| | | Looking for solder paste specification limits which have reliability data to support them. Currently, we are looking to define the min/max volume necessary to ensure a good joint on CBGA, PBGA, and Micro-BGA's. Any information will be greatly appreciated.. | | | | | Mike: Tessera.com talks about a lot of that stuff. Check it out. TTYL Dave F | | | The info that you saw Dave is it in the technical papers? I whent through the site and could not find anything on paste volumes but you did so.....is it to much to ask from a stranger and tell me which section you found it | ML: Yes it was in the techical papers, for instance (pasted from a paper):
Solder Stencil Development for CSP
Unlike the narrow rectangular land pattern geometry typical for attaching the leaded QFP while the contact sites for BGA devices are circular or square. Land pattern geometry recommended for chip-scale array device attachment is 0.25 to 0.30 mm (.010" to .012"). The stencil opening can be equal in size to the diameter of the land pattern or square and slightly larger as detailed in Figure 13, to meet specific requirements. To achieve a more robust solder connection, process engineers may specify an expanded stencil opening as shown in Figure 14 to furnish a higher solder paste volume at each attachment site as compared in Figure 15.
Defining Stencil Aperture and Geometry-
Changing the shape of the openings from round to square has proved beneficial for CSP applications.
Although the fine-pitch CSP land pattern is generally a circular shape matching the ball contact size, stencil openings that have a square geometry furnish better solder printing quality. The square stencil pattern can serve two purposes. It can increase the solder volume slightly and when adapting the smaller pitch CSP and, the square pattern promotes a more uniform release of the solder paste from the stencil surface.
Another technique that has improved paste transfer on the smaller land geometry�s is the tapered land pattern opening. The tapered wall is formed by chemical etching or with a laser. During stencil fabrication, the opening that will be closest to the board surface is made one or two mils wider than the opening at the top surface as illustrated in Figure 16.
The square stencil pattern serves two purposes:
1. It can increase the solder volume slightly when adapting the smaller pitch CSP 2. The square trapezoidal pattern releases paste from the stencil surface more uniformly.
CSP Device Placement
Placement of the chip-scale or chip-size device should be accurate within 0.1 mm (.004"). Even though perfect placement is desirable, the array device, because of the surface tension created when the solder is in a liquidus form, will self align during the reflow solder process.
Solder Process Description for CSP Program A:
188 I/O mBGA w/ .020" pitch Printer Type: Fuji w/ vision alignment Solder Paste/Supplier: Kester R244 Particle Size: Type 4 Flux Type: Low Solids RMA (no clean) Viscosity: 900-1000 kcps (cartridge) Stencil Thickness: .006" stainless Aperture Size: .012" sq. (chem etched) Aperture Shape: Square w/ .006" R corners Trapezoidal: .001" taper Reflow Type: Convection Oven Supplier: ABW Max. Surface Temp: 210�- 220�C Duration Above Liquid: 60 sec. max.
During process development, the finished condition of the solder connections under the array device may be of some concern. Product liability and reliability are key issues that must be confirmed using industry recognized methods.
Reflow Solder Processing
Forced air/gas convection soldering has proven advantages for reflow solder processing of the Chip-scale or chip-sized BGA package. The profile shown in Figure 17 represents temperature levels targeted for a multiple-zone forced (hot air/gas) convection process.
The peak reflow temperature and dwell time at liquidus is a critical factor. When using the eutectic 63Sn/37Pb solder alloy for example, the paste material will convert to a liquid at 183� C however, the temperature must continue to rise an additional 30-50� C to ensure that wetting takes place between device and mounting structure.
Please note: The dwell time above the liquidus point, may extend to 60 seconds to insure that all the flux materials separate from the alloy mass.
Solder Process Description for CSP Program B:
46 I/O mBGA w/ 0.75 mm pitch Printer Type: MPM Solder Print Inspection Cyberoptics in-line LSM Solder Paste/Supplier: Kester R244 Particle Size: Type 3 Flux Type: Low Solids RMA (no clean) Viscosity: 900-1000 kcps Stencil Thickness: .006" stainless Aperture Size: .014" (laser cut) Aperture Shape: Round Trapezoidal: .001" taper Reflow Type: Convection Oven Supplier: Vitronics Max. Surface Temp: 210�- 220�C Duration Above Liquid: 60 sec. max.
Actual measurement and profile or shape of the solder joint can be made with destructive as well as nondestructive techniques. The destructive process requires a cross-section typical of that furnished in Figure 18 that allows the inpection of the device through the solder joint. Other nondestructive methods for solder inspection may be more practical after a product is in production. Although useful during product development, x-ray inspection of critical segments of the assembly have proved to be very efficient for monitoring the processes.
What has proved to be most beneficial for solder process quality and uniformity control is inspection of the solder paste printing before attaching the CSP device. Unlike gull wing lead devices, solder rework is not practical and removal of the device is typically the only way to correct serious solder defects. Inspection of the solder printing should include measurement of both thickness and coverage. The final proof of process may require physical stress testing. Depending on the products use category, various methods of temperature cycling can be utilized to prove that the product can meet required operating conditions and life expectancy.
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Looking for solder paste specification limits which have r...
- Jul 27, 1999
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