With the advances in integrated technology and deep submicron technology in the 1990s, LSI, VLSI, and ULSI were invented in succession. Silicon single-chip integration continues to be improved. The requirements for IC packaging has become more and more stringent. I/O Pin count has increased dramatically. It also has increased the power consumption. In order to meet the developmental needs of the varieties on the basis of the original package, a new variety-ball grid array package, known as Ball Grid Array Package (BGA), has been introduced. In the following paragraphs we simply introduce such packaging production.
I、BGA preparation before production of the product:
1、 Stencil templates production requirements:
| A、 | Reducing the thickness of the template, which is different for different product. Generally it’s about 0.12 mm to 0.15 mm. |
| B、 | Trapezoidal steel mesh openings is adopted. The upper opening should be slightly larger than the lower one. Also a lower opening that is slightly smaller than the pad can be considered. |
| C、 | According to the pitch of the solder ball, Square openings can be used. |
2、 PCB and BGA baking:
| A、 | The moisture sensitivity of BGA Package is quite strong. Before the mount if it is not properly baked and kept dry, BGA phenomena of bow shape or crack would appear in reflow soldering. |
| B、 | When there is moisture in the PCB laminate, during the assembly there would be sudden exhaust of air, leading to partial delamination. The reliability of welding is reduced. |
| C、 | BGA process can refer to the instructions on the packaging. Generally it’s baked for 20 hours at 125 ± 5°C . For PCB, continuous baking for 12 hours at 100°C can be referred. |
3、 Solder paste requirement:
| A、 | Smaller particles of solder paste. |
| B、 | Slightly stronger viscosity and not easy to do, which means good printing ability. |
| C、 | Good weldability and low residue. |
II、 BGA process control in the production process:
1、PCB Printing:
| A、 | Angle of the blade is better at 60 degrees. Printing pressure is set at 3.5 to 10 kg 。 |
| B、 | Printing speed should be controlled at 10 mm/sec to 25mm/sec. The smaller the BGA solder ball pitch, the slower the printing speed. |
| C、 | Steel net off speed is set at 0.5mm/sec. |
| D、 | Workshop temperature should be controlled at 25°C. The humidity is less than 60% RH. And prevent the solder from being exposed to the air for too long. |
| E、 | Steel solder paste on the net should not be added in excess amount. Timely replenishment after use is needed, preferably in conjunction with steel mesh stir with a small amount of solder paste on, and then joins the steel printing online. |
| F、 | Times of steel washing is lower than 5pcs per time. (If balls pitch>1.0 mm, consider 8-10pcs/time). Steel grid is cleaned by wiping with alcohol, and every half an hour it is cleaned by toothbrush. |
| G、 | After printing PCB and before entering the Mounter, employee is appointed to check with a magnifying glass, BGA Solder print quality, and check for misprinting, little tin printing, Lian Xi or duoxi. Only when the printing is OK, PCB can enter the next process. |
2、BGA mount:
| A、 | Clear, complete visual image recognition parameter setting. The BGA preventing the solder ball from missing is directly mounted on the board. |
| B、 | Mount error value is set at lower than 30%, which prevents mounting deviation. |
| C、 | Drawing, placing speed should be slowed down, and the nozzle BGA time on the boards is 400 ms (turn off the vacuum state). |
| D、 | Mounting height is set at slightly downward pressure 0.1-0.3 mm. When BGA is placed, its solder balls can be fully contacted with the solder paste. This reduces the BGA a pin welding phenomena. But when the pitch is less than 0.65 mm, BGA should not be used, avoiding tin solder ball bridge connection. |
3、Requirements for BGA SMT reflow furnace inspection:
| A、 | After mounting BGA, it should not be moved, because doing so would tow the solder away, causing lead solder bridging. |
| B、 | When from the surface it looks like there is BGA deviation or warping of the mount PCBA, feedback should be given to the lines PE Analysis and it should be followed up. PE’s proposed remedy for defective products should be followed. Do not privately approach them with tweezers, vacuum pen or pinching them. |
| C、 | Sticker PCB Plate inspection that is OK should be placed immediately in the reflow oven. It should not be exposed in the air more than 30 minutes. |
4、Production notes of temperature control parameters of reflow oven profile:
| A、 | Making of BGA profile: Measuring point of the profile should be located in the central BGA location and that there should be two test lines. One is at BGA upper surface, and another is located in the solder ball between BGA and PCB (from PCB under the surface through BGA under the surface solder balls). Here’s a lead process for example: ① Warm-up slope < 2.5°C/sec ( not > 3°C/sec). ② 140-170°C is required to maintain for 60-120 sec. ③ Duration of more than 183°C is required to maintain for 90-110 sec. ④ Maximum temperature at the bottom is < 220°C. ⑤ Cooling slope required < 3°C/sec (maximum). ⑥ Temperature difference between the surface and bottom of BGA is approximately 5-6 degrees. |
| B、 | During the reflow soldering process if deviation from the pad is smaller than 50%, the component can automatically just itself. If there is a big deviation, there is a serious problem. Again it should be removed (check the solder) and go back to the line. |
| C、 | In this stage PCB must be evenly heated and stimulates and activates flux. Heating and cooling speed should not be too fast to prevent PCB from being heated up or cooled down too fast, which causes deformation. |
5、On the BGA after the completion of the inspection work
| A、 | X-ray tin-weld inspection. When in mass production we sample 20 PCs for every 100 PCs. We seek assurances on the bubble (also known as holes), tack weld, little tin, tin bridge, through-hole robbing series such as bad checks. |
| B、 | For problems that the X-ray examination cannot detect, such as tin, solder crack roughness, we use a new testing instruments, “ERSA SYSTEM 2500 BGA/CSP SMT Visual inspection system”. It can check the welding quality of solder balls from the side, and provides a more in-depth analysis of BGA Internal soldering results. |
