The latest complex technology in the SMT industry

When we look at the various professional SMT conferences held in various places, it is not difficult to understand the latest technologies used in electronic products.

CSP, 0201 passive components, lead-free soldering and optoelectronics are arguably the most advanced technologies that many companies have recently practiced and positively evaluated on PCBs. For example, how to handle the ultra-small openings commonly found in CSP and 0201 assembly (250um) The problem is the basic physical problem that solder paste has never had before. Board-level optoelectronic assembly, as a large area of communication and network technology, the process is very fine. Typical packaging is expensive and vulnerable, especially in After the device leads are formed, the design guidelines for these complex technologies are also very different from the normal SMT process, because the board design plays a more important role in ensuring assembly productivity and product reliability; for example, for CSP soldering interconnections In this case, reliability can be significantly improved simply by changing the size of the plate bond pad.

CSP application
One of the key technologies that is common today is CSP (Figure 1). The beauty of CSP technology lies in its many advantages, such as reduced package size, increased pin count, enhanced performance, and reworkability of the package. The advantages are: when used for board level assembly, it can cross the boundary of fine pitch (thin to 0.075mm) peripheral package and enter the larger pitch (1, 0.8, 0.75, 0.5, 0.4mm) area array structure.

Many CSP devices have been used in consumer telecom for many years, and they are widely considered to be low-cost solutions in the fields of SRAM and DRAM, medium-spin ASIC, flash memory and microprocessor. CSP can have four basic features. : rigid base, flexible base, lead frame base and wafer scale. CSP technology can replace SOIC and QFP devices and become mainstream component technology.
One problem with the CSP assembly process is that the bond pads of the solder interconnect are small. Typically, the bond pad size of a 0.5 mm pitch CSP is 0.250 to 0.275 mm. Such a small size is printed through an opening having an area ratio of 0.6 or less. Solder paste is very difficult. However, it can be successfully printed using a well-designed process. The failure usually occurs due to solder shortage caused by clogging of the template opening. Board-level reliability mainly depends on the package type, while CSP devices average. It can withstand a thermal cycle of -400 to 125 °C for 800 to 1200 times, without the need for underfilling. However, if a underfill material is used, the thermal reliability of most CSPs can be increased by 300%. CSP device failure is generally associated with solder fatigue cracking.

Advances in passive components
Another big emerging area is the 0201 passive component technology. Due to the market demand for reducing the board size, people are very concerned about the 0201 components. Since the introduction of the 0201 components in mid-1999, cellular phone manufacturers have assembled them with the CSP into the phone. The size of the plate is thus reduced by at least half. Dealing with such packages is quite cumbersome. To reduce the occurrence of post-process defects (such as bridging and erecting), pad size optimization and component spacing are key. As long as the design is reasonable, these packages can Placed close to each other, the spacing can be as small as 150?m.

In addition, the 0201 device can be placed under the BGA and the larger CSP. Figure 2 is a cross-sectional view of the 0201 under the 14mm CSP assembly with a 0.8mm pitch. Due to the small size of these small discrete components, assembly equipment manufacturers have The system planned to be updated is compatible with 0201.

Through hole assembly still has vitality
Optoelectronic packaging is widely used in the field of telecommunications and networking where high-speed data transmission is prevalent. Ordinary board-level optoelectronic devices are "butterfly-shaped" modules. Typical leads of these devices extend from the four sides of the package and expand horizontally. The assembly method and through-hole components The same, usually using a manual process - the lead is processed by a lead forming pressure tool and inserted into the plate via hole through the substrate.

The main problems in handling such devices are the lead damage that can occur during the lead forming process. Since such packages are expensive, care must be taken to avoid damage to the leads from being broken by the molding operation or by breaking the module package at the lead-to-device body connection. Ultimately, the best solution for incorporating optoelectronic components into standard SMT products is to use automated equipment so that components are removed from the disk, placed on a lead forming tool, and then the leaded device is removed from the molding machine. Put the module on the board. Given that this option requires considerable capital investment in equipment, most companies will continue to choose manual assembly processes.

Large-format printing plates (20 x 24") are also common in many manufacturing applications (Figure 3). Products such as set-top boxes and routing/switch printing plates are quite complex, including a mix of the various techniques discussed in this article, examples. For this type of board, large ceramic grid arrays (CCGA) and BGA devices up to 40 mm2 are often seen on this type of board.
The two main problems with this type of device are the large heat dissipation and heat-induced warpage effects. These components can act as large heat sinks, causing non-uniform heating under the package surface, due to furnace thermal control and heating curve control. A solder joint that causes no wetting near the center of the device. The warpage of the device and the plate caused by heat during processing can cause "non-wetting" such as separation of the component from the solder paste applied to the plate. Care must be taken when mapping the heating curves of these plates to ensure uniform heating of the BGA/CCGA surface and the entire plate surface.

Plate warpage factor
In order to avoid excessive bending of the printing plate, it is very important to properly support the printing plate in the reflow furnace. The plate warping is an important factor to be observed in the circuit assembly, and should be strictly described in the reflow cycle. Thermally induced BGA or substrate warpage can cause solder voids and leave a large amount of residual stress on the solder joint, causing early failure. This type of warpage can be easily described using a moiré fringe projection image system, which can be online or Offline operation, used to describe the pre-package and plate warpage. The offline system can also simulate the reflow environment by using the time/temperature coordinate based warp pattern set for the device and the plate in the furnace. .

Lead-free soldering
Lead-free soldering is another new technology that many companies have begun to adopt. This technology began in the European Union and Japanese industries, initially to remove lead from solder during PCB assembly. The date of implementation of this technology has been The change, originally proposed in 2004, was recently implemented in 2006. However, many companies are now seeking to own the technology in 2004, and some companies now offer lead-free products.
There are many lead-free solder alloys on the market today, and the most common alloy composition in the US and Europe is 95.6Sn ∕ 3.7Ag ∕ 0.7Cu. These solder alloys are not much different from the standard Sn/Pb solders. The printing and placement processes are the same, the main difference is the reflow process, that is, for most lead-free solders, higher liquidus temperatures must be used. Sn∕Ag∕Cu alloys generally require peak temperature ratios Sn/ Pb solder is about 30 ° C high. In addition, preliminary studies have shown that the reflow process window is much more stringent than standard Sn/Pb alloys.
For small passive components, reducing the surface energy also reduces the number of upright and bridging defects, especially for 0402 and 0201 size packages. In short, the reliability of lead-free assembly is completely comparable to Sn/Pb. Solder, except in high temperature environments, such as operating temperatures in automotive applications that may exceed 150 °C.

Flip film
When integrating current state-of-the-art technology into standard SMT components, the technology encounters the greatest difficulties. In the first-level package component applications, flip-chips are widely used in BGA and CSP, although BGA and CSP have adopted lead-frame technology. In board-level assembly, flip-chips offer many advantages, including reduced component size, improved performance, and reduced cost.
Unfortunately, the use of flip chip technology requires manufacturers to increase their investment to upgrade their machines and add specialized equipment for the flip chip process. These devices include placement systems that meet the high precision requirements of flip chip and Underfill dispensing system. Also includes X-ray and panning systems for post-reflow soldering inspection and post-filling cavity analysis. Pad design, including shape, size and mask definition, for manufacturability And testability (DFM/T) and meeting cost requirements are critical.
On-board flip chip (FCOB) is mainly used in products that are critical to miniaturization, such as Bluetooth module components or medical device applications. Figure 4 shows a Bluetooth module printer with the same features as the 0201 passive components. The package integrates flip-chip technology. The same high-speed placement and processing of flip-chip and 0201 devices is also possible to place solder balls around the perimeter of the package.