A Keyence VR-3200 3D measurement system was used to analyze waffle packs with COOP, measuring the flatness of the tray and lid materials. Figure 2 shows example surface flatness measurements of carbon loaded polypropylene and carbon loaded polycarbonate lids. The carbon loaded polypropylene lid has a flatness specification of 0.012 in., and the average of the samples measured was 0.0079 in. The carbon loaded polycarbonate lid has a tighter flatness specification of 0.004 in., and the average of the samples measured was 0.0024 in. The carbon loaded polypropylene tray and lid combination have flatness tolerances wide enough for 0.004 in. thick die to escape their pockets. Combining the 0.012 in. tray flatness and the 0.012 in. lid flatness specifications yields a worst-case tolerance of 0.024 in., twice the amount for die to migrate.

Figure 3

Figure 3 Measured internal lid surface flatness of the traditional waffle pack lid-clip system, showing 9.4 mil deflection between points A and B.

Figure 4

Figure 4 Misaligned, pinched polyethylene inserts in a waffle pack (a) create openings for COOP (b).

Figure 5

Figure 5 Waffle pack lid separating from the tray during normal handling (a), leading to COOP (b).

Additional analysis showed that the traditional waffle pack lid-clip system had an internal lid contour measurement of 0.0094 in., which is more than 2x the height of typical GaAs and GaN die (see Figure 3). These lid-clip combinations were tested with loose inserts to see if loose or misaligned inserts can increase COOP; the data shows the misalignment of loose inserts often results in die pinching and COOP (see Figure 4). The investigation showed that manipulation of a loaded waffle pack by the customer risks COOP, as expected. During clip attachment and removal, uneven pressure applied by an operator can cause the waffle pack to warp, enlarging the gaps between the pockets and lid (see Figure 5). Because traditional waffle packs are only secured by clips along two sides, any tilting during clip application creates pressure gradients. Combining the shock and vibration from handling with these pressure gradients can lead to COOP.

The study revealed whether packing at the supplier or unpacking by the customer, the risk of thin die migrating is high.

NEW PACKAGING SYSTEM

To prevent COOP, a bare die packaging system needs to encapsulate the die from when the supplier packs the waffle pack, through shipping and unpacking by the customer. As the traditional waffle pack system was designed in a previous era, when devices were much thicker, a simple fix or insert is unlikely to solve the tolerance, quality and structural deficiencies for the current die. To protect the increasingly expensive die they contain, a new packing system is necessary, one compatible with automated manufacturing. The key requirements of a reimagined lid-clip system are

  • Captivate the die fully during packing, shipping and unpacking
  • Compensate for waffle pack tray and lid flatness variation within their specified limits
  • Use an integrated industry interleaf with no additional modifications, materials or steps
  • Provide enhanced electrostatic discharge (ESD) protection, using low outgassing and static dissipative silicone-free materials meeting all relevant industry standards
  • Compress the lid uniformly around the full perimeter of the package.
Figure 6

Figure 6 Structure of the Gel-Pak LCS2 with gasketed tray pockets.

Figure 7

Figure 7 LCS2 components.

These goals led to the development of the LCS2, engineered to pair with standard, pocketed trays, to resolve the mechanical issues caused by traditional waffle packs. Gel-Pak developed the LCS2 to captivate each die in a pocket and prevent migration (see Figure 6). The LCS2 is fabricated using low outgassing, static dissipative and low density polyurethane foam, integrated with an industry-approved interleaf material and assembled into a static dissipative injection molded lid with a silicone-free, pressure sensitive adhesive. A new clip design provides uniform compression around the entire lid perimeter, ensuring a tight interface between the entire interleaf and a warped waffle tray surface (see Figures 7 and 8). The static dissipative materials were selected and tested to ANSI/ESD S11.11 and deliver ESD Class 000 protection, which is suited to high value devices with the lowest voltage susceptibility.

Figure 8

Figure 8 LCS2 lid on the waffle pack tray without the new clip compressing the foam gasket (a) and with the clip compressing the foam (b). Simulation courtesy of Forgione Engineering.

TESTING THE LCS2

Figure 9

Figure 9 X-ray images following 10 drops from 34 in. height: LCS2 (a) and new lid with the old industry standard clip (b), the latter leading to COOP and damaged die.

Testing the LCS2 has demonstrated its efficacy to protect thin semiconductor die. Drop test experiments using several lid-clip combinations and 0.002 in. thick GaN die were conducted, using a 34 in. height to represent catastrophic handling. X-rays verified the security of the die and whether COOP occurred. 100 drop tests were performed on 10 of the lid/clip products. The results showed no COOP (see Figure 9a). When the same test was performed using the historic industry clip instead of the new clip, COOP was observed (see Figure 9b). This test establishes the effectiveness of the new clip design.

Gel-Pak’s LCS2 Super System has demonstrated the capability to virtually eliminate COOP for all sized components, especially ultra-thin die less than 10 mils. The LCS2 reduces non-value-added supplier corrective action requests and return material authorizations. Though the new packaging system must be field tested, the evidence points to LCS2 eliminating millions of dollars in yield loss and improving supply-chain quality and efficiency.

References

  1. M. Marks, Z. Hassan and K.Y. Cheong, “Ultrathin Wafer Pre-Assembly and Assembly Process Technologies: A Review,” Critical Reviews in Solid State and Materials Sciences, 2015, pp. 1–40.
  2. P. Wadhwani and S. Yadav, “Thin Wafer Market Size,” Report ID: GMI5007, Global Market Insight, March 2021.
  3. Thinning Equipment Technology and Market Trends for Semiconductor Devices: Market and Technology Report, Yole Développement, 2020.