Saturday, January 29, 2011
【 Weak current College 】 what is ESD (electrostatic discharge) 】
Electrostatic discharge (ESD, electrostaticdischarge) is in the electronic Assembly and component damage in circuit board of a familiar but underestimated origins. It affects every manufacturer, no allowed size. Although many people think they are in ESD-safe environment, the production of the product, but in fact, ESD-related damage to continue to the world of electronics manufacturing industry billions of dollars each year.
ESD? electrostatic discharge (ESD) is defined as, to or from the original already have static (fixed) charge (e or surplus) discharge (electron flow). Electric charge in two condition is stable:
When it "in" electrical insulation; electrical conductivity; but, such as objects, there is a metal with plastic handle with a screwdriver.
When it stay in insulated surface (e.g. plastic), cannot flow above.
However, if the charge with a sufficiently high electrical insulation conductor (screwdriver), close to the contrary potential of integrated circuit (IC), charge the "crossover", caused by electrostatic discharge (ESD).
ESD to extremely high strength occurs very quickly, usually will generate enough heat to melt the internal circuit semiconductor chips, in electron micrograph outer appearance to blow out of small bullet holes, causing instant and irreversible damage.
More seriously, this kind of harm only one tenth of the bad to cause in the final testing of the entire component failure. The other 90%, ESD damage caused only part of the demotion-means that the damaged components can be silently through the last test, but is shipped to the customer after the premature failure of the site. The result is the loss of reputation, for a manufacturer to correct any manufacturing defect where most pay.
However, the main difficulties in ESD control is that it is not visible, but you can achieve the damage electronic components. Audible "ticking" a sound of discharge requires the accumulation of approximately 2000 volts rather larger charge and 3000 volts can feel small electric shock, 5000-Volt can see sparks.
For example, such as complementary metal oxide semiconductor (CMOS, complementarymetaloxidesemiconductor) or electrical programmable read only memory (EPROM, electricallprogrammableread-onlymemory) these common elements, were only 250 v and 100 v ESD electric potential difference is destroyed, but more and more sensitive of modern components, including the Pentium processor, as long as 5 volt can be destroyed.
The problem is caused by damage in a day of activities combined together. For example, from vinyl factory floor walk the floor surface and rubbing between shoes. The result is pure charge of objects, cumulative reach 3 to 2000 Volt charge, depending on local air humidity of equivalent.
Even workers in the stage of the natural movement of the friction can produce 400 ~ 6000 volts. If the voiding or packing foam cartridge or bubble bag of PCB, the worker has processed insulator, the workers body surface accumulated net charge up to around 26000 volts.
Therefore, as the main source of ESD damage, all entered the ESD protection zone (EPA, electrostaticprotectedarea) of staff must be grounded to prevent any charge accumulation, and all the surface should be ground to keep everything the same electric potential, prevent ESD occurs.
To prevent ESD's main product is a bowl of belt (wristband), kinky corduroy and dissipative surface or bedding-both of which must be properly grounded. Additional AIDS such as dissipative footwear or heel belt and suitable clothing, are designed to prevent personnel in ESD protection zone (EPA) cumulative and maintain when you move the net charge.
During and after the Assembly, PCB should prevent from internal and surface transport in ESD. There are a number of circuit board packaging products can be used for this connection, including shielding bags, shipping boxes and removable trolley. Although the correct use of these devices will prevent 90 percent of ESD-related issues, but for the last 10% takes another kind of protection: ionization.
And those who can produce the electrostatic charge of Assembly equipment and surfaces the most effective method is to use the ion generator (ionizer)-a device blowing out ionization air flow in the work area, and accumulated in the insulation of any charge.
A common fallacy is that because the workstation with the bowl with the insulator in the region, such as polystyrene Cup or cardboard boxes, the charge will be safely dissipated. By definition, an insulator is not electrically conductive, except through ionization cannot discharge.
If a charge of insulators remain in EPA, it will radiate an electrostatic field, raises net charge to any nearby object, thereby increasing the damage to the product of ESD. Although many manufacturers attempt to ban from the EPA, but the insulating material is very difficult to implement. Insulating material is daily life too much part-from the operator is located and comfortable foam pad, that some of the plastic cover.
As a result of the use of ion generator, manufacturers can accept some insulation material in the EPA in fact. Because ion system continuously and may occur in any charge above insulator, so for any of the ESD programme, they are a reasonable investment.
Standard electronic assembly equipment, ion occurs in two basic forms:
Desktop (a single fan)
Over the top of the device (in a single over the top of the unit, there is a series of fan)
There are indoor ion generator, but it is mainly used for clean room environment.
> Select the need to protect the area. Desktop ion generator will override the single work surface, and over the top-ion generator will cover two or three. Another advantage is the ion generator also prevent dust and electrostatic attached to product may give rise to an appearance of degraded.
However, without the validity of the ESD equipment and monitoring of normal testing, then none of the protection scheme is perfect. First-class ESD control and ionization expert reports on the use of (and therefore useless) ESD equipment and do not know the manufacturer of its failure.
In order to prevent this situation, in addition to the standard equipment of ESD, ESD vendors also provide constant monitor, if a performance is out of the automatic alarm. Monitor can be used as a stand-alone unit or in network connections. Also has automatic data acquisition of network software, real-time display of the relevant operator and workstation system performance.
The Profiler can eliminate many daily tasks to simplify the ESD programme, such as ensuring proper measurements per day with a bowl, ion generator of balance and the correct maintenance, no damage to Earth point of table.
Conclusions of the
Prevent ESD of the first step is to evaluate if neglected, how small details may result in irreparable damage. An effective scheme requires not only use effective ESD protection devices, and rigorous operational procedures to ensure that all factory ground personnel are ESD safe.
Although many manufacturers use automatic bowls with tester, but often you can see the operator because the bowl with a too loose, or by testing or failure. Many operator tries to use the other hand simple hold Tester close to their wrists to the test.
ESD protection circuit design
Electrostatic discharge (ESD) will give electronics fatal hazards, it not only reduces the reliability of the product, increased maintenance cost and is not in line with the European Community provides industry standard EN61000-4-2, the product cannot be sold in Europe. Therefore, electronic equipment manufacturers often at circuit design of the early consideration of ESD protection. This article will discuss the ESD protection circuit in several ways.
ESD is basically divided into three types, a variety of machines due to ESD, furniture move or move equipment, three due to ESD is human contact or equipment caused by ESD movement. These various ESD for semiconductor production and production of electronic products are very important. Electronic products in use are the most vulnerable third ESD damage, portable electronic products are particularly vulnerable to human contact damage of ESD. In normal circumstances ESD damage connected to interface devices, another case was subjected to ESD shock after the device may not be damaged, but the performance degradation cause products to fail prematurely.
When the integrated circuit (IC) to withstand ESD, discharge circuit resistance is usually very small, cannot limit the discharge current. For example, the static electricity cable plugged into a circuit interface, discharge circuit resistance almost zero, resulting in up to 10 amps peak instantaneous discharge current, flows into the IC PIN. The instantaneous current would seriously damage IC, local heating of heat even melt silicon chips. ESD damage to IC also includes internal metal connection was blowing, passivation layer, transistor unit is burned. ESD will cause the deadlock IC (LATCHUP). This effect and CMOS device internal structure similar to the thyristor unit is activated. High voltage to activate these structure, formation and high current channels, usually from VCC to land. Serial interface device deadlock currents up to 1A. Deadlock will remain current until the device is powered off. But by that time, the IC is usually already hot and burnt. ESD shock may exist two is not easy to be found, the average user and IEC test institutions use the traditional method of "feedback loop" and "Insert method" for testing, you typically cannot detect.
One problem is the RS-232 interface circuit sink on transmitter produces cross-talk. Products of the same RS-232 interface circuit ESD protection structure may be on a wave of ESD ESD or a failure, after impulse voltage ESD shock after input of the receiver and the transmitter output between the access road, causing the receiver to the transmitter produce pay increase (Figure 1). If RS-232 interface circuits for off road, then turn off period after ESD shock after more easily generate alternating current. Produced by alternating will lead to a communication failure and even shut off the status of the transmitter still have output, resulting in failure to turn off each other in the receiving State RS-232.
Another problem is the RS-232 interface circuits for power generation reverse driver. Some RS-232 interface circuit ESD protection structure through ESD shock may result in input and VCC power supply between the current path (Figure 2), on the reverse driver power supply. If the power supply does not have the ability to breathe in-current (typically power output circuit has a forward diode), which will cause the power supply voltage VCC rise, resulting in damage to the RS-232 interface circuits and other circuit within the system. Because the RS-232 interface circuit input voltage is 5V to 25V, between VCC is likely higher than the supply voltage 9V, exceeds the maximum area burned circuit. ESD protection circuit is the most effective protection measures is medium isolation: dielectric to internal circuit and isolated from the outside world. 1mm thick of ordinary plastic PVC, polyester or ABS 8KV protection ESD. But the actual media without gaps and joints, so materials creep and clearance distance is very important. LCD display screen, touch screen, has a very thick corners (12mm) isolation internal circuits.
ESD protection for the second method is a shield to prevent large ESD to storm the internal circuit. ESD shock metal shielding enclosures, initiallyA few milliseconds rather than protective ground voltage higher than many, shielding enclosures voltage ESD charge as the decline of the transfer, so the first few milliseconds on internal circuits secondary ESD shock, so only use external shield is not enough, the internal circuits and shielding enclosures must be shared, or the internal circuit for medium isolation. Electrical isolation also inhibits the ESD shock of an effective method, PCB mounting optical coupler or transformer, although it is not possible to completely eliminate the impact of the ESD, but combined with medium isolation and shielding can be very good on EDS, optical coupler and the transformer is particularly suitable for the power part. Signaling the best isolation is fibre, wireless, and infrared.
In the signal path on the use of another kind of protection is in each signal line plus RC components. Series resistor to limit the rush current, parallel to the sides of the capacitor can limit instantaneous peak voltage. This low cost, but limited defensive capabilities. ESD damage to a certain degree of restraint, but still exist. Since RC-component and cannot reduce peak voltage peak value, simply reduce the slope of the voltage to rise. But RC components also causes a distortion that limits the length of the communications cable and communication speed. External resistor/capacitor has also increased the circuit board area. Another popular method is applied voltage transient suppressor or TransZorb diode. This protection is very effective. There are some disadvantages: plus devices will increase the circuit board area; protective device capacitance effects will increase the signal line equivalent capacitance; cost is relatively high. Adopt internal integrated ESD protection function of serial interface device is an effective way. This type of diodes than ordinary non-protection function of the device's price is higher, but additional costs compared to the cost of additional protection against diodes. Internal ESD protection circuit integrated will not increase any input and output pins of equivalent capacitance, saving boardspace. Maxim company in recent years the development of an integrated ESD protection technology that provides a full range of ESD protection devices, including serial interface and the standard device is fully compatible with the product. Maxim also apply the same technology applied to the emulation switch and switch to shake the product. All of these devices of ESD protection ability meet ± 15kVIEC1000-4-2 (air gap discharge), ± 8kVIEC1000-4-2 (contact discharge), ± 15kV human body model (HBM) test standards.
Maxim's ESD protection of the European Community as provided for in the ESD protection have rigorous testing standards: ± 15kVESD human body model tests; ± 8kVESDIEC1000-4-2 contact discharge mode testing standards; ± 15kVESDIEC1000-4-2 air gap discharge mode testing standards; ± 4kVESDIEC1000-4-4 electrical fast transient/burst mode test standards. Of these, IEC1000-4-2 and ± 15kV human body model test standard primary difference between peak current; the same voltage IEC1000-4-2 shock absorption and current than the human body model, delivering up to five times more. ± 4kVESDIEC1000-4-4 electrical fast transient/burst mode test standard is simulating switch and relay the results of arc discharge. MAXIM devices can provide protection: ± 4kV twice IEC1000-4-4 standard ± 2kV pointer.
Because each device ESD threshold is different, each device in the normal working condition, off-State and powered by ESD threshold, so the Maxim we strictly follow the following steps to test:
1. starting from ± 200V, every increase in the 500V on each device that the impact of different polarity of the voltage and. 2, each time after impact, check the supply current to ensure that the devices have no atresia, check the transmitter and receiver is working properly;
3. repeat the above steps, until the device is damaged or meet ESD testers restrictions;
4. use the human body, IEC1000-4-2 contact discharge, IEC1000-4-2 air-gap discharge and IEC1000-4-4 fast/transient/pulse repeat these steps;
5, respectively, in the normal working condition, off-State and powered by repeating the test. Assurance reaching ESD protection testing standards, and not occurring by alternating and reverse driver problems.