Detailed analysis of assembly and rework knowledge of BGA components

Ball grid array (BGA) devices have undeniable advantages. But some of the issues in this technology are still to be discussed further and not immediately realized because it is difficult to trim the soldered ends. BGA interconnect integrity can only be tested by X-ray or electrical test circuit methods, both of which are expensive and time-consuming.

The two most common BGA packages are Plastic BGA (PBGA) and Ceramic BGA (CBGA). The PBGA has fusible solder balls, typically 0.762mm in diameter, that collapse into 0.406mm high solder joints between the package and the PCB during reflow (usually 215°C). CBGA uses infusible solder balls on components and printed boards (in fact, its melting point is much higher than the temperature of reflow soldering), the diameter of the solder balls is 0.889mm, and the height remains unchanged.

assembly problem

The great advantage of BGA assembly is that if the assembly method is correct, its yield rate is higher than that of traditional devices. This is because it has no leads, which simplifies the handling of the components and therefore reduces the possibility of damage to the device.

The BGA reflow soldering process is the same as the SMD reflow soldering process, but BGA reflow soldering requires precise temperature control and the establishment of an ideal temperature profile for each component. In addition, most BGA devices can be self-aligned on the pads during reflow soldering. Therefore, from a practical point of view, it is possible to assemble the BGA with the equipment that assembles the SMD.

However, since the solder joints of the BGA are invisible, the solder paste application must be carefully observed. The accuracy of solder paste application, especially for CBGA, will directly affect the assembly yield. Low yields are generally tolerated for SMD device assembly because rework is fast and inexpensive, but BGA devices do not have this advantage. To improve first-time yield, many high-volume BGA assemblers have purchased inspection systems and complex rework equipment. Inspection of solder paste application and component placement before reflow can reduce costs more than inspection after reflow, which is difficult and expensive to inspect after reflow.

Solder paste should be chosen carefully because the composition of the solder paste is not always ideal for BGA assembly, especially for PBGA assembly. Suppliers must ensure that their solder paste does not form solder joint voids. Likewise, if water-soluble solder paste is used, care should be taken to select the package type.

Since PBGA is sensitive to moisture, pretreatment measures should be taken before assembly. It is recommended that all packages be fully assembled and reflow soldered within 24 hours. Leaving the device out of the antistatic protective bag for an extended period of time will damage the device. CBGA is not sensitive to moisture, but care is still needed.

Repair

The basic steps for rework BGA are the same as those for rework traditional SMD:

Create a temperature curve for each component;

remove components;

Remove residual solder paste and clean this area;

Place new BGA devices. In some cases, BGA devices can be reused;

Reflow soldering.

Of course, these three main types of BGA all require slightly different adjustments to the process. For all BGAs, the establishment of the temperature profile is quite important. No attempt can be made to omit this step. If technicians do not have the right tools and are not trained to do so themselves, they can find it difficult to remove residual solder paste. Too frequent use of poorly designed desoldering braids, combined with poorly trained technicians, can lead to damage to the substrate and solder mask.

Create a temperature curve

Compared with traditional SMD, BGA has much higher requirements for temperature control. The entire BGA package must be heated incrementally to reflow the solder joints.

If the temperature, rate of temperature rise and hold time (2°C/s to 3°C/s) are not strictly controlled, reflow soldering will not occur simultaneously and may damage the device. Establishing a stable temperature profile for BGA removal requires some skill. Designers do not always have information on every package, and trying methods can 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 proper temperature profile. Drill holes in the PCB to expose the solder joints and attach thermocouples to the solder joints. In this way, a temperature profile can be established for each monitored solder joint. The technical data shows that the establishment of the printed board temperature curve is based on a printed board full of components, which uses a new thermocouple and a calibrated recording element, and the high and low temperature areas of the printed board are used. A thermocouple is installed. Once the temperature profile is established for the substrate and BGA, it can be programmed for repeated use.

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. The type of nozzle varies with equipment or technician preference. Some nozzles flow the hot air over the top and bottom of the BGA device, some move the hot air horizontally, and some spray the hot air just above the BGA. Others prefer to use hooded nozzles, which focus the hot air directly on the device, thereby protecting surrounding devices. It is important to maintain the temperature when removing the BGA. The key is to preheat the bottom of the PCB to prevent warping. Removing the BGA is a multi-point reflow and thus requires skill and patience. Additionally, reworking a BGA device typically takes 8 to 10 minutes, which is slower than reworking other surface mount components.

Cleaning and mounting position

Before mounting the BGA, the rework area should be cleaned. This step can only be done manually, so the skill of the technician is very important. If the cleaning is not sufficient, the new BGA will not reflow properly, and the substrate and solder mask may also be damaged beyond repair.

When reworking BGAs in large quantities, commonly used tools include desoldering irons and hot air desoldering devices. The hot air desoldering device first heats the surface of the pad, and then uses a vacuum device to suck away the molten solder paste. Desoldering irons are easy to use, but require skilled personnel to operate. Desoldering irons can easily damage printed boards and pads if used incorrectly.

When removing residual solder paste, many assemblers like to use tin removal braid. With the proper braid and method, the removal process is quick, safe, efficient and inexpensive.

Although it takes a certain amount of skill to use tin removal braid, it is not difficult. Touch the solder paste to be removed with the soldering iron and braid of choice so that the solder core is located between the soldering iron tip and the substrate. Direct contact of the soldering iron tip to the substrate may cause damage. Solder Paste-Solder Core BGA Removal Braid is designed to remove residual solder paste from BGA pads and components without damaging the solder mask or exposed traces. It optimally transfers the heat through the braid to the solder joints so that the possibility of pad displacement or PCB damage is minimized.

Since the solder core is very mobile in use, it is not necessary to drag the solder core to avoid thermal damage. Instead, place the solder wick between the substrate and the tip of the soldering iron, heat it for 2 to 3 seconds, then lift the braid and iron upwards. Lifting, rather than dragging, the braid minimizes the risk of damage to the pads. Braided tape 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. Residual flux can be removed with a brush. For proper reflow soldering of new devices, the PCB must be clean.

Mounting devices

Skilled technicians can "see" the placement of some components, but this method is not recommended. If a higher process yield is required, a split-opTIcs vision system must be used. Devices are placed, aligned, and reflowed with hot air using a vacuum pick-up tube. Here, 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 overlooked.

When remounting components, a completely different approach should be used. To avoid damaging the new BGA, preheat (100°C to 125°C), temperature ramp rate and temperature hold time are all critical. Compared to PBGA, CBGA is able to absorb more heat, but the heating rate is slower than the standard 2°C/s.