Chip-on-Board
Understanding Chip-on-Board
In conventional RF module assembly, MMIC die are first packaged in QFN, LCC, or BGA carriers, then soldered to a mother PCB. The package provides mechanical protection and standardized footprints but introduces parasitic elements: lead frame inductance (0.5 to 1.5 nH), bond pad capacitance (0.1 to 0.3 pF), and internal resonances. At frequencies below 10 GHz, these parasitics can be absorbed into matching networks with modest penalty. Above 20 GHz, however, the parasitic reactances become comparable to the characteristic impedance, making impedance matching impractical and degrading noise figure, gain, and output power.
COB assembly solves this by eliminating the package entirely. The bare die is attached directly to the module substrate using silver-loaded epoxy (for thermal/electrical ground), eutectic AuSn solder (280 degrees C reflow, for hermetic military modules), or non-conductive film (for flip-chip). Wire bonds of 25 micrometer gold wire connect die pads to substrate traces over distances of 250 to 500 micrometers, with inductance of approximately 1 nH/mm. For frequencies above 40 GHz, ribbon bonds (25 micrometer by 100 micrometer cross-section) reduce inductance by 30 to 40% compared to round wire. Flip-chip mounting inverts the die onto solder bumps, achieving the lowest possible parasitic with connections of 25 to 50 micrometers height and inductance below 50 pH. The finished COB module is protected by glob-top epoxy (commercial) or a hermetically sealed lid (military per MIL-PRF-38534).
Bond Wire Parasitic Analysis
L ≈ 1 nH/mm · l [nH, l in mm]
Reactance at Frequency f:
XL = 2πfL [Ω]
Return Loss from Series Inductance:
RL = -20 · log10(XL / (2Z0 + XL)) [dB, approx.]
Where l = wire length (mm), f = frequency (Hz), Z0 = system impedance (50 Ω). Example: 0.5 mm bond at 28 GHz gives L = 0.5 nH, XL = 88 Ω, RL = -10 dB. Flip-chip bumps (L < 0.05 nH) give XL = 8.8 Ω and RL = -21 dB at the same frequency.
Interconnect Technology Comparison
| Interconnect | Inductance | Max Frequency | Rework | Protection |
|---|---|---|---|---|
| QFN Package | 0.5 to 1.5 nH | ~15 GHz | Easy (solder) | Molded |
| COB Wire Bond | 0.1 to 0.3 nH | ~40 GHz | Moderate | Glob-top/lid |
| COB Ribbon Bond | 0.07 to 0.2 nH | ~60 GHz | Difficult | Glob-top/lid |
| COB Flip-Chip | < 0.05 nH | 110+ GHz | Very difficult | Underfill/lid |
Frequently Asked Questions
Why is COB preferred over packaged ICs for mmWave modules?
IC packages introduce 0.5 to 1.5 nH of lead inductance, which at 40 GHz represents 125 to 375 ohms of reactance, severely degrading matching and gain. COB wire bonds (250 to 500 μm) reduce inductance to 0.1 to 0.3 nH, and flip-chip drops it below 0.05 nH. This makes COB essential above Ka-band where MMIC amplifiers and mixers cannot tolerate package parasitics. The trade-off is that COB needs glob-top or hermetic encapsulation for environmental protection.
What is the difference between wire-bond and flip-chip COB?
Wire-bond COB mounts die face-up and uses 25 μm gold wire (0.15 to 0.3 nH per bond). Flip-chip inverts the die onto solder bumps (50 to 100 μm diameter, below 0.05 nH), providing superior thermal dissipation through the backside and eliminating wire resonances above 60 GHz. Flip-chip requires underfill for reliability but achieves controlled-impedance transitions that wire bonds cannot match at W-band and beyond.
How does wire bond length affect RF performance?
Bond wire inductance is approximately 1 nH/mm. At 28 GHz, a 0.5 mm wire presents 88 ohms of reactance, causing -10 dB return loss in a 50-ohm system. Compensation includes bond profile shaping, shunt capacitor pads, ribbon bonding (30 to 40% less inductance), and parallel wires. Above 60 GHz, wire bonding becomes impractical and flip-chip or embedded wafer-level packaging is required.