come back to high resistance after the circuit has switched off)

Advantages of using thermistors:

1. Reliable

2. Stable

3. Lower cost

4. Lesser footprint

5. Easy to include in the circuit and easy to select

6. Longer life time

Some of the disadvantages of using inrush current protection:

1. When thermistor is used, there will be a voltage drop across it.

2. For a small form factor boards, with sizable power consumption (current draw by load), we need to include a bigger size thermistor which may be a limiting factor

3. High recovery time for thermistors.

Important points of thermistor as Inrush current limiter:

1. Resistance drops to milli ohms within in milliseconds

2. The resistance offered by thermistor during idle state will be in hundreds of ohms.

How to select the resistance of inrush current limiter?

Let us assume that we have a circuit with input voltage 12V and load current of 1A. In such cases, the value of thermistor is selected such that it allows min. 1A and cuts off above that current. So, here resistance if V/I = 12 ohms. So, have a thermistor of at least 12 ohms.