Current Issue An Independent Journal Dedicated to the Advancement of Chip - Scale Electronics October 2001 Email the editor

By Shean Dalton, Speedline ACCEL, Plano, Texas

Figure 1. Reflow oven conveyor speed and product density govern throughput.

Figure 2. This unit, with its small footprint, exemplifies the new breed of space-saving reflow ovens.

With the proliferation of design conversions from standard leadframe technology to BGA, CSP and system-in-package (SiP), manufacturing engineers are setting up new infrastructures and re-tooling existing ones for advanced packaging capacity expansion.

Critical measures for efficient capacity expansion are optimized floor space utilization, high production yield and high product quality. While the recent business downturn has idled the rate of capacity expansion, manufacturing engineers now have the opportunity to analyze how production tools affect these critical factors, and thus gain an edge on efficiency.

In selecting a reflow oven for soldering bumps onto area array packages or components onto a SIP substrate, an analysis of throughput, thermal uniformity and cooling are fundamental considerations. Each item directly contributes to the total efficiency of a manufacturing capacity expansion investment.

Throughput

Throughput is typically measured in cycle time or UPH. Either measure is dependent on reflow oven conveyor speed and product density. Historically, speed has been governed by the "Big L"-oven length. Higher throughput meant increased oven length and more oven zones.

Common rectangular packaging formats allow reorienting products along their short width instead of their long length. Using the "short width" as the length of the part, higher throughput is achieved with increased product density. (See Figure 1.)

Another approach is to run a multi-lane, staggered array across the oven width. Either way, reorienting products increases product density and allows for slower conveyor speeds, decreased length and fewer zones. The bottom line is higher utilization of existing factory floor space (Figures 2 and 3).

Thermal Uniformity

Reflow profile specifications typically represent a ramp-up rate, a time above liquidus, a ramp-up rate from liquidus to peak temperature, a peak temperature, a time at peak temperature and a ramp down rate. As product passes through the oven creating profiles, temperature deviations (Delta T's) occur across the product's width, length or even from one product to the next. Tolerances are provided within reflow specifications to control temperature deviations for high production yield.

The product's composition largely contributes to temperature deltas. For example, a plastic area array package strip may appear thermally uniform. However, the pad located at the thin corner may experience a different profile from the pad located at the thicker center of the device.

A cross-section at the thin corner pad includes solder, copper and BT substrate with thermal conductivity for each material of 418, 50 and 0.19 W/m-°K respectively. The thicker center pad's cross-section also includes die attach adhesive, silicon die and mold compound with a thermal conductivity for each material of 1.38, 83, 0.67 W/m-°K respectively.

Temperature Deltas

An oven's configuration can also contribute to temperature deltas. Neglected thermal losses at the conveyor belt edges and through the oven's sidewalls are common sources for temperature variations across the product's width.

Figure 3. Smaller factory footprints can also be achieved by integrated systems. This configuration features sphere placement and reflow.

Ripples occurring along profile curves may be attributed to uneven impinging gas temperature or speed. In some cases, ripples may be attributed to oven components that absorb or emit thermal energy at specific localities or throughout the process length. Dips in profiles can be attributed to space between heating zones, or uneven (or blocked) forced convection.

As production starts with the product first entering the oven, the machine comes under loading, which continues with subsequent products until the last product exits the oven. The first, the last and all products in-between must be subjected to the same profile and tolerances.

Oven designs must achieve a robustness to compensate for light, medium and heavy loading conditions. Robustness is achieved through an effective closed-loop feedback circuit coupled with heater power and agility to compensate for loading changes.

Cooling

As product begins a ramp down from peak temperature, an oven design should accommodate the physical changes molten solder will experience as it solidifies.

Oven designs that benefit from these physical changes cause the formation of a uniform, blemish-free bump with a fine internal grain structure. Oven designs that don't produce these physical changes will result in a dimpled or slanted bump with a weaker, coarse grain structure (Figure 4).

Figure 4. This graphic illustrates the difference between a uniform and non-uniform bump.

In the worst cases, these bumps lack cosmetic appeal, result in problems with package co-planarity and exhibit poor shear strength characteristics.

With a greater understanding of a reflow oven's machine design related to throughput, thermal uniformity and final bump characteristics, manufacturing engineers will gain an edge on improving manufacturing capacity expansion efficiency.

This edge can lead the way to pushing out future capacity expansion investments, a reduction in cost per I/O, and a gain in product quality.