A robust development for QFP Cu wire necking prevention in automotive grade 0 temperature cycle reliability

Cu wire is increasing in usage in semiconductors due to continuous package cost reduction. Raising bar for reliability requirements, especially customers in automotive industry, has posted numerous challenges for Cu wire in meeting stringent quality requirements. This study is triggered by customer to development Grade 0 temperature cycles (TC) reliability with lower cost. Current package is running with Au wire Grade 1 TC reliability. In order to have better profit margin, internal decision was targeted on bare Cu wire to run with Grade 0 TC reliability at the initial stage. The focus of this paper is development of Cu wire for QFP robust wire necking prevention in Grade 0 temperature cycling (TC) reliability. Wire necking is one the major reliability concern in Grade 0 TC. The first failure of Grade 0 TC is observed with massive open failure after TC500X Grade 0 stress test. Further FA confirmed that wire is rapture due to wire necking. At the same time, the FA expert zooming into the each comer to quantify & classify the failure mode. The failure is localized at north/west of the wire bonded area this area is where the leadframe is not symmetry in design. Other area showed minor or no crack line no broken wire observed. In order to meet the Grade 0 TC, the investigation is streaming into 3 directions: First, process driven weaknesses for bare Cu wire. Second, comprehensive simulation was done in order to foster the development understanding and lastly, Cu wire material understanding. In process driven weaknesses, after series of wirebond & moulding process provocation, only two key indicators showed influence: vibration control during wirebond with different clamper design & symmetry leadframe design influence to stress distribution. Wirebond clamper with reduced vibration on leadfinger (spring loaded design) had significantly improved the zero hour on first bond with no micro line or surface dislocation. Despite also improve the second bond wedge peeling significantly. However, after TS 1000X, crack line is observed again on pin 86 & 92 (which is the bad corner). While in symmetry leadframe design, minor crack line is observed after TS 1000X. However, the bad comer effect was deleted as all location observed certain degree of minor crack line. At the same time, a comprehensive simulation with seven parameters & fifteen details items to be considered for further improvement. The simulation had also completed with priority on the key factors as die thickness, mould compound CTE & mould compound glass transition temperature (Tg). Thus, selected mould compound Tg & CTE did not showed positive contribution after TS1000X. The conclusion is then laid down to bare Cu wire intrinsic weaknesses lead to early failure. In term of Cu wire material understanding, based on in-house stress strength curve comparison between two type of bare Cu wire & 2 Type of Pd coated Cu wire, Pd coated wire exhibited some advantages over bare Cu wire: 1) Pd coated wire exhibit more superior in term of uniform elastic deformation as compared to bare Cu wire, mean advantages on higher degree of flexibility before it gone to uniform plastic deformation 2) Pd coated wire exhibit more superior in term of uniform plastic deformation as compared to bare Cu wire, means advantages for Pd coated wire to withstand higher stress load before proceed to necking condition 3) Under stress condition, wire necking is extremely faster for bare Cu wire as compared to Pd coated Cu wire, this also indicated that why Pd coated Cu wire is able to withstand stringent TC grade 0 reliability as compared to bare Cu wire. In extreme case, rupture of bare Cu wire is seen under longer cycles of TC grade 0 stress After combination of the 3 approaches, Pd coated Cu wire & symmetry leadframe design are parallel run in 2 projects. Due to current asymmetrical design was the qualified product, no change in leadframe design was done. Only change of Cu wire to Pd coated Cu wire. Based on the result, Pd Coated wire showed drastic improvement as compared to bare Cu wire to withstand TC grade 0 until 2000 cycles (vs requirements is 500 cycles). In addition, with symmetry leadframe design couple with Pd coated Cu wire also having positive result for TC grade 0 until 1000 cycles (no further result as project is still on-going) In this study, there was a good combination on FA, simulation; process & material expert effort in shorten the development cycle. The main conclusion from this paper is: 1. Symmetry leadframe design ~ proper design of leadframe to have even stress distribution will definitely help to improve package reliability. This is evidence when symmetry leadframe design deleted the comer wire rapture effect with less severe crack line simultaneously in all corners 2. Vibration reduction during wirebond ~ spring loaded clamper reduce the vibration during wire bonding for unsymmetry leadframe design. Spring loaded clamper served as secondary effects in improving stress mark at neck during wirebonding 3. Pd coated wire with more superior uniform elastic deformation, uniform plastic deformation and higher resistance to rapture under stress condition as compare to bare Cu wire could be the main contributor to sustain the Grade 0 TC.