Assembly and rework of BGA components

Most manufacturers believe that ball-and-spring array (BGA) devices have undeniable advantages. However, some of the problems in this technology remain to be discussed further, rather than immediately, because it is difficult to trim the soldered ends. Only X-rays or Electrical test circuit method to test BGA interconnect integrity, but both methods are expensive and time consuming.

Designers need to understand the performance characteristics of BGA, which is similar to earlier SMDs. PCB designers must know how to make changes to the design as the manufacturing process changes. For manufacturers, they are faced with different types of BGAs. The challenge of packaging and final process changes. In order to improve the yield, the assembler must consider establishing a new standard for handling BGA devices. Finally, to get the most cost-effective assembly, perhaps the key is BGA rework. personnel.

The two most common BGA packages are plastic BGA (PBGA) and ceramic BGA packages (CBGA). PBGAs have fusible solder balls with a diameter of usually 0.762 mm. During reflow (usually 215 ° C), these solder balls are in The package and the PCB collapsed to a 0.406mm high solder joint. The CBGA uses a non-welded solder ball on the component and the printed board (actually its melting point is much higher than the reflow soldering temperature), and the solder ball diameter is 0.889mm. The height remains the same.

The third BGA package is a ball grid array package (TBGA) package, which is now increasingly used in high performance components that require lighter, thinner devices. On polyimide tape, TBGA The number of I/O leads can exceed 700. TBGA can be processed using standard screen-printed solder paste and conventional infrared reflow soldering methods.

Assembly problem
The big advantage of BGA assembly is that if the assembly method is correct, the pass rate is higher than that of the conventional device. This is because it has no leads, which simplifies the processing of the components, thus reducing the possibility of damage to the device.

The BGA reflow process is the same as the SMD reflow process, but BGA reflow requires precise temperature control and an ideal temperature profile for each component. In addition, most of the BGA devices can be placed on the pads during reflow. Automatic alignment. Therefore, from a practical point of view, BGA can be assembled with equipment that assembles SMD.

However, since the solder joints of BGA are invisible, it is necessary to carefully observe the solder paste application. The accuracy of solder paste application, especially for CBGA, will directly affect the assembly yield. Generally, SMD device assembly is allowed to pass. Low condition, because its repair is fast and cheap, but BGA devices do not have this advantage. In order to improve the initial pass rate, many large batch BGA assemblers have purchased inspection systems and complex rework equipment. Before welding reflow soldering Paste coating and component placement are more cost effective than post-reflow soldering because it is difficult to detect after reflow and the equipment required is expensive.Careful selection of solder paste is required because the composition of the solder paste is not always ideal for BGA assembly, especially for PBGA assembly. The supplier must ensure that the solder paste does not form solder joint voids. Similarly, if water is used Solvent solder paste, should pay attention to the choice of package type.

Since PBGA is sensitive to moisture, pre-treatment measures should be taken prior to assembly. It is recommended that all packages be fully assembled and reflowed within 24 hours. The device will leave the antistatic bag for too long and will damage the device. CBGA does not affect moisture. Sensitive, but still need to be careful.

Rework
The basic steps for reworking a BGA are the same as those for a traditional SMD:
Establish a temperature profile for each component;
Remove components;
Remove residual solder paste and clean this area;
Mounting new BGA devices. In some cases, BGA devices can be reused;
Reflow soldering.

Of course, these three main types of BGAs require a slightly different adjustment of the process. For all BGAs, the establishment of the temperature profile is quite important. You can't try to save this step. If the technician does not have the right tools. And without the special training itself, it will be difficult to remove the residual solder paste. Too often use poorly designed desoldering woven tape, coupled with the lack of good training of technicians, will lead to the substrate and solder mask Damaged.

Establish temperature profile
Compared with traditional SMD, BGA requires much higher temperature control. The entire BGA package must be gradually heated to reflow the solder joint.
If the temperature, temperature rise rate and hold time (2 ° C / s to 3 ° C / s) are not strictly controlled, reflow soldering will not occur at the same time, and the device may be damaged. It is necessary to establish a stable temperature curve for removing the BGA. Tips. Designers don't always get information about each package. Attempts may cause thermal damage to the substrate, surrounding devices, or floating pads.

Technicians with extensive BGA rework experience rely primarily on destructive methods to determine the appropriate temperature profile. Drill holes in the PCB to expose the solder joints and then connect the thermocouples to the solder joints. This way, for each The monitored solder joint establishes a temperature profile. Technical data indicates that the printed board temperature profile is based on a printed board full of components, using a new thermocouple and a calibrated recording component. Thermocouples are installed in the high and low temperature areas of the printed board. Once the temperature profile is established for the substrate and the BGA, it can be programmed for reuse.

With some hot air rework systems, the BGA can be removed relatively easily. Typically, hot air at a certain temperature (determined by the temperature profile) is ejected from the nozzle to reflow the solder paste without damaging the substrate or surrounding components. Equipment or technicians have different preferences. Some nozzles allow hot air to flow in the upper and bottom of the BGA device, some nozzles move the hot air horizontally, and some nozzles only spray hot air over the BGA. Others prefer to use a hooded nozzle. It directly concentrates the hot air on the device, thus protecting the surrounding components. When removing the BGA, temperature maintenance is very important. The key is to preheat the bottom of the PCB to prevent warpage. Removing the BGA is more. Reflow requires skill and patience. In addition, it takes 8 to 10 minutes to rework a BGA device, slower than reworking other surface mount components.

Cleaning placement location
Before the BGA is placed, the repair area should be cleaned. This step can only be performed manually, so the skill of the technician is very important. If the cleaning is not enough, the new BGA will not be properly reflowed, and the substrate and solder mask may be damaged. Can't fix it.
When repairing BGA in large quantities, commonly used tools include desoldering soldering iron and hot air desoldering device. The hot air desoldering device first heats the surface of the pad and then vacuums away the molten solder paste. The soldering iron is easy to use, but requires skilled Personnel operation. If used improperly, the soldering iron can easily damage the printed board and the pad.
When removing residual solder paste, many assemblers prefer to use tin-removing tape. If a suitable braid is used and the method is correct, the removal process will be fast, safe, efficient and inexpensive.

Although the use of tin-removing tape requires certain skills, it is not difficult. Use a soldering iron and the selected braid to contact the solder paste that needs to be removed so that the core is between the tip and the substrate. Direct contact of the soldering iron with the substrate may cause damage. Solder Paste - Solder BGA Tin Strip is specifically designed to remove residual solder paste from BGA pads and components without damaging the solder mask or exposed traces. It allows heat to pass through the braid in the best possible way. Passed to the solder joint so that the possibility of pad displacement or PCB damage is minimized.

Since the core is very active during use, it is not necessary to drag the core to avoid thermal damage. Instead, place the core between the substrate and the tip, heat for 2 to 3 seconds, then lift the braid up And the soldering iron. Lifting rather than dragging the braid minimizes the risk of damage to the pad. The braid removes all residual solder paste, eliminating the possibility of bridging and shorting. After removing the residual solder paste, Clean this area with a suitable solvent. The residual flux can be brushed off with a brush. In order to properly reflow the new device, the PCB must be clean.

Mounting device
Skilled technicians can "see" the placement of some devices, but this method is not recommended. If higher process qualification rates are required, a split-optics vision system must be used. Vacuum pick-up tubes are used. Install and calibrate the device and reflow with hot air. At this point, a pre-programmed and precisely defined temperature profile is critical. When removing components, the BGA is most likely to fail, so its integrity may be ignored.

When reassembling components, a completely different approach should be used. To avoid damage to the new BGA, preheating (100 ° C to 125 ° C),

 temperature rise rate and temperature hold time are critical. Compared to PBGA, CBGA can absorb more More heat, but the rate of temperature rise is slower than the standard 2 ° C / s.

BGA has many advantages for modern high-speed assembly. BGA assembly may not require new processes, but requires existing processes for BGA assembly with hidden solder joints. To make BGA more cost effective, high yields must be achieved. And can effectively repair the components. Appropriate training of the rework technicians, the use of appropriate rework equipment, understanding of the key processes of BGA rework, will help achieve stable and effective rework.