Design and Development Notes 2004-2

• If the waveform on the scope droops then you may need to use a 10X attenuating probe or mode. this attenuator is at the tip of the probe hence better measurement possible, also very high frequency signals may be best transmitted over a 50 ohm impedance matched line to the scope.Never 'put or remove' ICs, cards or plug-in modules when the power to that system is on, power off system wait for some time for caps to discharge, then insert ICs or cards, applies to PCs too, USB and Firewire ports are exceptions as they are hotplug, anyway even in this, take a bit of care as some peripherals may cause some problems.
• Karl Ferdinand Braun - In 1897 he built the first cathode-ray tube...oscilloscope. CRT technology is to this day used by most television sets and computer monitors. The CRT is still called the "Braun tube" (Braunsche Röhre) in German-speaking countries (and in Japan: Buraun-kan).
• The inventor and developer of oscilloscope as a useful measuring instrument is Tektronix.
• Have a lathe lamp or table lamp with a 40W edison-filament lamp (ordinary bulb) on your workbench.
• Have an antistatic mat on the work or test table, if you cannot afford or get it then get a large aluminum sheet, laminate it with an insulating polycarbonate sheet and earth the metal plate with two 1M resistors in series.
• Use thin multi strand teflon wires for your jumpers within a PCB, these are not damaged by your iron too.
• Silicone shrink sleeves can withstand high temperature and use this in your work to make it safer and neat.
• When you make a connection with a wire you need mechanical strain relief or on use connection will open.
• Test a NPN transistor, DMM in 200K range. emmiter negative probe, collector positive probe, it should show open. while still holding that way press wet finger on base and collector, it may show some reading, then it is ok. for PNP swap probes polarity and do the same thing. the wet finger when pressed applies a base current in uA.
• Testing a Mosfet - Transistor Tester - Testing Transistors - Basic Testing of Semiconductor Devices
• Teflon tape can be used to make leak proof pneumatic or hydraulic connections if junctions are leaking.
• Gold plating is used in connectors because it has low contact resistance and does not corrode or react
• Persistence of vison around 16 frames a second (60mS) hence very fast events not seen, see in wikipedia.
• Use a 100uF and 0.1uF CD in parallel to filter because the inductance of 100uF is a lot (the electrodes are wound as in a coil) and high frequency passes over and is filtered by the 0.1uF which has negligible inductance.
• Try to use same family ICs in a circuit, like only LS or only HCT, if you mix up then you have to do a design review.

Electro Mechanical Points

• The tracks or components on a PCB should not be near the mounting holes, the tracks may form electrical connections with the cabinet earth if they touch the pillar, washer or nut-bolt.
• When you tighten the screws-nuts-bolt limit torque by feeling or with torque limit screwdrivers, or thread will slip or PCB-Cabinet break, or CRT-Glass-Plastic will crack-shatter.
• After assembly and testing is over on a board, do not drill-file it, it should have been done before. Else components crack or tracks break or small metal pieces will form shorts.
• The Mechanical and electro-mechanical must be streamlined before production, for example when fitting the board the screwdriver may slip and break a zener. The product should be engineered in such a way that it fits easily and the testing and quality checks need not be repeated.
• Some mechanical failures may not be visible but may show up as a field failure after even 3 months. this will make you lose further orders and ruin the reputation. Hence engineering is vital.
• Sparking at the high voltage mains supply power contacts like a plug can cause electronic gadgets to fail.
• When you drive nails into walls ensure that that part of the wall is free of mains, telephone or any wiring.
• Some pcbs can have hairline cuts or hairline shorts which will make troubleshooting difficult.
• Never 'put or remove' ICs, cards or plug-in modules when the power to that system is on, power off system wait for some time for caps to discharge, then insert ICs or cards, applies to PCs too, USB and Firewire ports are exceptions as they are hotplug, anyway even in this, take a bit of care as some peripherals may cause some problems.
  

Testing Points

Some Pilot Production And testing points when building electronic Projects

Components

• Use 1% MFR 100 ppm resistors for all analog instrumentation.
• Use plastic or-and multilayer capacitors as ceramics are leaky.
• Product should have minimum number of fasteners.

Assembly

1. Nuts, Bolts and screws locked with adhesive so that they do not slip and the product falls apart.
2. Potentiometers should move smoothly and not get twisted, strained or locked due to cabinets.
3. Use Strain relief for wires and strip the wire and solder properly where required.
4. Strip with a stripper do not use your teeth., Twist all the strands Tin the tip of the wire.
5. After Tweaking and setting presets or trimpot lock it with paint or varnish.
6. Big Components should have large pads and separate mechanical fixing to PCB.
7. Acrylic and plastic can get blurred or deformed with some solvents.

Testing

Do assembly of the card with a tested assembled 3D reference. Even after all the care if the card does not work, then after full assembly do visual inspection in this order First keep both the cards one reference and the other manufactured under a bright light.

• Compare resistor color patterns one by one.
• Compare Electrolytic capacitors physical appearance and POLARITY.
• Verify values of plastic and ceramic capacitors.
• Cross-check Diode sizes and POLARITY. Verify Zener values.
• Check IC and Regulator POLARITY and Exact type number / brand
• Check Transistor, MOSFET, LED, POLARITY. and verify display type No. and Polarity .
• Check for connector patterns and polarity and switches etc.
• Reverse the card and Check for solder bridges between close pins .
• Reverse card and Check for hairline shorts and solder streaks due to bad PCB processing or Soldering.
• Reverse card and check for hairline cuts or pads lifted off card.
• Closely observe for dry solders on big electrical parts (L and TX) a dry solder will cover the pin and will be dull in shine.
• before calibration allow for product to warm up for 15 mts.

If all the components were subject to incoming inspection and testing. If still unit does not work remove power and do cold check comparison with reference unit in DMM 200K R range, if still problem persists continue with power on testing and solve by DMM or scope tests.

Danger ! when DMM is in Ohms Mode, Even if one probe touches 230 V, DMM is Gone, so take extra care while doing this. test probe.

Electronic Product Production.

a. PCB Assembly.

First Inspect PCB for hairline shorts and cuts and also traces of unwanted copper in the edges and repair them. Also Check if all Drill holes & Slots are proper size and PCB is Fitting in its Place properly, This is because Drilling and Filing should not be done after assembly as it causes serious reliability problems.

Second populate all the resistors, Jumpers, Diodes and Ceramic Capacitors these are tough components. Then Insert all Transistors, LEDs, Displays, Electrolytic Capacitors and IC s (Bases), Here care should be taken not to overheat any component as it may damage them. Lastly Solder Connectors, Relays, Coils and Transformers which may require a high wattage Iron, and Mechanical Reinforcements.

b. Precautions during Production.

• Observe Polarity for Diodes, El-caps, Connectors, etc.
• Make sure of Pin 1 for IC s, Regulators, Transistors and Mosfets before insertion.
• Avoid bases for ICs in production as these fail on use.
• Make use of Electrostatic protection for CMOS devices.
• When Cutting Leads use protective Goggles and do it in a separate place as the cut leads fly all over the place.
• Use Iso Propyl Alcohol (IPA) as de-greasing agent on PCB.
• Water & Detergent wash is very good but only if coils, Transformers, Relays are hermetically sealed- Impregnated.

Use a Lacquer-Varnish or RTV rubber coat on both sides of PCB including on the components to prevent corrosion and also it helps maintain accuracy-precision.

Lead is highly poisonous, use gloves or wash hands with soap after work is over. Flux, IPA, Thinner Fumes can cause respiratory health problems. Work in a ventilated area use Exhaust Fans and Open Windows. Dispose of Metal, Plastic and Chemical waste separately and well packed as these can contaminate the environment and also cause Injury.