PCB ASSEMBLY
TECHNIQUES
PCB ASSEMBLY
TECHNIQUES
PCB ASSEMBLY
TECHNIQUES
PCB ASSEMBLY
TECHNIQUES
PCB ASSEMBLY
TECHNIQUES
PCB ASSEMBLY
TECHNIQUES
PCB ASSEMBLY
TECHNIQUES
PCB Assembly Techniques by Harvey Twyman
PCB ASSEMBLY
TECHNIQUES
PCB ASSEMBLY
TECHNIQUES
PCB ASSEMBLY
TECHNIQUES
PCB ASSEMBLY
TECHNIQUES
PCB ASSEMBLY
TECHNIQUES
PCB ASSEMBLY
TECHNIQUES
PCB ASSEMBLY
TECHNIQUES

Introduction
"A common misconception is that you have to invest heavily in PCB Assembly Equipment in order to succeed at all. 

However, you'll still need the ability to solder or repair the PCBs by HAND, at BOTH the prototyping AND production stage.
 

This Web Page demonstrates that by applying the SIMPLE PRACTICAL TECHNIQUES described you CAN assemble PCB's, WITH or WITHOUT the investment of costly Plated-Through-Hole and Surface Mount Equipment."


Harvey
Twyman

The solutions are in the form of
SPECIAL TECHNIQUES
that ONLY require
SIMPLE HAND TOOLS

The Techniques described are specifically useful for PROTOTYPING PCB DESIGNS where
manufacturing costly PLATED-THROUGH-HOLE PCBs for a ONE-OFF is uneconomic.

However the Techniques related to
Surface Mount Devices
apply to Manufacturing as well.

We know the term:
DESIGN for MANUFACTURE
However here we need to:
DESIGN for PROTOTYPE



Working Practices

The Techniques can only be SUCCESSFUL if the guidelines below are followed:

  • CAD Considerations
  • Lighting
  • Cleanliness
  • Hand Tools
CAD Considerations

The PCB Design will need modification at the CAD Level to enable these techniques to be WORKABLE:

  • VIA PADS need to be at least 55 to 60mil diameter IF the drilling is to be done BY HAND.

  •  
  • SMT Rectangular Pads MUST be LONGER so that they protude out and are VISABLE even when the device is sitting on the pad. This is so that you can observe and create a reliable joint when manual soldering.
Lighting

Use 2 standard 60W DESK LAMPS one either side of your work area and placed low to the bench to give maximum intensity. This will illuminate the work area adequately and not produce SHADOW.
 
 

Visual Inspection is the KEY to success.

Most faults are VISIBLE with
the lighting described above.

With these techniques you have to rely TOTALLY on YOUR OWN EYES for quality control.

THE MORE LIGHT YOU HAVE,
THE MORE YOU'LL SEE


Keeping Everything Clean

Before any soldering work is done on the pads they need to be cleaned with IPA (SEE BELOW) using a soft toothbrush style brush to remove oxides.

The PCB will re-oxidize within an hour, so
clean JUST BEFORE you intend to solder.

IPA (Isopropyl Alcohol) is used in AEROSOL SPRAYS as a diluent and solvent and is a common component of such products like MAGNETIC TAPE HEAD CLEANING SOLUTIONS. It has the advantage that it EVAPORATES quickly.

Hand Tools

Standard tools are adequate for the job, accept for the ones mentioned below:

  • Soldering Iron
    • Modern Temperature Controlled types are adequate. The Solder Tip needs to be pointed thus giving access to individual SMT joints.
      •  
    • Pointed Tips when used on SMT Pads have an added advantage of less heat capacity. So once placed on an SMT pad the temperature drops rapidly. This has the advantage that quick joints can be made on the tiny pads without them getting damaged.
      •  
  • Tweezers
    • DON'T use SHARP ones! These may DAMAGE delicate SMT resistors. Flat Ended types are preferable as rounded ones can't be used to pick up the smaller SMT devices.
      •  
  • Solder Braid
    • Use the narrowest grade you can. Using fat ones take too long to heat up which may damage the pads.
      •  
  • Inspection Eyeglass
    • These are available in different magnifications. A X8 magnification is adequate. Using any higher doesn't display enough board area. There are some types available with a built-in measuring facility. These are important for checking actual PCB pad and drill hole sizes.
      •  
  • Cut and Crop Tool
    • Also known as Cut and Clench or Cut and Crimp The tool automatically squashes the wire flat before cutting above it. This tool is used in the VIA Technique described later.

  • Heat Gun
    • This type of tool is required for removing (reworking) SMT components, This is the only tool that needs a certain amount of training to use successfully. This will be described later.

    • Heat Guns come in 2 forms:-

      • Paint Strippers
      • Hot Air Tools

    • Paint Strippers
      • These are relatively cheap and generate a wide column of very hot air with limited control over the air flow. These are more suited for removing the larger SMT devices or where heating a larger surface area of the PCB is required.
         
    • Hot Air Tools
      • These are designed to heat small areas and usually have a proper air velocity control to regulate air flow and thus the temperature. Thus these are more suited for removing the smaller SMT devices.

    • Generally though, as we're more interested here in a Low Cost solution, the techniques described later will be referring to the Paint Stripper type of tool.
    • Also known as Cut and Clench or Cut and Crimp. The tool automatically squashes the wire flat before cutting above it. This tool is used in the VIA Technique described.



 
A Badly Soldered Joint

The reasons for a bad joint:
 
  • The copper cladding is oxidised
      • Clean with non-abrasive methods:
        • Scrub with small brush using IPA solvent
        • Re-solder within a few minutes before oxidisation occurs again
    • Soldering iron bit needs Wetting
      • Press the tip into the special wetting paste provided.

      •  
    • Not applying the solder tip and solder to the joint simultaneously

    •  
    • Not enough heat being applied to both the component leg and pad.

    A Good Soldered Joint


    Facts about PCBs and Soldering

  • The copper cladding is glued to the fibre glass laminate

  •  
  • The melting point of the solder is 180°C

  •  
  • The melting point of the glue is 150°C
  • Therefore when the solder is molten, the
    copper cladding is floating on molten glue!

    The reason why the glue's temperature is LOWER than the solder's is BY DESIGN.This is to insure that the copper cladding's EXPANSION is UNIMPEDED by the glue.

    This means that PCB pads are VERY VULNERABLE during soldering.


    Rules of Soldering:

    • Take care not to press TOO HEAVILY on the pads, otherwise the pads will lift up and will be very difficult to repair.

    •  
    • Moving components BEFORE the solder has solidified may again either LIFT the copper tracks or cause a DRY JOINT explained previously.

    •  
    • Apply the soldering iron tip and solder to the joint SIMULTANEOUSLY and keep there until the solder flows correctly over the joint.
    • Remove the soldering iron tip IMMEDIATELY after the smoke from the flux within the solder disappears approximately 2 seconds after the solder melts.

    • Only use SILVER-LOADED SOLDER for SMT joints ( 2% Silver )


    Tin Plating

    • Don’t Tin Plate your PCB UNLESS the plating solution is NEW! 

    •  
    • The solution can become heavily contaminated from being used just a few times. These contaminants make it MUCH MORE DIFFICULT to solder. 

    •  
    • This rule is ESSENTIAL particularly for SMT PCBs

    •  
    • Prototype PCBs don't need Tin Plating anyway. They still remain clean looking YEARS LATER. They WON'T go GREEN unless exposed to severe climatic conditions. The PCB Image BELOW is over 2 Years Old and STILL clean!

    PAD Specification for SMT Components
     

    The SMT Pad MUST be designed in the CAD STAGE SPECIFICALLY for HAND SOLDERING and:

    • CAN be the same WIDTH or wider as the SMT leg.
    • MUST be LONGER than the IC LEG positioned on it, to allow ease of VISUAL INSPECTION and hand soldering of the joint.
     The same rules apply to the J-Lead SMT package


    PCB Assembly Limitations

    Some of the tracks on a double sided PCB connect from one side of the board to the other. Most of them connect via a component’s leg, but some don’t. These connections that don’t are called VIAs.

    On commercially manufactured PCBs these VIAs are produced by a system called Plated-Thru-Hole.  This process electro-plates all the inside of the holes with copper.

    However a simple technique is described below that doesn't require the EXPENSIVE plated-thru-hole technology.
     


    VIA Connection Technique

    A special Cut and Crop Tool  is used to produce a reliable connection through the VIA hole pads.

    There are various names used for this tool:

    • Cut and Clench
    • Cut and Crimp

    Using 24 SWG TINNED COPPER WIRE:

    • Place the wire in the VIA hole. (Hole should be 0.7mm diameter)

    •  
    • Use the cut and crop tool to produce the result shown above

    •  
    • Solder both sides of the wire link.

    IC Socket Assembly Techniques

    There are 2 COMMON types of IC SOCKETS in use today:-

    • DIL - Dual-in-Line
    • PLCC - Plastic-Leaded-Chip-Carrier
    DIL SOCKETS

    Available in various sizes from 8 to 64 pins. However their QUALITY can vary. By FAR the BEST is the TURNED PIN TYPE. Unfortunately they're the MOST EXPENSIVE. However they do give BETTER ELECTRICAL CONTACT with the IC leg, which is VERY IMPORTANT for RELIABILITY.

    When creating a DOUBLE-SIDED PCB tracks need to be connected to the DIP socket from BOTH sides of the board. The SOLDER SIDE connections are EASY, but the COMPONENT SIDE connections get OBSTRUCTED by the socket's PLASTIC BODY, making soldering difficult if not IMPOSSIBLE.

    A SPECIAL JIG described below is used to EXTRACT the pins from their FORMER. This makes the soldering of the COMPONENT SIDE pin MUCH easier.

    PLCC SOCKETS

    Available in various sizes from 20 to 84 pins. The SURFACE MOUNT type described here are purchased WITH a CENTRE SECTION as shown in the LEFT PHOTO below.

    The CENTRE SECTION needs to be REMOVED by gently pressing it out with a SCREWDRIVER. The result is shown in the RIGHT PHOTO below.

    The reason for REMOVING the PLCC socket's centre is so that VIA HOLE LINKS, discussed PREVIOUSLY, can be placed in the CENTRE of the PLCC socket,  i.e. UNDERNEATH the PLCC device.

    The socket RAISES the PLCC device OFF the PCB ENOUGH to accommodate for the HEIGHT of the VIA LINKS but ONLY when the CENTRE SECTION of the PLCC socket is REMOVED.



    If PLATED-THRU-HOLE TECHNOLOGY isn't available to you, but you need to produce DOUBLE SIDED PCBs, then this technique described is a solution.

    The JIG described below is for EXTRACTING the DIL IC SOCKET PINS. The IC sockets used are a 16 WAY TURNED PIN type. These are used as they represent the CHEAPEST WAY of purchasing the socket pins.

    The JIG shown below can be constructed from a 4" VICE MOUNTED VERTICALLY.
     

    DIL IC Socket Pins Extraction Jig



    PLCC Socket Modification
     

    As Purchased
    Centre Removed

    Soldering SMT Components

    Surface mount pads need to be tinned with solder prior to mounting the component. When this is completed the pads look like this:


    However observe what happens if you place a SMT component on soldered pads like these.

    and solder one pad... 



    Solder Levelling an SMT Pad

    To overcome this problem above, commercial PCB manufacturers use a process called Hot Solder Levelling which produces a flatter solder joint thus:

    If this process is not available there is an alternative technique using Solder Braid to achieve the same result.

    • Scrub the pads with a small brush while applying IPA solvent

    •  
    • Apply the solder (MUST use the 2% silver type) to the SMT pad

    •  
    • Place the solder braid on the pad near to the track connecting the pad

    •  
    • Apply the soldering iron to the top of the braid resting on the pad

    •  
    • While the solder is molten, slowly drag the braid away from the pad

    • in the opposite direction to the track connecting it
    Take care when dragging the solder braid. As stated previously, the pad is floating on molten glue which is holding the pad on the board. This is the reason for dragging the braid AWAY from the track that connects to the pad. The track prevents the pad from moving out of position.



    Soldering a 2 Padded SMT Component
    • Tin BOTH Pads


    • Solder Level only ONE of the 2 pads as described previously, leaving the other pad just TINNED


    • Hold the sides of the component with tweezers so its terminals aren't OBSTRUCTED


    • Apply solder and heat the TINNED pad


    • While the solder is molten, move the component to align on BOTH pads


    • If alignment is proving to be DIFFICULT then remove the iron OFTEN to prevent the pads from being DAMAGED by the excessive heat


    • When the component is correctly ALIGNED on the pads then solder the LEVELLED PAD properly


    • Finally RESOLDER the first pad AGAIN as almost certainly that joint will be DRY from all the extra heating during the alignment procedure


    Soldering a Multi Padded SMT Component  
    • Tin ALL the component's pads


    • Solder Level all pads EXCEPT one CORNER pad


    • Hold the component with tweezers so that its terminals aren't OBSTRUCTED


    • Apply solder and heat to the UNLEVELLED corner pad


    • While the solder is molten, move the component to align on ALL its pads


    • If alignment is proving to be DIFFICULT then remove the iron OFTEN to prevent the pads from being DAMAGED by the excessive heat


    • When the component is correctly ALIGNED on ALL the pads then solder the DIAGONALLY OPPOSITE corner pad and RE-CHECK alignment


    • REPEAT the task of soldering the corner pads until alignment is correct


    • Now solder all the OTHER pads


    • Finally RESOLDER the ORIGINAL 2 corner pads AGAIN as almost certainly those joints will be DRY from all the extra heating during the alignment process

    Soldering the Very Small Fine Pitch SMT Devices

    The fact that SMT components are getting smaller is common knowledge. The problem is that the larger SMT components are becoming UNAVAILABLE. Therefore the techniques described HAVE to cope with the SMALLER more common components as well.

    A Special Technique:

    • Normally the device pads are soldered INDIVIDUALLY. However this is NOT POSSIBLE with these FINE PITCH devices as the pitch between the device's leads are TOO SMALL.

    •  
    • Therefore place the soldering iron BETWEEN 2 ADJACENT leads.

    •  
    • Apply solder and heat as normal. REMEMBER not to keep the heat applied for VERY LONG, particularly as the pads are SO SMALL, the GLUE will MELT holding the pad. See ABOVE

    •  
    • This will almost certainly SHORT CIRCUIT the 2 leads TOGETHER.

    •  
    • The excess solder can be REMOVED using SOLDER BRAID described in the SOLDER LEVELLING Technique mentioned previously 

    Handling SMT Components

    The physical size of SMT components demands the use of tweezers. However SMT components are very fragile and thus a blunt nose type is essential. Sharp tweezers can easily damage SMT resistors. 

    The resistive film is very thin and damage could either change the value or even make it completely open circuit!

     Resistors come in various physical sizes:
    • 1206 – 0.12” by 0.06” – This size is the easiest to handle
    • 0603 – 0.06” by 0.03”
    • 0402 – 0.04” by 0.02” – This size will blow away if breathed on!
    The component value is printed on them in the form:
    • 1K5 1500 Ohm
    • 390R 390 Ohm
    • 1M8 1,800,000 Ohm
    Capacitors are available in the same sizes as above but have NO IDENTIFICATION marks on them at all! So DON'T MIX THEM UP!
     
    It is important that you use the largest of the
    SMT components available, for ease of handling.

    Some devices are so small and light that just breathing on them will send them flying away! The smaller devices may damage more easily as described. 

    The pads on SMT components are pre-tinned and sealed in airtight packaging. This is to prevent the pads oxidising. They also have use-by dates similar to food stuffs. Therefore:

     ONLY OPEN THE PACKAGING JUST BEFORE USE

    The use of CAROUSELS to hold lots of UNPACKED SMTs
    is NOT ADVISED for the above reasons. 

    ALL SMT components MUST ONLY be touched with TWEEZERS! 

    Fingers will CONTAMINATE them with grease, which will be very difficult to remove and subsequently DIFFICULT to solder.

     

    ELECTROSTATIC DAMAGE can occur when CMOS inputs are
    exposed to high voltages as low as 60 Volts. However it’s
    more likely to be KVolts.

    The use of wrist straps and earth mats are essential while handling any CMOS device.

    When handling devices away from the electrostatic-free area ALWAYS:

    Hold the device tightly in the palm of your hand,
    keeping its pins discharged through your damp skin.



    Removing SMT Components

    This is called REWORK in the manufacturing trade, as the job has to be repeated. Reworking a job costs a company MONEY.

    Therefore it’s important to get things right every time by monitoring QUALITY at all stages of assembly to REDUCE the necessity for rework. 

    EXCESSIVE HEAT during rework may damage the:-

    • Components

    • The PCB's PADS

    • De-Laminate the Fibre Glass PCB itself i.e. a bulge appears in the PCB due to trapped gases expanding. 
    Therefore priorities have to be established when reworking as to which is more important, saving the COMPONENT or the PCB’s PADS.

    The PADS are usually treated MORE IMPORTANTLY than the COMPONENTS. PCB repair is always MORE DIFFICULT and TIME CONSUMING when compared to the cost of purchasing new components.
     

    Removing a 2 Legged SMT Device

    Removing Multi Pinned SMT Devices

    There are several options, but the CHEAPEST are discussed below:

    • Using a HEAT GUN

    •  
      • This may cause DAMAGE to the surrounding components and PCB if TOO MUCH heat is applied. This can be a VERY EFFECTIVE method if the surrounding area is PROTECTED from the heat. This can be achieved by either using special HIGH TEMPERATURE SELOTAPE availalble or by keeping the Heat Gun IN MOTION over the PCB area to control the temperature.

      • As long as care is taken to LOCALISE heating, the only PROBLEM will be that the solder on the ADJACENT SMALL COMPONENTS may ALSO become MOLTEN. They may even get BLOWN AWAY! However as long as the heat gun has a WIDE funnel of air, the air pressure shouldn't be enough for this to occur.

      •  
      • Once the solder is MOLTEN the component can be either taken off with TWEEZERS or more easily by TAPPING the PCB on the bench. However BEWARE, this may also DISLODGE other adjacent components that have molten solder.

      •  
    • CUTTING the chip's LEGS OFF

    •  
      • This method involves sacrificing the component, to save the PCB:-

      • Cut all the legs off the component using either a fine pair of SIDE CUTTERS or if the legs are very small then a SHARP KNIFE can be used

      •  
      • Great CARE is required in not DAMAGING the PCB's PADS when pressing down with the knife.

      •  
      • Using SOLDER WICK, prepare the pads for a new component by applying the SOLDER LEVELLING TECHNIQUE described previously.
    Of the 2 methods described above the HEAT GUN method has an advantage that:-
    • It's LESS DAMAGING to the PCB's pads from the SHARP KNIFE as long as the Heat Gun is kept IN MOTION as described above.
    And Finally:-
      • Another popular method of rework is to use a proprietry product called LOW MELT™. This is a special LOW MELTING POINT solder but DOES REQUIRE extra equipment support so isn't discussed here in detail. Further information is available from the manufacturers at www.zephyrtronics.com.

    An Example of a Hand Soldered PCB

    A Typical Double Sided PCB Project Soldered with Hand Tools Only


    Frequently Asked Questions

    • Question:

    • You mention using a POINTED TIPPED soldering iron - surely wouldn't a WIDE TIP be better?
       
      • Answer:

      • If your PCB has a SOLDER MASK LAYER then a wide tip can be used. A Solder Mask Layer will encourage the solder to FALL either side of the mask so reducing BRIDGEING.

        This Web Page is intended for PROTOTYPE PCBs that don't have a Solder Mask Layer. Therefore a pointed tip is easier to solder individual SMT pads.

        Observation:
        The fine tip has a very low THERMAL CAPACITY because of it size. Therefore the temperature drops extremely quickly when it comes into contact with the pad.

        Once off the pad it regains its higher tempereature also very quickly. The higher thermal capacity of the rest of the bit CHARGES up the tip quickly as well.

        This DROP in tempereture has the advantage that the solder melts quickly on the pad but the temperature dropping protects the pad and its glue from overheating and causing damage.



         
    • Question:

    • You use SOLDER WIRE intead of SOLDER PASTE - Why? 
       
      • Answer:

      • SOLDER PASTE requires a lot of handling considerations:-

        • Storage in a REFRIDGERATOR
        • Thawing out for at least 30 minutes to room temperture
        • Manual application to SMD pads using applicators is MESSY due to its VERY THICK consistency

        •  
        Manual Soldering using solder paste can ALSO cause SOLDER BALLING where the temperature is accelerated TOO HIGH that causes the solder to BOIL, BUBBLE and SPIT its solder balls EVERYWHERE over the board with a possibility of causing SHORT CIRCUITS.

        However by using SOLDER WIRE - NONE of these occur.


    • Question:

    • Wouldn't it be better to use a HOT AIR PENCIL instead of a Soldering Iron?
       
      • Answer:

      • Hot Air Pencils are DIFFICULT to set up in terms of their AIR TEMPERATURE. The Air velocity controls are usually very fiddly to adjust the flow to achieve the correct temperature.

        If the air pressure is set TOO HIGH, it can be strong enough to BLOW the SMT devices OFF their pads.

        You are usually heating an AREA of the PCB so it take longer to melt the solder.

        However a SOLDERING IRON solders quickly WITHOUT all the other problems mentioned.


    Another PCB Example is Shown Here:

    http://www.Makaton-Signs.org.uk/uPL-Trainer


    Printing this Document

    To print this document successfully you must display the Non-Frames version by CLICKING HERE and then printing it in Landscape Mode.


    See the Surface Mount Techniques in Action:

    THE SURFACE MOUNT VIDEO SHOW
    at
    http://www.Twyman.org.uk/The-SMT-Video-Show

    demonstrates the surface mount techniques mentioned above.

    There's also AUDIO NARRATION available which
    explains key areas of the videos in more detail.



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    About the Author

    For further information about:

    Harvey
    Twyman
    Please visit:

    www.Twyman.org.uk/CV



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