In-situ stress state measurements during chip-on-board assembly

Abstract

In this work, die stresses in wire bonded chipon-board (COB) packages have been measured using special (111) silicon stress test chips. The test die incorporate an array of optimized eight-element dual polarity piezoresistive sensor rosettes, which are uniquely capable of evaluating the complete stress state (six stress components) at points on the surface of the die. Sensor resistance measurements were recorded before packaging, after die attachment, and throughout the encapsulant cure process. Using the appropriate theoretical equations, the stresses at sites on the die surface have been calculated from the raw sensor resistance data. Also, three-dimensional (3-D) nonlinear finite element simulations of the chip-on-board packages were performed, and the stress predictions were correlated with the experimental test chip data.

Application of piezoresistive stress sensors in ultra thin device handling and characterization

“…The average˘n 11 +˘n 12 is found to be −1.98 × 10 −4 MPa −1 and the average˘n 44 is found to be −1.06 × 10 −4 MPa −1 . The values match the previous work reported [7] as we doped the resistors with the same doping dose as the work of [7]. The doping level is 1.0 × 10 17 cm −3 .…”

“…The doping level is 1.0 × 10 17 cm −3 . From previous reports [7], the˘p 11 +˘p 12 is found to be 5.5 × 10 −5 MPa −1 and the˘p 44 is found to be 13.8 × 10 −4 MPa −1 . The˘n 11 +˘n 12 is found to be −3.9 × 10 −4 MPa −1 and the˘n 44 is found to be −1.2 × 10 −4 MPa −1 .…”

“…(1), L is the loading span, d is the supporting span, h is the width of the sensor strip, and t is the thickness of the sensor strip. This formula works well if the deflection due to the load (F) is not significant and the t and h are small as compared to the d and L [7]. Thus in the present work, L, d, h, and t were chosen to be of 50 mm, 20 mm, 10 mm, and 0.73 mm, respectively.…”

Microelectronic Bonding Processes Monitored by Integrated Sensors

“…The experimental results were used to calibrate a finite element (FE) model, which in turn can be used to predict the creep behavior for different device geometries, materials, and process variables. More recently, in situ stresses were measured before and after die attach and during curing of the liquid encapsulant of a chip on board (COB) process [27]. It was shown that die attach stresses were small compared with the encapsulation stresses.…”

“…Test chips with integrated microsensors for plastic package analysis were reported in considerable detail [11,25]. Such microsensors measure thermal stress [26,27], corrosion [28][29][30][31], and moisture [25,31]. Various test chips are commercially avail-able [38][39][40].…”