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Printed Circuit Board - Montana State University

By Lewis Williams,2014-07-09 19:36
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Printed Circuit Board - Montana State University

    ASSEMBLING

    the

    ECEbot

    Printed Circuit Board: Part One

    Due Date

    The Part One assembly steps must be completed prior to:

    Prepared by R.C. Maher

    October 2008

    Copyright ? 2008 Department of Electrical and Computer Engineering, Montana State University

    All Rights Reserved

    20081010RCM

PCB Assembly Part One 2

    BEFORE STARTING: be sure to put your name on your kit box! Also be sure to

    label your robot PCB with a marker pen. This will help get the items returned if they are lost or misplaced.

    1 Printed Circuit Board Preliminaries

    This section describes the initial steps to begin soldering the ECEbot’s circuit board. Look at both sides of the Printed Circuit Board (PCB). The front side has printed lines and labels called the “silk screen,” while the back side doesn’t have any printing on it. Notice that there are many

    small labels on the front (silk screen) side of the board. The instructions will often refer to the components by these labels, like diode “LED3”, resistor “R11”, or header “J17”. Most of the

    components will be inserted in the board from the front side by slipping the wires or legs through the drill holes, then soldering them to the back side.

     Figure 1-1: PCB Layout: Front of Main Board

PCB Assembly Part One 3

    The assembly and soldering steps are quite intricate and you must keep alert to avoid placing the components incorrectly: be sure to double-check each step before doing the assembly. The instructions include a “stop sign” whenever there is a particularly important set of instructions:

1.1 Separating the header pins

    Many of the electrical signals on the PCB will appear on a header, or a row of vertical

    needle-like pins. Find the three sets of 36-pin headers in your parts kit.

    Only a few header pins are needed this week, but it makes sense to go ahead and break the 36-pin sets into the smaller segments that are needed later. Clip the 36-pin header strips into the following groups by carefully snapping or cutting between the pins. Be sure to hold all of the

    pieces as you separate the sections so that nothing pops off into your eye.

     Set 1: 30, 4, 2

     Set 2: 30, 4, 2

     Set 3 6, 4, 4, 3, 3, 2, 2, 2, 2, 1, 1, plus spares

    You should now have: 2@30, 1@6, 4@4, 2@3, 6@2, and 2@1. Place all of the separated

    header pieces into a container in your lab kit for later.

    Main Bumper Chassis 1.2 Separating the PCB sections

    The main robot components consist of three

    separate boards, but for ease of manufacture the

    boards are produced as one connected piece.

    Carefully flex the board to snap and separate the

    sections: main board, bumper switch board, and

    the chassis board. You may want to file the

    rough edges or smooth the corners with a hobby

    knife.

    Identify the main board (Figure 1-1) and place the other two boards in your lab kit for later use.

PCB Assembly Part One 4

    1.3 Soldering Step 2: Installing the Discrete Resistors

    Do not proceed until the instructor has demonstrated how to use the soldering iron and you have practiced making several solder joints using one of the scrap circuit boards in the lab.

    There are 19 individual resistors that need to be soldered in place on the main PCB.

    Locate the following discrete resistors in your parts kit:

    Qty Resistor Type Color Bands Position on PCB

    8 Orange Black Brown Gold R12, R13, R14, R15, R16, R17, R18, R19 300;, 5%, ? W (small)

    5 Yellow Violet Brown Gold R10, R21, R22, R24, R25 470;, 5%, ? W

    4 Brown Black Red Gold R4, R5, R6, R7 1 k;, 5%, ? W

    4 Brown Black Orange Gold R20, R23 (and R8 and R9 for bumpers) 10 k;, 5%, ? W

    There are several different resistances (ohms) that are used, so be careful to verify the correct value resistor is in the proper mounting position.

    Hint: It is a good idea to place the resistors so that the color code bands start at the top of the board. The

    resistor is not a polarized component so it would work in either orientation, but keeping all the bands in the

    same direction makes it easier to verify the circuit or to find errors later onand it also makes for a neat,

    tidy, and professional looking assembly.

    To start, hold the board with the front side up and oriented so that you can read the printed silk screen labels from top to bottom, like in Figure 1-1. Select one of the 10 k; resistors and

    locate position R23 near the upper left corner of the PCB.

    Carefully bend the wire leads and insert the ends from the

    front of the board through the proper pair of holes. It is always

    wise to double-check the component and its placement before

    soldering, since removing soldered parts is not easy and often

    results in damage to the board and to the component. The

    resistors and other passive components should not be soldered

    precisely flush with the board: leave a small gap between the

    component and the board.

    Hint: Once the component leads are through the board, bend the

    ends slightly on the back side of the board so that the component

    won’t slide out of the holes when you flip the board over.

PCB Assembly Part One 5

     Flip the board to the back side and

    solder the two wire leads of the resistor to the

    Soldering Iron pads on the back side. Solder

    Only use enough heat and solder to make a

    clean, shiny, and uniform joint that fills each

    pad but does not seep over to touch any of the

    adjacent pads. The solder joint should look

    like a tiny wrapped Hershey's Kiss candy. Back Side Have your instructor or TA look at your

    soldered component and approve your

    technique before continuing.

    After the solder cools, gently clip off the excess wires on the back side just above each solder joint: be sure to hold the wire so it doesn't go shooting across the room as you clip it!

     Now locate the proper place for each of the 18 other resistors and systematically solder them in place. If you are in doubt about where a resistor goes, be sure to ask the instructor or lab TA for assistance.

    1.4 Soldering Step 3: Attach the 60-pin Socket

    Hold the board with the front side up. The row of holes just above the silk screen label "Montana State" (J52 label) is the location of the 60-pin socket that will hold the microcontroller board. A socket provides electrical connections to another component and aids in removing and replacing that component if necessary.

    Socket Location:

    bottom row

    Figure 1-2: Socket location and orientation on the PCB

    Find the 60-pin socket in your lab kit. Carefully line up all 60 pins of the socket with the holes on the PCB (see Figure 1-3) and hold the socket flush against the board as you flip it over.

PCB Assembly Part One 6

    60-pin Socket

    PCB Front Side

    Figure 1-3: Placing the 60-pin socket onto the PCB

    Once flipped over, you should now see the 60 pins protruding from the holes on the back side of the board (Figure 1-4).

    PCB Back Side

    60-pin Socket

    (hold flush against front side

    Figure 1-4: Socket pins protruding from the back of the PCB

    Hint: You may be able to hold the socket in place by resting the board face up flat on the bench top, with the socket sandwiched underneath. You might also choose to use some tape to hold the socket in place temporarily while you do the soldering.

     Carefully solder each of the 60 pins where they protrude from the back surface. Only use enough heat and solder to make a clean, shiny, and uniform joint that fills each pad but does not seep over to touch any of the adjacent pads. Have your instructor or TA look at your soldered socket and approve your technique before continuing.

PCB Assembly Part One 7

    1.5 Soldering Step 4: The Power System Components

    In this section, the power system for the robot will be introduced. Hold the PCB with the front side up, the silk screen oriented so you can read the main labels, and locate the power section of the board in the lower left portion (see Figure 1-5).

    Figure 1-5: Power system section (front side, bottom left)

    To begin with, be sure you understand the concept of polarized or directional components, meaning components that have a positive and negative direction: the diodes, the voltage regulator, and the electrolytic capacitor. This concept will be important in this sub-system.

    If you have any questions do not hesitate to ask the instructor.

1.5.1 Components list

    The components you will use for this step are listed below. The bold code refers to the

    component's name on the PCB silk screen. Find each of the power sub-system components from your parts kit:

    104J

     Two 0.1F capacitors, C1 and C2 (non-polarized plastic film)

     One 120F capacitor, C5 (polarized aluminum electrolytic)

     One silicon power diode, D5 (1N4004)

     One red or green LED, LED1

     One on/off switch, S2

     One 4-pin header, J4

     One 2-pin headers, J21

     Two 1-pin headers, GND and +5V

PCB Assembly Part One 8

     One +5V regulator, VR1 (LM7805) 7805

     One shorting jumper

1.5.2 Installing the power sub-system headers

    Headers are used with mating connectors and wires to reach other parts of the board, or with small shorting jumpers that slide on pairs of pins to make a connection. The silk screen shows headers with the abbreviation J.

    First, pick out the required power system header sets: one group of 4 (J4), one group of 2

    (J21), and two single-pin header (J24 GND and J16 +5V). Soldering the headers is a little

    trickier than the resistors and capacitors.

    There are a few options for holding the header in position while you solder the pins.

    1) If you are pretty comfortable with the soldering iron you can hold the header in place

    with a tweezers (or for multi-pin headers, your fingerbut just touch one of the pins that

    is not being heated!) and solder the pin to the board. After securing the first pin of a

    multi-pin header, the section will hold itself in place so you can solder the rest of the pins.

    2) Find something to support the header while you solder the back side pins. At this stage

    of assembly you can probably just use the bench top, but later other components may

    inhibit this method of soldering headers.

    3) A piece of tape can be used to hold the header in place. In some ways this seems like the

    best option, but it is often hard to keep the header aligned perpendicularly to the board.

     Place the 4-pin power system header (J4) into position by sliding the short ends of

    the jumper pins through the matching holes, decide how you will hold it in place, and

    then solder it from the back side.

     Do the same for the 2-pin header J21.

     The single pin J24 attached to the board’s ground potential (GND). J24

    is located over in the middle left side of the PCB just above the power section.

     The single pin J16 (+5V) attached to the board’s regulated power output

    potential is located in the middle of the power section.

    As you will see later, there are a lot of headers to install on the board!

PCB Assembly Part One 9

    1.5.3 Power supply connector "key"

    When the entire kit is assembled you will be powering your robot using a battery pack. The pack will connect to the PCB using the four header pins (J4) you just soldered in place. The positive

    (+) battery wire will connect to the top pin and the negative (-) battery wire will connect to the bottom pin. In order to make it less likely to connect the power backwards, we will key the

    connection by removing one of the header pins. The corresponding hole in the battery connector is plugged, so the connector can only be installed in the proper orientation.

     Carefully clip off the next-to-the-bottom header pin of J4. The hole plug in the

    mating battery connector to make sure the positive and negative connections are correct.

1.5.4 Installing the passive components

    First identify the correct holes for the two 0.1F (104) plastic film capacitors C1 104J

    and C2.

    Hint: The plastic film capacitors look a bit like Chiclets candy and do not have a polarization: they can go

    into the circuit in either orientation. However, be sure to choose the capacitor orientation so that the "104J" label will still be visible after installation.

     Insert the legs from the front side of the board so that you can see the capacitor's label and then carefully bend the capacitor backward to lie flush against the board. Now flip the board to the back side, and solder the capacitor legs in place on the back. Carefully clip off the excess wire.

    Now place the 120F aluminum electrolytic capacitor

    C5 into its holes: make sure the negative lead and positive

    lead are oriented correctly.

    Hint: The electrolytic capacitor looks like a small metal can and is

    polarized: one of the leads is labeled "-", for negative, on the side

    of the capacitor. Make sure the other lead (positive) goes into the

    hole with the square pad near the "+" silk screen mark. Your

    board will not work if the electrolytic capacitor is inserted

    backwards!

     After double-checking the orientation, carefully bend the C5

    capacitor over to lie flush against the PCB, then solder the Front Side legs in place on the back. Carefully snip off the excess wire.

    Next, locate the holes for the light-emitting diode (LED1) and the power diode (D5) in the lower

    left area of the board.

PCB Assembly Part One 10

    Hint: The LED (light emitting diode) is directional. The long wire is the anode and the short wire is the cathode. You may also be able to see the larger cathode element inside the LED package. Make sure the LED is inserted so the short wire (cathode end) goes through the square pad

     Front Side cathode: short wire anode: long wire

     Short wire… …square pad

    Hint: The proper direction for the power diode stripe corresponds to the white mark on the silk screen.

     Solder the LED and the power diode into position. Both before and after soldering, verify the placement and orientation of each polarized component, and seek help if you notice any errors.

    1.5.5 Installing the voltage regulator

    The next piece that goes in the power sub-system is the 3-pin voltage

    regulator. The regulator takes the voltage from the batteries (which will vary with temperature, load, and as the batteries discharge), and

    electrically converts it to the constant +5 volts that is needed by the rest of the circuitry. The first pin is the input voltage, the second pin is the circuit

    ground, and the third pin is the regulated 5 volts.

     Insert the component from the front side of the PCB oriented

     so that the tab is toward the top and the label toward the bottom,

    carefully bend the regulator back flat against the PCB, and then solder the three pins on the back side. Gently clip off the excess wires after soldering.

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