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chapter3_MIT_6270_Manual - Chapter 3 Electronic Assembly...

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Unformatted text preview: Chapter 3 Electronic Assembly Technique 3.1 Electronic Assembly Technique If there are places in life where neatness counts," electronic assembly is one of them. A neatly-built and carefully soldered board will perform well for years; a sloppily- and hastily-assembled board will cause on-going problems and failures on inopportune occasions. This section will cover the basics of electronic assembly: proper soldering technique, component mounting technique, and component polarities. By following the instructions and guidelines presented here, you will make your life more enjoyable when debugging time rolls around. A rule of thumb is that a job may take one hour to solder, but if there is a mistake, it take 3 hours to undo. A little extra care will save you time in the long run. 3.1.1 Soldering Technique Figure 3.1 shows proper soldering technique. The diagram shows the tip of the soldering iron being inserted into the joint such that it touches both the lead being soldered and the surface of the PC board. Then, solder is applied into the joint, not to the iron directly. This way, the solder is melted by the joint, and both metal surfaces of the joint the lead and the PC pad are heated to the necessary temperature to bond chemically with the solder. The solder will melt into the hole and should ll the hole entirely. Air or gaps in the hole can cause static discharges which may damage some components. Figure 3.2 shows the typical result of a bad solder joint. This gure shows what happens if the solder is painted" onto the joint after being applied to the iron directly. The solder has balled up," refusing to bond with the pad which did not receive enough heat from the iron. 37 38 CHAPTER 3. ELECTRONIC ASSEMBLY TECHNIQUE Feed solder on opposite side from soldering iron so that the solder is melted into the joint. Soldering iron positioned so that tip touches both the pad on the PC board and the component lead coming through the hole Figure 3.1: Proper Soldering Technique If you feed the solder into the soldering iron rather than the joint, the solder will ball up, refusing to bond with the improperly heated PC board pad. Figure 3.2: Improper Soldering Technique 3.1. ELECTRONIC ASSEMBLY TECHNIQUE 39 With this technique in mind, please read the following list of pointers about electronic assembly. All of these items are important and will help develop good skills in assembly: 1. Keep the soldering iron tips away from everything except the point to be soldered. The iron is hot and can easily damage parts, cause burns, or even start a re. Keep the soldering iron in its holder when it is not being held. 2. Make sure that there is a damp sponge available used for cleaning o and tinning the tip. Soldering is basically a chemical process and even a small amount of contaminants can prevent a good joint from being made. 3. Always make sure that the tip is tinned when the iron is on. Tinning protects the tip and improves heat transfer. To tin the iron, clean the tip and wipe it on a damp sponge and then immediately melt some fresh solder onto the tip. The tip should be shiny and coated with solder. If the iron has been idle for a while, always clean and then re-tin the tip before continuing. 4. The tips of the irons are nickel-plated, so do not le them or the protective plating on the tips will be removed. 5. A cold solder joint is a joint where an air bubble or other impurity has entered the joint during cooling. Cold solder joints can be identi ed by their dull and mottled nish. The solder does not ow and wrap around the terminal like it should. Cold joints are brittle and make poor electrical connection. To x such a joint, apply the tip at the joint until the solder re-melts and ows into the terminal. If a cold solder joint reappears, remove solder with desoldering pump, and resolder the joint. 6. Do not hold the iron against the joint for an extended period of time more than 10 seconds, since many electronic components or the printed circuit board itself can be damaged by prolonged, excessive heat. Too much heat can cause the traces on the printed circuit board to burn o . Some components that are particularly sensitive to heat damage are: diodes, ICs, and transistors. 7. It is good practice to tin stranded wire before soldering to other components. To tin the wire, rst strip the insulation, and twist the strands. Apply heat with the soldering iron and let the solder ow between the strands. 40 CHAPTER 3. ELECTRONIC ASSEMBLY TECHNIQUE 8. After a component has been soldered, clip the component's leads wires coming 1 out of the component away from the printed circuit board. Leave about a 8 of the lead sticking out of the board. When clipping the leads, face the board and the lead down into a garbage bag or into your hand. Leads tend to shoot o at high speeds, and can y into someone's eye. 00 3.1.2 Desoldering Technique It takes about ten times as long to desolder a component than it did to solder it in the rst place. This is a good reason to be careful and take one's time when assembling boards; however, errors will inevitably occur, and it's important to know how to x them. The primary reasons for performing desoldering are removing an incorrectly-placed component, removing a burnt-out component, and removing solder from a cold solder joint to try again with fresh solder. Two main methods of desoldering are most common: desoldering pumps and desoldering wick. The 6.270 toolkit includes a desoldering pump as standard equipment. To use a desoldering pump, rst load the pump by depressing the plunger until it latches. Grasp the pump in one hand and the soldering iron the other, and apply heat to the bad joint. When the solder melts, quickly remove the soldering iron and bring in the pump in one continuous motion. Trigger the pump to suck up the solder while it is still molten. Adding additional solder to a troublesome joint can be helpful in removing the last traces of solder. This works because the additional solder helps the heat to ow fully into the joint. The additional solder should be applied and de-soldered as quickly as possible. Don't wait for the solder to cool o before attempting to suck it away. The desoldering pump tip is made of Te on. While te on is heat-resistant, it is not invincible, so not jam the te on tip directly into the soldering iron. Solder will not stick to Te on, so the desoldering operation should suck the solder into the body of the pump. Desoldering works e ectively when the joint is hot, and there is ample solder to be removed. Additional solder can be added to joints that are di cult to desolder. The additional solder transfers heat to the existing solder, allowing it to be de-soldered more easily. 3.1.3 Component Types and Polarity There will be a variety of electronic components in use when assembling the boards. This section provides a brief introduction to these components with the goal of teaching you how to properly identify and install these parts when building the boards. 3.1. ELECTRONIC ASSEMBLY TECHNIQUE 41 Component Polarity Polarity refers to the concept that many electronic components are not symmetric electrically. A polarized device has a right way and a wrong way to be mounted. Polarized components that are mounted backwards will not work, and in some cases will be damaged or may damage other parts of the circuit. The following components are always polarized: diodes LEDs, regular diodes, other types transistors integrated circuits Capacitors are an interesting case, because some are polarized while others are not. Fortunately, there is a rule: large capacitors values 1 F and greater are generally polarized, while smaller ones are not. Resistors are a good example of a non-polarized component: they don't care which direction electricity ows through them. However, in the 6.270 board, there are resistor packages, and these have non-symmetric internal wiring con gurations, making them polarized from a mounting point of view. The following paragraphs discuss the aforementioned components individually, explaining standardized component markings for identifying a component's polarity. Resistors Resistors are small cylindrical devices with color-coded bands indicating their value how to read color-coding is explained in a subsequent section. Most of the resistors in the 6.270 kit are rated for 1 watts, which is a very low 8 power rating. Hence they are quite tiny devices. A few resistors are much larger. A 2 watt resistor is a large cylindrical device, while a 5 watt resistor has a large, rectangular package. Resistor Packs Resistor packs are at, rectangular packages with anywhere from six to ten leads. There are two basic types of resistor pack: Isolated Element. Discrete resistors; usually three, four, or ve per package. Common Terminal. Resistors with one pin tied together and the other pin free. Any number from three to nine resistors per package. Figure 3.3 illustrates the internal wiring of an 8-pin resistor pack of each style. 42 CHAPTER 3. ELECTRONIC ASSEMBLY TECHNIQUE Isolated Element 4-pack Common Terminal 7-Pack Figure 3.3: Resistor Pack Internal Wiring Cathode Anode Figure 3.4: Typical Diode Package Diodes Diodes have two leads, called the anode and cathode. When the anode is connected to positive voltage with respect to the cathode, current can ow through the diode. If polarity is reversed, no current ows through the diode. A diode package usually provides a marking that is closer to one lead than the other a band around a cylindrical package, for example. This marked lead is always the cathode. Figure 3.4 shows a typical diode package. LEDs LED is an acronym for light emitting diode," so it should not come as a surprise that LEDs are diodes too. An LED's cathode is marked either by a small at edge along the circumference of the diode casing, or the shorter of two leads. Figure 3.5 shows a typical LED package. Integrated Circuits Integrated circuits, or ICs, come in a variety of package styles. Two common types, both of which are used in the 6.270 board design, are called the DIP for dual-inline package, and the PLCC for plastic leaded chip carrier. In both types, a marking on the component package signi es pin 1" of the component's circuit. This marking may be a small dot, notch, or ridge in the package. After pin 1 is identi ed, pin numbering proceeds sequentially in a counter-clockwise fashion around the chip package. 3.1. ELECTRONIC ASSEMBLY TECHNIQUE 43 Side View Cathode Anode (-) Bottom View (+) Cathode Anode (-) (+) Flatted rim indicates cathode. Short lead indicates cathode. Figure 3.5: Identifying LED Leads Pin 14 Pin 8 Notch marking Pin 1 Pin 7 Figure 3.6: Top View of 14-pin DIP 44 CHAPTER 3. ELECTRONIC ASSEMBLY TECHNIQUE Pin 1 Marking M Motorola 68HC11A0 Figure 3.7: Top View of 52-pin PLCC Figure 3.6 shows the typical marking on a DIP package. Figure 3.7 is a drawing of the PLCC package. DIP Sockets Most of the integrated circuits ICs are socketed. This means that they are not permanently soldered to the 6.270 board. Components that are socketed can be easily removed from the board if they are damaged or defective. Do not place the components into the sockets before you mount the sockets onto the board! Sockets are also used to avoid the need to solder directly to ICs, reducing the likelihood of heat damage. DIP sockets also have a similar marking to those found on the components they will be holding. DIP sockets are not mechanically polarized, but the marking indicates how the chip should be mounted into the socket after the socket has been soldered into the board. PLCC Sockets PLCC sockets are polarized, however: a PLCC chip can only be inserted into the its socket the correct" way. Of course, this way is only correct if the socket is mounted right in the rst place. When assembling the 6.270 board, a marking printed onto the board indicates the correct orientation of the PLCC socket. There are smaller corner holes that will help you orient the socket. Place the socket on the board and double check the polarity before soldering. Capacitors Quite a few di erent kinds of capacitors are made, each having di erent properties. There are three di erent types of capacitors in the 6.270 kit: 3.1. ELECTRONIC ASSEMBLY TECHNIQUE 45 Monolithic. These are very small-sized capacitors that are about the size and shape of the head of a match from a matchbook. They are excellent for use when small values are needed 0.1 F and less. They are inexpensive and a fairly new capacitor technology. Monolithic capacitors are always non-polarized. Electrolytic. These capacitors look like miniature tin cans with a plastic wrapper. They are good for large values 1.0 F or greater. They become bulky as the values increase, but they are the most inexpensive for large capacitances. Electrolytics can have extremely large values 1000 F and up. They are usually polarized except for special cases; all the electrolytics in the 6.270 kit are polarized. Tantalum. These capacitors are compact, bulb-shaped units. They are excellent for larger values 1.0 F or greater, as they are smaller and more reliable than electrolytic. Unfortunately they are decidedly more expensive. Tantalum capacitors are always polarized. As indicated, some capacitors are non-polarized while other types are polarized. It's important to mount polarized capacitors correctly. On the 6.270 boards, all polarized capacitor placements are marked with a plus symbol + and a minus symbol ,. The pads on the boards are also marked di erently. The negative lead , goes through a square hole and the positive lead + goes through the round hole. The capacitors themselves are sometimes are obviously marked and sometimes are not. One or both of the positive or negative leads may be marked, using + and , symbols. In this case, install the lead marked + in the hole marked +. Some capacitors may not be marked with + and , symbols. In this case, one lead will be marked with a dot or with a vertical bar. This lead will be the positive + lead. Polarized capacitors that are mounted backwards won't work. In fact, they often overheat and explode. Please take care to mount them correctly. Inductors The inductor used in the 6.270 kit looks like a miniature coil of wire wound about a thin plastic core. It is about the size of a resistor. Some inductors are coated with epoxy and look quite like resistors. Others are big bulky coils with iron cores. Inductors are not polarized. three-wire devices. The larger the transistor is used for larger currents. Transistors are polarized devices. Transistors There are two types of transistors used in the 6.270 kit. Both are 46 CHAPTER 3. ELECTRONIC ASSEMBLY TECHNIQUE The table shown in Figure 3.8 summarizes this discussion with regard to polarity issues. Resistor Isolated R-Pack Common R-Pack Diode LED Monolithic capacitor Tantalum capacitor Electrolytic capacitor DIP socket PLCC socket Integrated circuit Inductor Transistor Device Polarized? E ect of Mounting Incorrectly no no yes yes yes no yes yes yes yes yes no yes circuit doesn't work circuit doesn't work device doesn't work explodes explodes user confusion 52-pin severe frustration overheating; permanent damage circuit doesn't work Figure 3.8: Summary of Polarization E ects 3.1.4 Component Mounting Good Bad Ugly Figure 3.9: Flat Component Mounting When mounting components, the general rule is to try to mount them as close to the board as possible. The main exception are components that must be folded over before being soldered; some capacitors fall into this category. Components come in two standard packaging types: axial and radial. Axial mounts, shown in gure 3.9 and gure 3.10, generally t right into the holes in the PC board. The capacitors and LEDs in the 6.270 kit are all radial components. The leads of axial components must be bent or modi ed to mount the component. The resistors, diodes and inductor are all axial components. 3.1. ELECTRONIC ASSEMBLY TECHNIQUE 47 Good Bad Ugly Figure 3.10: Upright Component Mounting Most resistors and diodes must be mounted upright while others may lay at. If space has been provided to mount the component at, then do so, and try to keep it as close to the board as possible. If not, then just bend one lead over parallel to the component, and mount the component tightly. See Figures 3.9 and 3.10 for clari cation. 3.1.5 Component Value Markings Various electronic components have their values marked on them in di erent ways. For the same type of component, say, a resistor, there could be several di erent ways that its value would be marked. This section explains how to read the markings on resistors and capacitors. Other devices, such as transistors and integrated circuits, have their part number printed clearly on the device package. Resistors The largest resistors|in terms of wattage, not resistive value|simply have their value printed on them. For example, there are two large, rectangular 7.5 resistors in the 6.270 kit that are marked in this fashion. Other resistors are labelled using a standard color code. This color code consists of three value bands plus a tolerance band. The rst two of the three value bands form the value mantissa. The nal value band is an exponent. It's easiest to locate the tolerance band rst. This is a metallic silver- or goldcolored band. If it is silver, the resistor has a tolerance of 10; if it is gold, the resistor has a tolerance of 5. If the tolerance band is missing, the tolerance is 20. The more signi cant mantissa band begins opposite the tolerance band. If there is no tolerance band, the more signi cant mantissa band is the one nearer to an end of the resistor. Figure 3.11 shows the meaning of the colors used in reading resistors. A few examples should make this clear. brown, black, red: 1,000 , or 1k . 48 CHAPTER 3. ELECTRONIC ASSEMBLY TECHNIQUE Black 0 1 Brown 1 10 Red 2 100 Orange 3 1000 Yellow 4 10,000 Green 5 100,000 Blue 6 1,000,000 Violet 7 Grey 8 White 9 Figure 3.11: Resistor Color Code Table Color Mantissa Multiplier Value Value yellow, violet, orange: 47,000 , or 47k . brown, black, orange: 10,000 , or 10k . Capacitors Reading capacitor values can be confusing because there often are numbers printed on the capacitor that have nothing to do with its value. So the rst task is to determine which are the relevant numbers and which are the irrelevant ones. For large capacitors values of 1F and greater, the value is often printed plainly on the package; for example, 4.7F". Sometime the " symbol acts as a decimal point; e.g., 47" for a 4.7F value. Capacitors smaller than 1F have their values printed in picofarads pF. There are 1,000,000 pF in one F. Capacitor values are similar to resistor values in that there are two digits of mantissa followed by one digit of exponent. Hence the value 472" indicates 47 102 picofarads, which is 4700 picofarads or 0.0047 F. ...
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