PCB Design Tips for Replacing ICs

In PCB design, when you need to replace the IC, then let’s talk about the skills when replacing the IC in PCB design to help the PCB design engineer to be more perfect in the PCB Design.



Direct replacement

Direct replacement refers to the direct replace the original IC with other ICs without any modification, and the replacement does not affect the main performance and indicators of the machine.

Direct Replacement Principle: All the function, performance index, package form, pin usage, pin number and interval of the replacement IC are the same. The same function of the IC not only means function, but also same logic polarity, that is, the output and input level polarity, voltage, and current amplitude must be the same. The performance index refers to the main electrical parameters (or main characteristic curve) of the IC, the maximum power dissipation, the maximum working voltage, the frequency range, and the input and output impedance of each signal, which should be similar to the original IC. Substitutes with low power should increase the heat sink.

1. Replacement of the same type of IC
The replacement of the same type of IC is generally reliable. When installing the integrated PCB circuit, pay attention to the direction not to be wrong, otherwise, the integrated PCB circuit is likely to be burned when the power is turned on. Some single-in-line power amplifier ICs have the same model, function, and characteristics, but the direction of the pin arrangement is different. For instance, the two-channel power amplifier ICLA4507, the pins are divided into “positive” and “reverse”, and the direction of the starting foot label (color dot or pit) is different: No suffix and the suffix “R”, IC,such as M5115P and M5115RP.

2. Replacement of ICs with the same model prefix letters and different numbers
This kind of replacement can be directly replaced as long as the functions of the pins are exactly the same, and the internal PCB circuit and electrical parameters are slightly different. Such as sound amplifier ICLA1363 and LA1365, the latter has a zener diode inside IC pin 5 than the former, and the others are exactly the same.

In general, the prefix letter indicates the manufacturer and the category of the PCB circuit. The numbers after the prefix letter are the same, and most of them can be directly replaced. But there are also a few special cases, although the number is the same, but the function is completely different. For example, HA1364 is a audio IC, and uPC1364 is a color decoding IC; The digital is 4558, the 8-pin is the operational amplifier NJM4558, and the 14-pin is the CD4558 digital PCB circuit; Therefore, the two cannot be replaced at all. It indicates that pin function should also pay attention to when replacement.

Some manufacturers introduce unpackaged IC chips, and then process them into products named after their own factories, as well as products that are improved in order to improve certain parameters. These products are often named with different models or distinguished by model suffixes. For example, AN380 and uPC1380 can be directly replaced, and AN5620, TEA5620, DG5620 can be directly replaced.



Indirect Replacement

Indirect replacement refers to the method of slightly modifying the peripheral PCB circuit of the IC that cannot be directly replaced, changing the arrangement of the original pins or adding or removing individual components just in order to make it replaceable.

Indirect Replacement Principle: The IC used for replacement can be different in function and shape from the original IC, but the function should be the same and the characteristics should be similar; The replacement should not affect the performance of the original one.

1. Replacement of ICs in different packages
For same type IC but with different package shapes: When replacing, just reshape the pins of the new IC according to the shape and arrangement of the pins of the original IC. For example, AFTPCB circuits CA3064 and CA3064E, the former is a circular package with radial pins; The latter is a dual in-line plastic package, the internal characteristics of the two are exactly the same, and the connection can be made in the light of the pin function. The two-row ICAN7114 and AN7115 are basically the same package as LA4100 and LA4102, and the pins and heat sinks are exactly 180 degrees apart. The aforementioned AN5620 dual-in-line 16-pin package with heat sink, TEA5620 dual-in-line 18-pin package, pins 9 and 10 are located on the right side of the integrated PCB circuit, which is equivalent to the heat sink of AN5620. The other pins of the two are arranged in the same way. Connect the 9th and 10th feet to the ground and you can use it.

2. Same PCB circuit function but different functions in certain pins
The replacement can be carried out according to the specific parameters and descriptions of each type of IC. For example, the AGC and video signal outputs in the TV have positive and negative polarities, which can be replaced as long as an inverter is connected to the output.

3. Replacement of ICs with the same plastic but different pin functions
This kind of replacement need to change the peripheral PCB circuit and pin arrangement, so it requires certain theoretical knowledge, complete information and rich practical experience and skills.

4. Some empty pin should not be grounded without authorization
Some pins in the internal equivalent PCB circuit and application PCB circuit are not marked. When encountering empty pins, they should not be grounded without authorization. These pins are replacement or spare pins, and sometimes they are also used as internal connections.



5. Combination Replacement
Combination replacement is a method of recombining the undamaged PCB circuit parts of multiple ICs of the same model into a complete IC to replace a malfunctioning IC. It is quite applicable when the original IC is not available. However, it is required that the intact PCB circuit inside the IC used must have interface pins.

The key to non-direct replacement is to find out the basic electrical parameters of the two ICs to be replaced, the internal equivalent PCB circuit, the function of each pin, and the connection relationship between the IC components. Pay attention to when operating.

(1) The number sequence of the integrated PCB circuit pins must not be wrongly connected;
(2) In order to adapt to the characteristics of the replaced IC, the components of the peripheral PCB circuit connected should be changed accordingly;
(3) The power supply voltage should be consistent with the replaced IC. If the power supply voltage in the original PCB circuit is high, try to reduce the voltage; If the voltage is low, it depends on whether the replacement IC can work or not;
(4) After the replacement, the quiescent working current of the IC should be measured. If the current is much larger than the normal value, it means that the PCB circuit may be self-excited. At this time, decoupling and adjustment are required. If the gain is different from the original, the resistance value of the feedback resistor can be adjusted;
(5) After the replacement, the input and output impedance of the IC should match the original PCB circuit; check its driving ability;
(6) Make full use of the pin holes and leads on the original PCB circuit board when making changes, and the external leads should be neat and avoid crossovers in order to check and prevent the self-excitation of the PCB circuit, especially to prevent high-frequency self-excitation;
(7) It is best to connect a DC ammeter in series in the power supply Vcc loop before power-on, and observe whether the change of the total current of the integrated PCB circuit is normal from the resistance value of the step-down resistor from large to small.

6. Replacing ICs with discrete components
Damaged parts of an IC can sometimes be replaced with discrete components to restore functionality. Before replacement, you should understand the internal functional principle of the IC, the normal voltage of each pin, the waveform diagram and the working principle of the PCB circuit formed with peripheral components. Also consider:

(1) Whether the signal can be taken out from the industrial C and connected to the input end of the peripheral PCB circuit;
(2) Whether the signal processed by the peripheral PCB circuit can be connected to the next level inside the integrated PCB circuit for reprocessing (the signal matching during connection should not affect its main parameters and performance). If the intermediate amplifier IC is damaged, from the perspective of the typical application PCB circuit and internal PCB circuit, it is composed of the audio intermediate amplifier, frequency discrimination and frequency amplification. The damaged part can be found by the signal input method. If the audio amplifier part is damaged, discrete components can be used. replace.


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