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A Comparative Study of Ball Grid Array and Ultra Fine-pitch Qfp Technologies Using Solder Paste Stencil Printing
| Content Provider | Semantic Scholar |
|---|---|
| Author | Rörgren, Roger Carlsson, Per Liu, Johan |
| Copyright Year | 1999 |
| Abstract | The present miniaturization trend toward higherperformance, smaller and lighter products has resulted in an increasing demand for smaller component packages and/or higher pin counts. This paper addresses some important properties of solder pastes for assembling BGA and ultra fine-pitch QFP components. Viscosity measurements and analyses using Differential Scanning Calorimetry are presented, and give some insight into the complex rheology and thermal behavior of solder pastes. Gold plated test boards were assembled using nitrogen reflow of 1.5 mm pitch BGAs and QFPs with a pitch range from 0.3 mm (11.8 mil) to 0.65 mm (25.6 mil), and the yield in terms of solder defects and solder balls was studied. A fractional factorial experiment to study the solder balling phenomenon was carried out using IR reflow in air and pretinned boards. INTRODUCTION The Quad Flat Pack (QFP) and the Ball Grid Array (BGA) packages today both offer a large number of I/Os, as required by modern IC technology. The BGA concept has received much appreciation owing to its inherent, potential benefits to surface mount production. In order to accommodate the increasing number of I/Os needed, the peripheral QFP technology is forced to an ever finer lead pitch with thinner and more fragile leads. The BGA, taking advantage of the area under the package for the solder sphere interconnections, satisfies the I/O demand using a far coarser pitch. Additionally, the package size and the board real estate required are usually smaller. The relationship between BGA and QFP packages' size and I/O count is illustrated in Figure 1. A typical 0.65 mm (25.6 mil) fine-pitch QFP with 160 leads measures 28x28 mm. Modern portable electronics asking for the same number of leads in a package 14x14 mm ends up at 0.3 mm (11.8 mil) pitch with a space between the leads of only 0.15 mm (6 mils). Alternatively, increasing the number of I/Os while retaining the 0.65 mm pitch, means e.g. 232 leads in a 40x40 mm body. A 27x27 mm plastic BGA (PBGA) houses 225 I/Os with a coarse 1.5 mm pitch. The distance between adjacent solder spheres is approximately 0.8 mm. The more I/Os needed, the better off with the BGA in terms of package size since the dimensions only grow as the square root of the I/O count for a given pitch, and not linearly as is the case for QFPs. Replacing QFPs with BGAs not only means that higher pin counts or smaller packages can be achieved, but also that a considerably higher manufacturing process yield can be reached. Today, the manufacturing aspects seem to be the major driving forces for the BGA technology, although issues like cost, reliability, and rework and inspection will probably soon push the technology further. Even though a choice between the BGA and QFP technologies seems easy from a production point of view, as illustrated in Figure 2, the alternatives still have to be considered and pertinent issues should be scrutinized. Purpose The purpose of this work is to evaluate the suitability of Figure 1.Size comparison of square QFP and BGA packages of different types and with different I/Os. Figure 2.A 0.3 mm (11.8 mil) QFP placed on a grid of 1.5 mm pitch solder spheres (bottom side of PBGA225) existing manufacturing processes for production using BGAs and/or ultra fine-pitch QFPs. Since a key to success is believed to be the solder paste printing process, much effort has been devoted to different solder pastes and their properties. Test boards were fabricated using stencil printing on flash gold boards and assembly of QFP packages with component lead pitch ranging from 0.3 mm to 0.65 mm. Optimization of the processes for e.g. 0.3 mm pitch QFP or BGA has not been included, but rather the comparison of existing processes for the two technologies. Particular emphasis was put on the print quality and on the process yield in terms of shorts and solder balls after reflow. A factorial experiment was designed and carried out using IR reflow in air of pretinned boards to find the factors influencing the solder balling rate under the BGA package and its consequences to the technology. The solder pastes included in the tests are described in more detail at the end of the paper. SOLDER PASTE CHARACTERIZATION The properties of the solder paste is of great importance for the final result of the surface mount assembly, especially when component lead pitches below 0.65 mm (25.6 mil) are addressed. A solder paste may typically contain up to 20 ingredients providing separate functions, e.g. a binding agent, a fluxing agent, rheology controllers and modifiers, which unfortunately most often are interrelated with respect to performance. A typical paste contains 85 to 95 % (by weight) of solder particles and 5 to 15 % of fluxing and other agents or additives and solvents making up the final product. The paste viscosity is different for different application methods: stencil printing requires about 1 000 Pas (=1 000 000 cps), screen printing 800 Pas and dispensing approximately 500 Pas according to Brookfield measurements For fine-pitch SMT assembly, it is also important that the size of the solder particles is sufficiently small and the that its distribution is tightly controlled. The IPC type 3 is usually recommended for fine-pitch applications. Type 3 implies a particle size with 80 % (by weight) of the sample ranging from 20 to 45 μm (mesh -325+500), see Table I. For ultra fine-pitch, which involves stencil apertures on the order of 0.15 mm (6 mils), even 3 or 4 solder particles in a row are sufficient to effectively block the aperture and cause a meagre print. There are modern pastes available containing very fine powder-like solder particles less than 20 μm in size. It should be noted, however, that such pastes are more likely to oxidize due to their larger surface area. What is gained in printability may thus be lost in inferior wetting and an unacceptable soldering performance. The viscosity is perhaps the single most important parameter relating to the rheology and the printing performance of a paste. For instance, from a production point of view, the paste should be easily printed in a standard stencil printing process through very small apertures while still not sagging. This is sometimes described in a more elaborate way in terms of shear thinning, thixotropy, and yield point. Furthermore, the paste must not dry out on the stencil, which also would block the apertures, but retain its tackiness in order to keep the components in place in the wet paste between placement and reflow. During reflow the boards are first heated to evaporate solvents and to activate the solder paste flux system. The flux then removes oxides on the solder particles and the metal surfaces to be soldered while preventing their re-oxidation. Pastes are generally affected by changes in ambient temperature and humidity and the viscosity also changes with time as the material is being used in a normal stencil printing operation. Viscosity is commonly measured using dedicated equipment such as the Brookfield or the Malcom viscometers. However, different operating principles of different equipment as well as different measurement parameters often make it impossible to compare the values. Under these conditions, values from single-point measurements, i.e. taken at one shear rate only, are not of any use. In this study, a Brookfield DV-II viscometer with a reversible helipath stand was used with a T-spindle (TF) at different shear rates to register the pastes' dynamical viscosity, η, at 5 RPM and to calculate a Thixotropy Index, T.I.=Log[η (1 RPM)/η (10 RPM)]. To investigate the pseudo-plastic and thixotropic behavior of the pastes, a circular spindle (no.7) was employed using shear rates between 0.5 and 10 RPM. A rheology profile was recorded by shearing for 15 s at each shear rate prior to measuring and without allowing the material to recover between the measurements. The viscosity was also measured under constant shear stress conditions for 5 minutes at each shear rate used. Using Differential Scanning Calorimetry (DSC), it is possible to study the thermal properties of a solder paste, simulating a soldering process. During DSC measurements, phase transformations and chemical reactions causing heat generation (exothermic reaction) or Table I.Size classification of solder paste ( IPC-SP-819). |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://extra.ivf.se/ngl/documents/ChapterE/smi95ivf.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
