Quality Control of Ceramic PGA Microprocessor Packages

The Challenge

Ceramic pin grid arrays (PGAs) package high-power microprocessor chips in hermetic environments. These multilayer co-fired structures with internal metal planes and vias experience thermal expansion mismatches during fabrication and subsequent processing. Combined with ceramic’s brittle nature, these stresses can cause package cracking that propagates to loss of hermeticity. Digital Equipment Corporation needed to detect flaws in incoming packages, during assembly, and critically, on packages with live die where destructive testing is impossible.

The Solution

GrindoSonic resonant frequency testing proved fastest, least expensive, and best suited for high-volume manufacturing. Frequencies were repeatable to ~1 Hz for lowest modes. Finite element modeling guided test setup for each vibration mode. Testing 750 unassembled packages confirmed that manufacturing dimensional tolerances are the largest source of frequency variability. A key insight: frequency shifts from cracks follow different patterns across vibration modes than shifts from dimensional variations. Measuring multiple modes can differentiate the two.

Key takeaway: Measuring four vibration modes instead of one distinguishes crack-induced frequency shifts from dimensional tolerance variations, improving detection sensitivity by an order of magnitude.

Results

Packages tested before and after mechanical loading showed frequency drops correlating with crack length measured by C-Scan. The smallest detectable cracks were approximately 0.2” (the C-Scan limit). For single-frequency testing, cracks must exceed ~0.5” through full thickness to distinguish from tolerance variations in 2” packages. However, measuring frequencies of four vibration modes enables detection of much smaller cracks by distinguishing crack-induced shifts from dimensional shifts.

Testing packages with live chips during production confirmed that observed flaws were not created by electrical testing, a determination impossible with any other NDT method.