Step 7 - Solder bumping
08/01/2000
Solder bumping is a crucial step in the assembly of high-density packages because it provides an interconnecting medium. A successful solder-bumping operation requires that close attention be paid to the electroless-nickel and solder-paste-printing processes.
By Tennyson A. Nguty and Ndy N. Ekere
The explosive growth in the use of high-density packaging, such as chip scale packages (CSPs) and flip-chip technologies, is having a massive impact on the electronics assembly and manufacturing industry. This can be attributed to the cost, density, assembly yield and performance advantages that high-density packages offer when compared with conventional surface mount devices, such as quad flat packs (QFPs).
 Figure 1. Process route for solder bumping of wafers. |
These packaging formats are becoming more complex. Increasingly, bumped die are directly assembled, bypassing the packaging area. As a result, companies that used to manufacture packaging equipment are now getting involved in the solder bumping of wafers. These changes are driven by performance (measured by the number of I/Os and operating speeds) and package size (which is application-specific). In the assembly of high-density packages, solder bumping is an important step because it provides an interconnecting medium. Following is a discussion of the solder-bumping process of wafers through solder-paste printing. The process route involves the steps illustrated in Figure 1, and includes both electroless-nickel and solder-paste-printing processes.
Wafer Design and Description
The layout of a typical die is shown in Figure 2. In this case, the individual die measures 6 x 6 mm. In addition to the daisy-chain pads, resistor and heater structures are also incorporated into the design. The Al bond pads are octagonal in shape, with a distance across the flats of 90 µm. The openings in the passivation over the pads is circular in shape and 75 µm in diameter (Figure 2b).
Electroless-nickel Process
To apply a solderable metallization layer to the Al bond pads of the die for solder interconnection, an electroless nickel-plating process is applied that involves a number of cleaning and activation steps (Figure 3). The process is applied on a wafer level with the wafers supported in a plastic carrier designed to fit inside the plating tanks. The carrier can hold two wafers and, in most instances, two wafers are processed simultaneously. For special treatments, individual wafers are bumped.
 Figure 2. Layout of die, (a) overall view of single chip, (b) schematic diagram of single daisy chain and (c) SEM of a single Al bond pad showing the passivation opening.1 |
Pre-treatments: Before beginning the electroless nickel-bumping process, wafers are briefly rinsed with deionized water to remove any loose debris from the surface. Then an initial etching treatment is applied to the Al bond pads by immersing the wafer into a solution of sodium hydroxide (NaOH). This step is carried out with gentle agitation of the wafer within the solution. Following every chemical treatment step, the wafers are immediately rinsed by first pouring deionized water over the surfaces to remove excess chemicals, then immersing them in a tank containing deionized water. The samples are removed and immediately taken to the next stage without drying.
 Figure 3. Electroless-nickel process steps.1 |
After the sodium-hydroxide treatment, the wafers are immersed in a bath of 50-percent nitric acid for less than 60 seconds with gentle manual agitation. The first zincate activation treatment of the wafers is carried out for 20 seconds and involves gentle agitation of the wafers in a zincate bath. If the wafers are to have a single zincate pre-treatment, they are rinsed and immediately passed to the electroless nickel bath for bumping. For double zincate pre-treatments, a number of further steps are applied (Figure 3).
 Figure 4. SEM of electroless Ni bumps (top view and cross section).1 |
After activation of the Al bond pads and final rinsing, the wafers are immediately immersed in the electroless nickel bath. The plating rate of this bath is calibrated and the plating time is monitored to determine the thickness of Ni deposited. The solution is agitated throughout the plating period by slow stirring with a magnetic stirrer bar, and the bath temperature and pH are maintained at 85°C and 4.6, respectively. The growth rate of Ni is isotropic, growing at the same rate in the x and y direction, producing the mushroom-like structure shown in Figure 4.
 Figure 5. Schematic of the printing process. |
In addition to the basic bath composition, a stabilizer also is added to reduce the extent of unwanted plating onto clear areas. This is added in low concentrations (less than 1 mgl-1) by adding small quantities of thiourea (NH2CSNH2) dissolved in deionized water. The stabilizer not only prevents extra plating but also generates smoother deposits on the bumps. Previous experience has shown that the addition of too much stabilizer can result in poor quality deposits that are badly shaped, so the amount of stabilizer in the bath should be restricted. Because monitoring of this small concentration in a large volume of other chemicals is not possible, the stabilizer concentration is adjusted by observing the quality of the deposits already produced and making small additions of the stabilizer solution to keep the bath at its optimum performance. This requires the initial plating of test pieces during bath setup before wafer bumping begins.
 Figure 6, above. Solder paste printed on wafers. |
After the electroless nickel bumping of the wafers is complete, the wafers are thoroughly rinsed with deionized water and then dried with a hot-air blower. After inspection, they are stored in a wafer carrier until required for solder-paste printing.
Solder Bumping
The solder-bumping process requires printing solder paste onto the electroless nickel bond pads of the wafer (Figure 5). A stencil is designed with apertures to match the nickel bumps. The amount of solder paste required to yield a specified solder bump dimension can be estimated and used during stencil design.2 The printing process is conducted using a stencil printer. A wafer carrier holds the wafers in place by vacuum during printing and also protects them from any damage.
 Figure 7. Typical reflow profile of oven. |
Solder-paste printing is the process by which paste is deposited onto the wafers ready for reflow. The printing machine has two squeegees: one for the forward stroke and the other for the reverse stroke. During printing, solder paste rolls in front of the squeegee, filling the apertures in the stencil some distance ahead of the squeegee. The squeegee then shears off the paste in the apertures as it moves over the stencil. Hydrodynamic pressure generated by the squeegee in the paste roll injects paste into the apertures. Once the print stroke is complete, the wafer carrier (holding the wafer) is separated mechanically from the stencil.
 Figure 8, left. Solder bumped wafer.1 |
To obtain optimum yield in printing, the process parameters, including squeegee speed and pressure, are altered. After printing, the wafer is inspected (Figure 6), and reflowed in a nitrogen atmosphere. A typical temperature profile for the reflow oven is shown in Figure 7. During reflow, the volatile elements in solder paste evaporate, leaving behind molten solder that assumes a spherical shape on solidification (Figure 8) because of its surface tension. The residue around the solder bumps can be removed using a suitable solvent, such as water in the case of a water-soluble flux. At this point, the wafer is ready for dicing to produce single chips that can be used for flip-chip assemblies.
AP
Acknowledgements:
The authors acknowledge the support of industrial partner Multicore Solders Ltd. UK, DEK Printing Machines, Celestica UK, Matra BAe, Intarsia, and the Engineering and Physical Sciences Research Council (EPSRC) UK who are funding this work under Grant No. GR/L66113. Special acknowledgement goes to Dr. David Hutt (Loughborough University, UK) for the electroless nickel process, and Mike Hendriksen (Celestica, UK).
References:
- T. A. Nguty, D. A. Hutt and M. Hendriksen, "Process Route Documentation for Flip-chip Assembly," Project Review Report, 1999.
- T. A. Nguty and N. N. Ekere, "Modelling a Low-cost Wafer Bumping Technique for Flip Chip Application," International Journal of Microcircuits and Electronic Packaging, Vol. 22, No. 4, 1999, pp. 327-333.
TENNYSON A. NGUTY, Ph.D., senior design engineer, can be contacted at Bookham Technology, 90 Milton Park, Abingdon, Oxfordshire OX14 4RY, UK; Fax: +44-123-582-7201; E-mail: [email protected]. NDY N. EKERE, Ph.D., professor and director of research, can be contacted at the School of Aeronautical, Mechanical and Manufacturing Engineering, University of Salford, Salford, Manchester M5 4WT, UK; Fax: +44-161-295-5575; E-mail: [email protected].
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