Archive for September, 2011

PostHeaderIcon Heat dissipation through thermal conductance can be realized through below ways

1>   Use high heat conducting material;

2>   Keep distance from heat source to heat dissipation the shortest way;

3>   Ensure stable heat connecting among each conducting routes;

4>   Install as large printed circuit conductor as possible in the route of heat dissipation;

PostHeaderIcon Hole diameter of PAD when design a printed circuit board

In order to achieve the minimum diameter when design PCB according to the requirement of PAD, the diameter at least should be 0.5mmlarger than maximum diameter of soldering pin outer flange.

Designer has to provide testing PAD for all of joints according to ANSI/IPC 2221. JOINT means the electric connection among 2 or above components. One testing PAD need to use one signal node, coordinate location of testing PAD (also need to specify which side the testing PAD at) and X-Y coordinate axis of printed circuit board datum mark. Designer need to provide fixing installation data for SMT, as well as PCB layout technology, through which the testability of circuit diagram can be improved by the assistance of IN-CIRCUIT test fixing installation.

Below conditions are required:

1>    Diameter of PAD specify for in-circuit electric testing shouldn’t larger than0.9mm;

2>    Space around testing PAD should larger than0.6mmbut smaller than5mm, if the height of component greater than6.7mm, then this testing PAD should be placed5mmaway from this component;

3>    No components or testing PAD would be allowed to placed within3mmarea;

4>    Testing PAD should be placed in the center of2.5mmdiameter, if possible, apply a standard probe and more reliable fixing device is allowable.

5>    Don’t use the edge of connector’s pin to instruct PAD testing, since the testing probe could readily damage the gold-plated pins.

Aspect ratio of plated-through hole has great impact on effectively through-hole plating capability for manufacturer, it is also very critical for ensuring the structure of PTH/PTV. When the diameter of hole smaller than 1/4 of basic printed circuit board thickness, tolerance should increase0.05mm. When drilling hole diameter is0.35mmor even smaller, aspect ratio could be 4:1 or even larger, manufacturer should use the proper way to shield or block the through-hole to prevent entering of the solder, generally speaking, printed circuit board thickness and plated through hole ratio should less than 5:1.

PostHeaderIcon How to choose a proper inspection method

Test speed is an important factor to choose the right test device. Nail-bed test device can test thousands of point in one time, and flying probe can only test 2 or 4 points in one time. Furthermore, nail-bed test device will consume only 10-150usd for single side, it also depends on the complexity level of PCB. But flying probe will need over 1 hour or even more time to test the same of work. Shipley(1991) has ever explained that even mass production PCB manufacturer will blame flying probe test for its low processing rate, but it is a good choice for low volume and complex PCB manufacturer.

For bare board test, we have dedicated device, one more optimized way for cost is universal device, although this device originally could be more expensive than dedicated device, but it is cost can be offset by installation. Refers to universal grid, PCB with component lead and SMT stand grid is2.5mm. so the test PAD should be greater or equal to1.3mm. for1mmgrid, test PAD should be greater than0.7mm. If grid and probe is so small and fragile that easy to damage. Therefore we had better choose above2.5mmgrid. If universal device and flying probe device can be applied simultaneously, can ensure both commercial and precise measure for high density PCB, another way is conducted rubber test device, this technology can detect those points deviate grid. However, PAD height can vary after HASL which will bring trouble for test point connection.

Normally three different inspection should be committed:

1 bare board test;

2 incircuit test;

3 functional test;

PostHeaderIcon Inspection technology of PCB 1

With the development of surface mount technology, the mounting density of PCB is increasing very fast, as a result of that, even for PCB with less density, automatic inspection is not only basic requirement, but also commercial. During the inspection for complex PCB inspection, 2 normal ways are nail-bed test and flying probe test.

1 Nail-bed test

This kind of way is using probe with spring to connect with each test point on the PCB. Spring can let the probe withstand 100-200gpress force and to ensure each test point working properly, all the nail probe array together can be viewed as nail bed. Under the control of inspection software, we can program test point and test signal, graphic below is a typical structure of nail bed inspection, inspector can acquire all the information about test point. Although both side of PCB can be tested simultaneously by using nail-bed test method, designer should still consider to install all the test point on the solder side of PCB. Nail-bed test device is very expensive and hard to repair.

There is basic universal array processor constitute by drilled bare board, the distance among each pin center is 100mil, 75mil and 50mil. These pins play the role like probe, and use electrical connector or joint on the PCB to make mechanic communication. If PAD on PCB is compatible with test array, then polyamide film which has been drilled orderly can be placed among them for the sake of inspection design. Continual inspection is through get access to end point of network grid. Since all the networks on the PCB can execute continually inspection, then a isolated inspection can be finished. However the distance of each probe constrain the functionality of each nail-bed.

PostHeaderIcon Layout draft of printed circuit board

Layout draft of printed circuit board can describe the final drawing of each components and layout design of interconnected circuit, it can also provide sufficient information for the final prototype drawing. Moreover, layout draft also include component hole, wire width and space.

 

Before designer begin to make out plan for printed circuit board layout draft, we suggest to prepare a trial draft, also takes below factors into consideration:

1>    Size of printed circuit board—depends on the space of equipment or use standard size;

2>    Profile of component—attain from data handbook;

3>    Installation datasheet of component—if install professional version, need to take data handbook as reference, if has thermal limitation need to use heat-sink or high volume capacitor;

4>    Interconnect graphics;

5>    Width and space of wire—it depends on the functional requirement of wire;

6>    Edge line—Normally will leave 0.5cm for each edge;

7>    Connector—Size and type of connector depends on the outer connection, such as power input and output signal, grounding and device requirement.

8>    Fixing gadget—screw and clip;

Layout design is always a process of experiment and error-checking, before the final result come out, it need to modify and adjust again and again.

PostHeaderIcon Attentions of Printed circuit board layout design

Hole and PAD on printed circuit board need to array according to standard system, it is more important in multilayer printed circuit board design, because during the process of manufacturing, each layer need to align with other layers. As seen from standard system as picture below, it use 2 mutual related graphics, one is standard or one reference line, as well as standard symbol, all the components array should base upon these symbols. Below factors should be taken into consideration:

1> Fix 0/0 fiducial location, ensure all or most part of printed circuit board could be covered within the area of cartesian coordinate.

2> Fix most of components space and locations, and install the original diagram network. Specify all components which are not suitable for identify in the graphic.

3> Refers to the manufacturing and assembling aspects, we have to consider the locations requirement of holes. If size and density of printed circuit board prohibit add internal hole, then designer has to integrate panel board with external hole and view it as part of design.

PostHeaderIcon Rational installation of power wire ground wire of printed circuit board

Power wire should has enough width to ensure low resistance and small inductance, however, value of capacitance coupling will increase accompany with its width.

On the same printed circuit board, simulated circuit and digital circuit should strictly separated by ground wire network. Moreover, reference voltage normally would be quite sensitive to vibration of ground level, so it should be separated from power wire, connect directly to the incoming interface of printed circuit board, furthermore, its ground wire should connect to an isolated and stable reference ground point of device.

Generally speaking, ground wire not only has resistance, but also inductance, additionally, it will has unknown current pass by, voltage of resistor will decrease when current pass it. CAD program has great number of defects on ground wire design of printed circuit board, because they would design all the wires as thin as they can for the sake of saving more copper size, this will bring much higher ground resistance. The most obvious way to substitute the thin ground wire is connecting all the ground wires on the copper clad layer to form a continual grounding side. Since 0.001inch (0.025mm) copper resistance is around 0.67 mΩ/in2, so under normal circumstance it is enough already.

PostHeaderIcon Welding current of French SAF welding machine can’t shift higher

Breakerdown Appearances: Welding current of French SAF welding machine can’t shift higher;

 

Troubleshooting: After PCB Repair of key component we have found the current control circuit board is malfunctions, inspect this PCB and found a operational amplifier chip LF347 has failed, because we don’t have this chip in-stock, so use LM124 as substitute to maintain the machine can keep on working, after 15 days later LM124 fail and LF347 has been bought back not long later and assemble LF347, machine can working properly.

PostHeaderIcon PCB Repair Daily 1

Breakerdown Appearances:

MAZAK CNC Lathe output disorder pointillization, digital enter point status bounce, sometimes is 1 and sometimes is 0.

Troubleshooting:

After PCB Repair, a storage PCB of Mitsubishi CNC system has faulty, there are lots of 74 series chips like 74LS640/244/32/00, additional, one array of RAM chips. Use on-line testing instrument verify all 74 series chips are no problem, but one of RAMs being found has faulty, change this RAM and re-load the parameter, machine can work properly.

PostHeaderIcon Process of Schematic Diagram Drawing

1 Add new SCH file:

Before entering SCH design system, first you should have general image of the schematic diagram you are about to draw, that is to say you have to understand what circuit need to use on the design of your schematic diagram, and then use PROTEL DXP to draw schematic diagram;

2 Install working environment:

You should choose the right size of drawing paper according to the complexity level of circuit pattern. During the whole process of circuit design, size of drawing paper can be adjusted so set up the proper size of drawing paper could be defined as the 1st step of schematic diagram drawing.

3 Component placement:

Choose components from library, dissipate them onto proper location of drawing and define their name, package method, Adjust and modify the locations of each components according to their electronic connection and circuit relations to make it more reliable and easy to understand.

4 Layout of Schematic Diagram:

Layout schematic diagram according to the requirement of circuit pattern by using different kinds of tools and instruction provide by SCH software, connect all the components on the surface through their electrical connection by circuit and symbol, to form a complete schematic diagram.