Download

PDF Abstract Text:

LFBGA APPLICATION NOTE ATO INNOVATION, PHILIPS SEMICONDUCTORS MARCH 2000

BGA Application Note

APPLICATION INFORMATION AN01026
(LF)BGA APPLICATION NOTE ATO INNOVATION, PHILIPS SEMICONDUCTORS MARCH 2000
Philips Semiconductors
BGA Application Note
Philips Semiconductors
BGA Application Note
Philips Semiconductors offers several (LF)BGA outline versions with different pin count, pitch and body dimensions. An overview of our current range is show in our Data Handbook IC26: Integrated Circuit Packages. This application note contains all aspects for processing the (LF)BGA, including footprint design, stencil printing, automatic placement, reflow soldering process and rework. 3 REFLOW SOLDERING PROCESS
The most important items of (LF)BGA technology are shown below. Printed circuit board · conductor lines: 100 µm, spacing: 100 µm · solder resist resolution: 100 µm · placement accuracy: - 75 µm for BGA - 62.5 µm for LFBGA · multilayer
The reflow soldering process comprises three steps: 1. Stencil printing 2. Component placement 3. Reflow soldering.
Philips Semiconductors
BGA Application Note
3.1 Stencil printing
squeegee solder paste stencil solder land
For (LF)BGA assembly, stencil printing is the most critical process step. This is because that, after soldering, the joints cannot be inspected as is the case with other surface mounted devices.If one of the balls is not properly soldered, it can only be detected using electrical testing, X-ray or destructive analysis. Stencil printing can be divided into three sub-processes: · Filling of apertures · Levelling · Stencil release. These three steps are shown in Fig.1. 3.1.1 FILLING OF APERTURES
board
filling
The filling of the apertures is determined by a complex interaction between the material properties of the stencil squeegee, solder paste and the machine settings, such as pressure and speed. To ensure the apertures are properly filled, the solder paste must "roll" on the stencil in front of the squeegee. By rolling the paste, the highest pressure arises at the point where the squeegee is in contact with the stencil. This is the place where the solder paste can flow into the stencil apertures. 3.1.2 LEVELLING
levelling
The elevating force on the squeegee determines whether the squeegee correctly levels the paste in the stencil openings. This force is determined by factors such as the amount of paste, the squeegee angle and stiffness, the print speed, and the applied pressure on the squeegee. The last two, which are machine parameters, can be controlled by the operator until a satisfactory result is obtained. The process window can be increased in two ways: by increasing the pressure on the squeegee and / or by using a stiff squeegee. For example, increasing the pressure above the level that is simply required to clean the stencil ensures sufficient paste is forced through the stencil aperture onto the solder land, which in turn increases the process window. Likewise, using a stiff squeegee increases the process window. However, a careful balance has to be maintained because if the squeegee is too stiff, it would not be able to follow the shallow contours on the board. Refer to Section 5 for more information on the process window.
release
MSB905
Fig.1 Applying solder paste by stencilling
Philips Semiconductors
BGA Application Note
3.1.3 STENCIL RELEASE 3.2 Component placement
When the apertures are filled with solder paste, the stencil and board are separated. The way of separating determines the smallest printable opening. Though experimentation, we have found that stencil printing in combination with a slow, controlled release speed gives the best results, compared with printing with zero snap-off. The highest resolution is obtained as the separation speed is slow and with small "jerks". This ensures that any excess paste is removed from the apertures caused by the resulting vibrating motion. 3.1.4 PRINT RESULTS
The key factor in component placement is the size of the component. For example, small passive components are usually placed with a chip shooter, whereas larger components, such as ICs, are placed with an IC placer. (LF)BGAs are considered large components, irrespective of their actual size. The major process deliverable of a placing machine are its speed and placement accuracy. The latter of which is determined by the vision alignment system. In addition, placement force is also important it ensure optimum contact. Too high a force and the component may crack damage the solder land to low a force and the component will have poor contact with the solder paste.
The circular stencil apertures are relevant. Figure 2 shows the typical appearance of solder paste deposits as the diameter of the stencil openings increase. Also, owing to process vibrations, the deposition may look different even when using only one stencil opening.
handbook, full pagewidth
Stencil thickness 0 1 2 3 4 5 6
MGS886
Fig.2 Different deposition appearances with increasing stencil opening diameter.
Philips Semiconductors
BGA Application Note
During soldering, metal pads are joined by molten solder that flows between their adjacent surfaces, which have a higher melting point than the solder itself. The parts to be joined, and the solder paste are heated so that the flux can remove the oxides. After this, the solder is then brought above its melting point. As the solder melts, it flows around the ball contacts of the BGA and forms a meniscus. After solidification, the final joint is formed. To achieve a suitable soldered joint, all parts of the board must be subject to an accurate temperature / time profile. Figure .3 shows a suitable profile framework.
handbook, full pagewidth
temperature Tp max Tp min TR TE tE tM tR
PCB damage
organic finish affection
MGS889
Fig.3 Reflow soldering process requirements for boards and paste.
Philips Semiconductors
BGA Application Note
dbook, full pagewidth
temperature Tp max Tp min TR TE tM tR
MGS890
Fig.4 Process window for a specific component.
handbook, full pagewidth temperature
PCB damage
Tp max
organic finish affection
Tp min TR TE tE tR
MGS891
Fig.5 Example of a narrowed process window because of component restrictions.
Philips Semiconductors
BGA Application Note
4 FOOTPRINT DESIGN 4.2 Component placement
An important step in the design of a printed circuit board is the choice of footprints. A well-chosen footprint is the basis for a reliable solder joint. 4.1 Print board dimensions and pattern positions
The (LF)BGA package owes much of its popularity to the fact the component is self aligning, i.e. during reflow, an (LF)BGA that has not been properly placed, will float back to its optimal position on the solder lands thanks to surface tension forces (see Fig.6). The maximum displacement for e.g. a BGA256 is 400 µm and for a LFBGA64 is 125 µm. Furthermore, as the (LF)BGA does not have leads that can be bent, the placement force can have a large tolerance of roughly between 3 N and 10 N for BGAs, and between 1.25 N and 10 N for LFBGAs. This force is dependent on the board support and the construction of the placement force-control unit.
handbook, full pagewidth
reflow
MGS888
Fig.6 (LF)BGAs are correctly positioned during reflow soldering.
Philips Semiconductors
BGA Application Note
4.3 Reliability 4.4 Routing
handbook, full pagewidth
MGS887
Fig.7 Different solder joint layouts (a) solder resist defined (b) copper etched defined.
Philips Semiconductors
BGA Application Note
5 5.1 PROCESS WINDOW Stencil printing 6 REWORK
For SMD assembly on printed circuit boards, one of the requirements after it has been placed on the solder paste is that all the leads (or terminations in the case of leadless components) must be in contact with the solder. This is because if a lead is not in contact with the paste, then the wetting of the lead by the liquid solder is not always good. In practice, this means that the solder pasted deposits should have the same height, i.e. their height should be equal to the stencil thickness (see Fig.2). The fundamental difference between an (LF)BGA and normal SMD is that the leads of (LF)BGAs consist entirely of eutectic solder, whereas the leads of the standard SMDs, in most cases, only had a eutectic or tin-plating of a few tenths of microns on the leads (or terminations). Also, the tolerance on coplanarity for an (LF)BGA package is larger than for a QFP with a comparable amount of input and outputs. The maximum coplanarity tolerance for BGAs is 150 µm, and for QFPs is 80 µm. At a non-coplanarity of 150 µm, some termination balls of the BGA will not touch the solder paste, even if a 200 µm thick stencil is used. This is because the ball will be pressed into the paste to a depth of only 80 µm. 5.2 Reflow soldering
Although (LF)BGA assembly yields are very high, there may still be a requirement for component rework. By rework, we mean the process of removing the component from the pcb and replacing it with a new component. If an (LF)BGA is removed from a pcb, the solder balls of the component are deformed drastically so the removed (LF)BGA has to be discarded. 6.1 Device removal
As is the case with any component, it is essential when removing an (LF)BGA that the board, tracks, solder lands or surrounding components are not damaged. To remove an (LF)BGA, the board must be uniformly heated to a temperature close to the reflow soldering temperature. A uniform temperature reduces the chance of warping the pcb. To do this we recommend that the board is heated until it is certain that all the joints are molten. Then carefully pull the component off the board with a vacuum nozzle. 6.2 Site separation
The peak temperature for reflow soldering should remain below 230 °C (typically the peak temperature is between 200 and 205 °C) and the dwell time above 183 °C should not exceed 70 seconds, with a preference for temperatures at the higher ends to permit good wetting and ball shear. If any moisture is present in the plastic package during soldering, it may turn into steam and expand rapidly. Under certain circumstances, the force exerted by this expansion can cause internal delamination or, in the more severe cases, may result in internal or external crack (known as the popcorn effect). To avoid this problem, components should not be removed from their drypack longer than is specified on the box label. The reflow soldering profile is shown in Fig.3. Although the minimum peak temperature is defined as 205 °C, to enlarge the process window and to be less sensitive to oven temperatures, in practice the minimum peak temperature of 215 °C is often defined.
When the component has been removed, the vacant I site must then be cleaned before replacing the (LF)BGA. Removing an IC often leaves varying amounts of solder on the mounting lands. This excessive solder can be removed with either a solder sucker or solder wick. The remaining flux can be removed with a brush and cleaning agent. It is recommended that both sides of the board are cleaned to ensure maximum success. After the board is properly cleaned and inspected, apply flux on the solder land and on the connection balls of the (LF)BGA. Do not apply solder paste as this has shown to result in problems during re-soldering. 6.3 Device replacement
The last step in the repair process is to solder the new component on the board. Ideally, the (LF)BGA should be aligned under a microscope or magnifying glass. If this is not possible, try to align the (LF)BGA with any board markers. To reflow the solder, apply a temperature profile that is as close as possible to the profile shown in Fig.4. So as not to damage neighbouring components, it may be necessary to reduce some temperatures and times.
Philips Semiconductors
BGA Application Note
7 (LF)BGA FOOTPRINTS
handbook, full pagewidth
see detail
ball pitch
ball diameter
MSD412
Fig.8 Generic footprint diagram of an (LF)BGA.
Ball pitch 0.50 mm, ball diameter 0.32 mm
Philips Semiconductors
BGA Application Note
7.2 Ball pitch 0.80 mm, ball diameter 0.46 mm / 0.40 mm
Philips Semiconductors
BGA Application Note
7.4 Ball pitch 1.27 mm, ball diameter 0.75 mm