PCBA Tech - Rework of ordinary SMD in SMT patch processing

The principle of the ordinary SMD rework system. The hot air flow is concentrated on the pins and pads of the surface mount device (SMD) to melt the solder joints or reflow the solder paste to complete the disassembly and soldering functions.

The main difference between the rework systems of different manufacturers is that the heating source is different, or the hot air method is different, and some nozzles make the hot air above the SMD. From the perspective of protection devices, the airflow should be better to flow around the PCB. In order to prevent the PCB from warping, a rework system with the function of preheating the PCB should be selected.

Second BGA repair

Use HT996 to carry out BGA repair steps:

..1 Remove BGA

Use a soldering iron to clean and level the remaining solder on the PCB pad. Desoldering braid and spade-shaped soldering iron tip can be used to clean it. Be careful not to damage the pad and solder mask during operation.

Use a special cleaning agent to clean the flux residue.

..2 Dehumidification treatment

Because PBGA is sensitive to moisture, it is necessary to check whether the device is damp before assembling, and dehumidify the damp device.

..3Printing solder paste

Because other components are already installed on the surface assembly board, a special small template for BGA must be used. The thickness of the template and the size of the opening must be determined according to the ball diameter and ball distance. After printing, the printing quality must be checked. If it is unqualified, the PCB must be cleaned. Reprint after clean and dry. For CSP with a ball pitch of 0.4mm or less, no solder paste is required, so there is no need to process a template for rework, and paste flux is directly applied to the PCB pad. Put the PCB that needs to be removed into the soldering furnace, press the reflow button, wait for the machine to finish according to the set procedure, press the in and out button when the temperature is highest, and use the vacuum suction pen to remove the components to be removed, PCB The plate can be cooled.

..4 Cleaning the pad

Use a soldering iron to clean and level the remaining solder on the PCB pad. Desoldering braid and flat spade-shaped soldering iron head can be used to clean it. Be careful not to damage the pad and the solder mask during operation.

..5 Anti-damp treatment

Because PBGA is sensitive to moisture, it is necessary to check whether the device is damp before assembling, and dehumidify the damp device.

..6 printing solder paste

Because other components are already installed on the surface assembly board, a special small template for BGA must be used. The thickness of the template and the size of the opening must be determined according to the ball diameter and ball distance. After printing, the printing quality must be checked. If it is unqualified, the PCB must be cleaned. Reprint after clean and dry. For CSP with a ball pitch of 0.4mm or less, no solder paste is required, so there is no need to process a template for rework, and paste flux is directly applied to the PCB pad.

.. 7-mount BGA

If it is a new BGA, it must be checked whether it is damp, if it has been damp, it should be dehumidified before mounting.

The removed BGA device can be reused in general, but it must be used after ball planting. The steps for mounting BGA devices are as follows:

A Place the surface mount board with solder paste printed on the workbench

B Select the appropriate nozzle and turn on the vacuum pump. The BGA device is sucked up, the bottom of the BGA device is completely overlapped with the PCB pad, and the suction nozzle is moved down, the BGA device is mounted on the PCB, and then the vacuum pump is turned off.

..8 Reflow soldering

The soldering temperature can be set according to the size of the device, the thickness of the PCB and other specific conditions. The soldering temperature of BGA is about 15 degrees higher than that of traditional SMD.

..9 inspection

BGA welding quality inspection requires X-ray or ultrasonic inspection equipment. In the absence of inspection equipment, the welding quality can be judged through functional tests, or it can be inspected based on experience.

Lift the surface assembly board of the soldered BGA, look around the BGA, observe whether the light is transmitted, the distance between the BGA and the PCB is the same, whether the solder paste is completely melted, whether the shape of the solder ball is correct, and the solder ball The degree of collapse and so on.

..--If there is no light, it means there is a bridge or there is a solder ball between the solder balls;

..--If the shape of the solder ball is not correct and there is distortion, it means that the temperature is not enough, the soldering is insufficient, and the self-positioning effect is not fully exerted when the solder reflows;

..--The degree of solder ball collapse: the degree of collapse is related to the soldering temperature, the amount of solder paste, and the size of the pad. When the pad design is reasonable, it is normal that the distance between the bottom of the BGA and the PCB after reflow soldering is 1/5-1/3 collapsed than before soldering. If the solder ball collapses too much, the temperature is too high and bridging is prone to occur.

..--If the distance between the BGA circumference and the PCB circuit board is inconsistent, it means that the surrounding temperature is uneven.