Using Open-Cavity Plastic Packages in Avionics Applications

Figure 1. Qualification test plan for 64QFP OCPP

February 1, 2022

Semiconductor makers invest significant resources in the development and manufacture of their devices, including packaging and assembly techniques that accommodate their material, mechanical, electrical, reliability, and footprint specifications. They must meet very tight time-to-market windows as cost-effectively as possible.

Fast-turn prototype package assembly is frequently employed for rapid IC design verification. Open-cavity plastic packages (OCPP) are an attractive quick turn assembly option when commercial off-the-shelf (COTS) prototype packages are not available. QP Technologies can convert any existing IC plastic packages, whether dummies, electrical test rejects or excess inventory, into open-cavity packages, ready to be assembled with new die. By converting a range of popular package types, sizes and pin counts, new packages can be created that will fit an established footprint. This allows them to be easily inserted into test sockets and boards, so that no retooling or redesign is required.

OCPP is also viable for production package assembly. It becomes an attractive option when there are no onshore sources for the particular plastic package assembly, and lead time and ITAR ^* constraints prohibit offshore assembly. When avionics ASIC provider Device Engineering, Inc. (DEI) ran into this problem, QP Technologies’ OCPP approach was an effective solution for DEI’s crucial production schedule, reliability requirements, and ITAR supply constraints.

OCPP vs. OmPP

QP Technologies’ two key types of package offerings are OCPP and its patented Open-molded Plastic Package (OmPP™) technology. What’s the difference?

OCPP is analogous to a quality “pre-owned car.” Using OCPP allows us to reclaim existing “dummy” packages, e.g., electrical rejects, test packages, or excess inventory. After removing all plastic and existing die, down to the copper, a new package is then built within the OCPP shell. OCPP is well suited for cost-sensitive projects or those that only require small batches.
OmPP is the “new car.” Using these packages, we start with the bare lead-frame and copper, perform die attach, then wire bond and overmold the die, and then we singulate and trim the device. Tested to RF frequencies of 40+GHz, OmPP is a proven, cost-effective alternative to industry-standard ceramic for most applications.

Another key application for OCPP is to employ it as an interim solution, optimizing the package and working out any kinks, before transitioning to OmPP.

DEI’s Challenge

DEI turned to OCPP technology for production assembly of a 64-lead TQFP [thin quad flat pack] 10 × 10 mm ePad product when its onshore assembly provider ceased doing business. DEI sourced suitable dummy (no die) IC packages from an offshore supplier, and QP Technologies converted them to OCPP, developing the assembly process and materials to achieve the required package performance.

The IC was characterized for JEDEC moisture sensitivity level (MSL), as determined by J-STD-020, and qualified via environmental stress-based accelerated reliability tests. The IC was qualified in the end product and has since entered production.

QP Technologies’ Methodology

To find the right process that would meet DEI’s specifications, QP Technologies experimented to evaluate various types of OCPP. First, 64-lead TQFP packages were opened and sent out to plating to protect the copper with a nickel-gold (Ni+Au) alloy. Nickel acts as a diffusion barrier, with gold protecting the circuit from elements.

Various die-attach epoxies were then evaluated. These included Ablebond 84-1lmi and H70E, both electrically conductive epoxies with silver particles. In both cases, epoxy bleed-out later created delamination between encapsulation and surface of the package, with parts failing solder stress tests as detected by CSAM [scanning acoustic microscopy] imaging.

Next, another design-of-experiment sets (DOEs) was started, working with unplated OCPP packages. Again, various epoxies were evaluated, and Ablebond QMI529HT was determined the winner. This newly formulated epoxy is designed specifically for copper, features minimal bleed-out, and doesn’t affect the adhesion of encapsulation to the copper, as indicated by CSAM. The epoxy is also compliant with the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment directive (RoHS). [Although originated in the EU, California has its own RoHS laws governing electronic devices sold in the state, to which QP Technologies adheres.]

QP Technologies then pursued multiple iterations in order to hit on the ideal recipe for building packages that would pass all-important moisture sensitivity level (MSL) testing. To ensure repeatability of the process, a traveler was created to document the recipe, including the exact steps and order in which they must be performed – epoxy type, how the package is opened, wire-bonding process, encapsulation, material, etc.

First, a lot of 50 packages was developed, on which DEI performed pre-compliance (confidence) reliability testing. After passing this testing, several hundred packages were assembled as a qualification lot. These were production screened by DEI, including temperature cycling, burn-in and electrical tests. Finally, the leads and ePads were hot solder dipped to remove the native matte tin plating. Several of the screened parts were submitted for qualification testing. Figure 1 illustrates the qualification tests and flow.

Results

The OCPP packages passed testing with results equivalent to factory-fresh parts. The tables in Figure 2 provide further insight into the testing process and results. As noted above, the parts were subjected to a range of tests and test conditions – not only did they pass, but none were rejected following any of the tests, illustrating the robustness of the OCPP solution.

The completed OCPP-based packages that QP Technologies developed for DEI have proved highly reliable, and DEI continues to use them to ensure continuity of performance for its customers.

Although QP Technologies is now developing overmolded parts, i.e., OmPPs, for DEI from scratch (a long-lead-time activity), the high-performing OCPPs continue to be produced as well. OCPPs serve as a reliable insurance policy so that DEI and its customers are covered.

Conclusion

While customers increasingly choose OmPP, creating packaged devices from scratch involves many steps, which can be time- and cost-intensive. Therefore, for customers that need only a small quantity or, like DEI, need to recreate an existing package design in larger quantities using a standard package type and size, using OCPP with reclaimed packages is a cost-effective, quick-turn option. To put it into other words, OCPP is where technology, innovation and creativity intersect.

This article was written by Sam Sadri, Senior Packaging Engineer, QP Technologies (Escondido, CA), and Tom Tammen, VP of Engineering, Device Engineering Inc. (DEI) (Tempe, AZ). For more information, visit here .

Reference

International Traffic in Arms Regulations, the set of U.S. government regulations with which all manufacturers of defense- and military-related products and services must comply in order to safeguard national security.