Monthly Archives: August 2016

What is Vapor Phase Reflow Soldering?

Vapor Phase Reflow Soldering is an advanced soldering technology. This is fast replacing other forms of soldering processes manufacturers presently use for assembling printed circuit boards in high volumes for all sorts of electronic products. Soldering electronic components to printed circuit boards is a complex physical and chemical process requiring high temperatures. With the introduction of lead-free soldering, the process is more stringent, required still higher temperatures and shorter times. All the while, components are becoming smaller, making the process more complicated.

Manufacturers face soldering problems because of many reasons. Main among them is the introduction of lead-free components and the lead-free process of soldering. The other reason is boards often can contain different masses of components. The heat stored by these components during the soldering process varies according to their mass, resulting in uneven heat distribution leading to warping of the printed boards.

With Vapor Phase reflow soldering, the board and components face the lowest possible maximum temperatures necessary for proper soldering. Therefore, there is no overheating of components. The process offers the best wetting of components with solder and the soldering process happens in an inert atmosphere devoid of oxygen – resulting in the highest quality of soldering. The entire process is environment friendly and cost effective.

In the Vapor Phase Reflow Soldering process, the soldering chamber initially contains Galden, an inert liquid, with a boiling point of 230°C. This is same as the process temperature for lead-free Sn-Ag solders. During start up, Galden is heated up to its boiling point, causing a layer of vapor above the liquid surface, displacing the ambient air upwards. As the vapor has a higher molecular weight, it stays just above the liquid surface, ensuring an inert vapor zone.

A printed circuit board and components introduced in this inert vapor zone faces the phase change of the Galden vapor trying to cool back its liquid form. The change of phase from vapor to liquid involves the release of a large amount of thermal energy. As the vapor encompasses the entire PCB and components, there is no difference in temperature even for high-mass parts. Everything inside the vapor is thoroughly heated up to the vapor temperature. This is the biggest advantage of the vapor phase soldering process.

The heat transfer coefficients during condensation of the vapor ranges from 100-400Wm-3K-1. This is nearly 10 times higher than heat transfer coefficients involved in convection or radiation and about 10 times lower than that with contact during liquid soldering processes. The excellent heat transfer rate prevents any excessive or uneven heat transfer and the soldering temperature of the vapor phase reflow process stays at a constant 235°C.

There are several advantages from the Vapor Phase Reflow Soldering process. Soldering inside the vapor zone ensures there can be no overheating. As the vapor completely encompasses the components, there are no cold solders due to uneven heat transfer and shadowing. The inert vapor phase process precludes the use of nitrogen. Controlled heating up of the vapor consumes only one-fifth the usual direct energy consumption, and saves in air-conditioning costs.

As the entire process is a closed one, there is no creation of hazardous gasses such as from burnt flux. Additionally, Galden is a neutral process fluid and environment friendly.

A Raspberry Pi HAT with 16-Channel PWM Servo

DC servo motors are a few of the things that the single board computer, Raspberry Pi or RBPi, finds uncomfortable. The reason being the specific and repetitive timing pulses these motors require for setting their position, which the RBPi is unable to provide in the absence of a real time clock. Although the Linux kernel can do the job, it leaves the RBPi rather over taxed.

A HAT or Hardware Attached on Top board eases the situation. It takes care of all the timing requirements, runs and controls 16 Servos, and is capable of delivering pulse width modulated or PWM signals up to 1.6 KHz using 12-bit precision. Additionally, all this is completely free running that leaves the RBPi to handle everything else.

The 16-Channel 12-bit PWM/Servo HAT from Adafruit can drive 16 servos simultaneously or output PWM signals. It communicates with the RBPi through only two pins using the I2C protocol. Additional RBPi processing overhead is not required for the on-board PWM controller on the HAT board to drive all the 16 channels at a time. Moreover, you can stack more HAT boards – up to 62 of them and control 992 servos – all with only the same two pins.

Adafruit offers a Python library that you can use to immediately set up and run the servos to make your robotic system come to life. When you need to run several servos, this HAT and the Python library to go with it are the simplest and perfect solution.

The HAT board requires two levels of DC voltages. The 3V3 DC comes from the RBPi to power the PWM chip and to decide the logic levels for the PWM signals and the I2C signals. The voltage is available as soon as you plug in the RBPi – shown by the PWR or the red LED on the RBPi.

The other voltage is required for the servos, for which you need to supply 5-6V DC. Usually, most servos will be happy with only 5V, and will work a little more strongly if you give them 6V. You can connect this supply via the DC jack or the blue terminals on the HAT board. A reverse-polarity diode protects the board in case you have the wires connected in reverse. However, do not use both the DC jack and the terminal block at the same time.

Keep in mind that servos need a lot of current from the 6V DC supply. Even if you are using micro servos, they will draw several hundred mA when moving. Larger servos will need more power and you should have provision of about 2A for up to four servos. That means it is not recommended drawing this power from the 5V supply of the RBPi, as it could cause your RBPi to behave erratically. Keeping the servo power supply and the RBPi power supply totally separate gives good results.

On the RBPi, there is a place for soldering a through-hole capacitor. It is a good idea to use one if you are driving many servos. Switching motors generate dips and spikes on the power lines and these can upset the RBPi. A capacitor takes care of the sudden variations – use n*100µF, where n is the number of servos.

A Stamp-Sized Radar Sensor from NXP

Radio waves are used for different purposes other than transmitting audio, video and for communication. One of their primary uses includes detecting the presence of objects in the atmosphere, including aircraft, clouds, and precipitation. This is done mainly through Radio Detection and Ranging or RADAR. By noting the time of flight that a single pulse takes to return after reflection from an atmospheric object, it is possible to estimate the distance of the object.

To detect a target, radar systems generate an electromagnetic pulse, focus it, and transmit it using an antenna. Objects in the path of the transmitted pulse scatter most of its energy. However, some of this scattered energy returns to the radar system and is gathered by the same antenna, which then feeds it into a receiver.

The receiver determines the time taken for the pulse to make a round trip from the radar to the target and back. As the electromagnetic pulse travels at the speed of light, its multiplication with the time of travel gives the total distance travelled by the pulse. Therefore, the actual distance to the target is half this total distance.

Manufacturers feel radar is a versatile gadget for use in automobiles. For instance, it can help the driver estimate the distance between his/her vehicle and other objects in front, sides, or back – promoting safer driving. Following this lead, manufacturers have been shrinking the size of the radar system to make it suitable for use in automobiles.

At present, the smallest radar is the 77GHz radar transceiver from NXP Semiconductors N.V. It is a single chip device, roughly the size of a postage stamp. Consequently, manufacturers can place the chip anywhere in the vehicle. This is a very big advantage to vehicle designers, as they are targeting driverless, fully automated driving in the near future, and need increasing numbers of sensors within the vehicle. In fact, Google engineers are already field testing working prototypes of the NXP device for their self-driving cars project.

The reference design from NXP is a 35×35 mm printed circuit board and it has a radar front-end, two MCUs for signal processing, and supporting components. Designers can use this in their self-driving cars, in the form of a cocoon comprising 10-20 tiny radar sensors all around the vehicle to provide a high-resolution, 360-degree view of the environment around it.

ADAS or Advanced Driver Assistance Systems also use radar as their core technology, using it to make driving easier and safer. For instance, they use it for adaptive cruise control, lane change assist warnings, forward collision warnings, blind spot monitoring, emergency braking, and automated braking. According to IHS Research, the market for radar-based ADAS will grow by 23 percent year-on-year, increasing from the current year to more than 50 million radar sensors.

Although alternate technologies presently exist for avoiding collisions, mostly in the form of laser-light and ultrasonics based systems, the 77GHz radar offers a superior performance under adverse conditions such as road grime, fog, and rain. So far, bulky hardware had made it difficult to use radars in vehicles, but not anymore.

How Do You Read Resistor Values?

Resistors range from huge multi-watt giants to sub-miniature surface mount devices (SMDs) and parts with different types of leads in between. The larger varieties do not pose much of a problem as they usually have a big-enough surface for printing the value of the resistance, its tolerance, and other necessary specifications. For smaller sizes, codes are generally used for letting the user know the details of the resistor.

Two common methods are under use for identifying resistors – color coding for resistors with leads and number coding for SMD resistors. Color coding is an easy way to convey a lot of information concisely and effectively. One of the advantages is that specifications of the resistor are visible irrespective of its orientation on the PCB – very useful for overcrowded boards. As SMD resistors have only limited surfaces, number coding is more suitable.

Color coding for resistors

Resistors with color coding come with one of two standard codes – the 4-band code or the 5-band code. The 4-band coding is used more with resistors of low precision with 5, 10, and 20% tolerances. Higher precision resistors with tolerances of 1% and lower are marked with 5-band color codes.

The colors used have their own values. For example, Black represents zero, Brown represents one, Red represents two, Orange represents three, Yellow represents four, Green represents five, Blue represents six, Violet represents seven, Gray represents eight, White represents nine, Gold represents 0.1, and Silver represents 0.01.

For tolerances, Gray represents ±0.05%, Violet represents ±0.1%, Blue represents ±0.25%, Green represents ±0.5%, Brown represents ±1%, Red represents ±2%, Gold represents ±5%, Silver represents ±10%, while an absence of color represents ±20%.

The 4-Band color coding scheme

The 4-band color coding has thee color bands crowded on one side with the fourth band separated from the others. One has to read the code from the left to right beginning with the crowded colors on the left and the separated color band on the extreme right. Starting from the left, the first two color bands represent the most significant digits of the resistance value, while the third band represents the multiplier digit. The isolated fourth band is the tolerance band. As an example, a resistor of 4.7KΩ, 5% value will have the colors bands Yellow, Violet, and Red representing 4700Ω, with a fourth band of Golden color. In cases where there are only three color bands, it means the resistor has a ±20% tolerance.

The 5-band color coding scheme

High quality, high precision resistors with tolerances of 2%, 1% or lower are represented by five color bands, with the first three denoting the three most significant digits of the resistance value. The fourth band represents the multiplier value, while the fifth stripe gives the tolerance. Some resistors have an additional sixth band denoting the reliability or the temperature coefficient.

Number coding for SMD resistors

SMD resistors usually have three or four numbers on them, depending on whether they are of 5% or 1% tolerance. The last number is the multiplier with the others representing the most significant digits of the resistance value. In some cases, an alphabet is used, representing the resistor’s tolerance. However, if the alphabet is an R, it represents a decimal at its position. For more details, refer to this web site.

How Gray Are The Gray Codes?

Many data acquisition systems and rotary encoders use the Gray Codes for their operation. As only one bit changes state while the numbers progress, read errors from timing and mechanical issues are minimized. Initially, use of Gray Codes was limited to specific applications, but now this versatile coding scheme is extensively used in Karnaugh maps, error detection systems, and in rotary and optical encoders.

In general, a Gray Code represents numbers using the binary encoding scheme and it groups a sequence of bits such that only one bit in the group changes from the number before and after it. Frank Gray, a researcher from Bell Labs, described the code in his 1947 patent, where he called it the Binary Reflected Code. After the patent was granted in 1953, the encoding system was referred to as the Gray Code.

Being an un-weighted code, the columns of bits in the Gray Code do not imply any base weight in contrast to the Binary number system. For instance, in the Binary number system, the right most column holds the most significant bit and carries a weight of 20=1; the second column has the weight of 21=2; the third 22=4, and so on. That means each column represents a base (2 in Binary) raised to a power, with the final value calculated by multiplying the bit by the weight of its column and adding the results of the columns.

Although columns in the Gray Code are also positional, they are not weighted, as the Gray Code is a numeric representation of a cyclic encoding scheme. The code rolls over and repeats, therefore, it is unsuitable for mathematical operations. To be used in displays or in mathematical computations, Gray Code sequences need to be converted to Binary or Binary Coded Decimal (BCD).

Gray Codes are a member of unit-distant, minimal-change codes. That means only a single bit of the sequence changes with the progress of the number count. Therefore, Gray Codes are more flexible during synchronization and misalignment as they limit the maximum read error to one unit. This property makes them useful in error detection schemes as well. Communication systems use Gray Codes in preference to parity check, as detection of unexpected changes in data is better with Gray Codes. If you sum up the bits in a number, the sum of the next number will change only by one, with the sum alternating even and odd.

Rapidly changing values can lead to errors due to interfacing or hardware constraints. This is where the Gray Code is most useful, as only a single bit quantifies the change. That is also the reason most mechanical rotary and optical encoders offer Gray Code outputs. However, Gray Codes have progressed farther than the encoding mask that Frank Gray documented in his patent.

For example, aircrafts use mechanical altimeters where the encoding disk is synchronized to the dials, producing a sort of Gray Code output known as the Gillham Code. The specialized code offers a single-bit change for each increment of 100 feet – allowing an easy tracking of the altitude.