Mold Compound Optimization and Certification for Cu Wire Leaded Package

Teng Seng Kiong, Serene Teh Seoh Hian, Navas Khan Freescale Semiconductor Malaysia Sdn. Bhd.

2, Jln. SS8/2, Free Industrial Zone Sungei Way, 47300 P.J. Selangor, Malaysia sengkiong.teng@freescale.com

603-78734283

Abstract Semiconductor industry is moving rapidly from gold (Au)

wire to cooper (Cu) wire for several packages mainly due to gold price increase and remains volatile in recent years. The package encapsulation material (mold compound) needs to be studied to understand the interaction of it with Cu wire bond for package reliability and not to introduce new failure mechanism. Various studies shows that the mold compound pH and Chloride (Cl-) content are the primary factors affecting Cu wire package reliability performance [1, 2]. The purpose of this study is to certify our existing green mold compound and to have single mold compound for Au & Cu parts. In this work, we investigated the impact of mold compound properties for Cu wire reliability performance particularly Highly Accelerated Stress Test (HAST) 130 degree C/ 85% humidity and Temperature Cycle (TC) -65 degree C to 150 degree C. The test vehicle chosen for the evaluation is 80 lead LQFP 14 x 14mm package with 4N bare Cu wire and wire diameter of 25um. Two material properties of mold compound which are pH and Cl- level are the focus in this study. We evaluated compound lots with different pH and Cl- for our study and submitted for reliability test. Effects of mold compound pH and Cl- content on the reliability failure rate and how we define the specification range of pH and Cl- level for Cu wire package will be discussed. From result, we also understand Cu wire bond IMC coverage and device bias voltage are also major factor for Cu wire package reliability performance.

1. Introduction Mold compound properties are known to affect Cu wire

bond package reliability. Current mold compound running high volume manufacturing for our study is not Cu wire compatible as it has high amount of ionic content particularly Cl- and wide range of pH. It is estimated only 40% of current mold compound batch able to meet the desired pH and Cl- range recommended for Cu wire package. In order to optimize current mold compound for Cu wire packages with the targeted pH and Cl- range from above study, collaboration with compound supplier is needed. The approach is using high purity resin, increase of Ion Catcher (IC) content as well as pH adjuster to meet our requirements. Metal Hydroxide (MH) used as pH adjuster in this study in compound formulation. The study compared two re-formulated compounds with higher pH and lower Cl- level, and down selected one mold compound for Cu wire production based on good manufacturability and passed reliability requirement of Moisture Sensitivity Level (MSL) 3, TC and HAST tests. The mold process responses such as void, delamination, flag tilt etc are measured and found comparable to our base line mold

compound used for similar devices using Au wire. The results are very encouraging and the modified mold compound is meeting all the requirements for the Cu wire. One of the challenges we faced was higher viscosity of re-formulated compound causing significant wire sweep, and it required process parameter optimization. The full process characterization and performance comparison is discussed. The package warpage and lead co-planarity with the modified mold compound have been measured and found acceptable for the customer board mounting.

The Cu wire reliability is a major concern for most of the OEMs and this work demonstrates an epoxy mold compound with right combination of pH & Cl- meeting the stringent reliability requirements of the automotive environment. The material has been thoroughly characterized using DOE and high volume manufacturing requirements have been verified by multiple lots of validation. The reliability of the Cu wires in LQFP & QFP were tested and shows comparable performances with Au is demonstrated.

2. Experiments A. Re-formulated Mold Compound In this study, two options were evaluated, which is not

major change on current compound formulation yet able maintain current compound grade. The basic mold compound properties comparison shown in Table I.

Table I: Epoxy Mold Compound (EMC) Properties

EMC A is the current mold compound using in high

volume manufacturing for gold wire package, which is the control in this study. EMC B and C are reformulated mold compound for the Cu wire package certification. EMC B reformulated using high purity resin and increase Ion Catcher content from current 0.03% to 0.5%, whereby EMC C reformulated using the same approach as EMC B but with addition of 0.5% Metal Hydroxide in order to achieve higher level of pH.

B. Moldablity, Reliability and Compound Robustness

542978-1-4673-4552-1/12/$31.00 c©2012 IEEE

Both reformulated mold compound winitial mold-ability test such as wire moisture related failure etc. The parts wreliability test up to Moisture Sensitivity260°C. Final selection based on mold abiperformance. EMC A was included as conEMC C is able to yield higher pH level whiCu wire package reliability.

Selected reformulated mold compound hfull mold compound qualification using compound batches from supplier. Mold abwire sweep, void, delamination, packagstudied. Apart from this, units from 3 evsubmitted for reliability stresses which incluTemperature Cycle (TC) 500 cycle (-65°Highly Accelerated Stress Test (HAST) 85%RH with electrical test to assess the Cperformance.

Furthermore, to ensure reformulated mocomparable mold-ability robustness acompound, several studies were carried outsupplier had carried out adhesion test and mtest to compare both existing and reformulaaddition, continuous molding evaluation wa500 shots for both compounds to assess anDesign of Experiment (DOE) was used to parameter in order to provide optimum settuse. Full mold cleaning, waxing and dummprior to the mold-ability evaluation. Moldfor instance: wire sweep, void, flag tilt andchecked in this experiment.

As for mold compound pH and Cl- spewire package, this study compared reliabilitand electrical test data with several mold cCl- value. Device biased voltage and wire bCompound (IMC) coverage were assessed aon Cu wire package reliability. A mold comspecification is proposed based on above package. The mold compound specificatipackage have been finalized based on supplier capability matching the proposed sfrom this study.

3. Result and Discussion The results from experiments are

following sections. A. Mold ability and Reliability Test

Compound Initial molding test with both re

compound options as well as current mobeen carried out using existing molding ability result summarized in Table II.

Table II: Mold ability Result at Time Zero (T

were evaluated for sweep, adhesion,

were submitted for y Level (MSL) 3 lity and reliability ntrol in the study. ich is favorable for

had then gone thru 3 different mold

bility test such as ge warpage were aluation lots were ude MSL3 260 °C, °C ~ 150°C) and 96 hours 130°C/

Cu wire reliability

old compound has as current mold t. Mold compound

mold release-ability ated compound. In as performed up to ny stickiness issue. optimize the mold ting for production

my shots were done d-ability responses, d resin bleed were

ecifications for Cu ty HAST 96 hours compound pH and bond Inter Metallic as well for its affect mpound pH and Cl-

data for Cu wire ions for Cu wire

mold compound specification value

presented in the

for Reformulated

eformulated mold old compound had

parameter. Mold

T0) and Post MSL

Both EMC B and C showedA. However, wire sweep level compounds compared to EMCbehavior could be explained dEMC B and C. Reformulated mCatcher content; which is insHydroxide in formulation cviscosity, however the contentsmall amount in the formulatshowed historical lots data for Efrom 80 – 194 poise based on lthat viscosity for EMC B (162within the viscosity range of EMC C was selected basedreliability result and higher pH

Figure 1: Wire Sweep Resuand Existing Compound

In order to certify EMC C fof EMC C were requested fromqualification build, mold compoIII. Mold compound viscosity Figure 2, which is well specification. All 3 batches sholevel are below 15ppm.

Table III: Mold Compound Pro

d comparable result with EMC is higher for both reformulated

C A as show in Figure 1. This due to higher viscosity for the mold compound has higher Ion soluble and addition of Metal can potentially yield higher t of IC and MH are relatively tion. Mold compound supplier EMC A having viscosity ranges ast two years data, which mean 2 poise) and C (163 poise) are EMC A. From above results,

d on good mold ability and level.

ult for Reformulated Compound

for Cu wire package, 3 batches m mold compound supplier for ound properties shown in Table and Spiral Flow data shown in

within current compound ows pH level close to 7 and Cl-

perties of 3 Batches EMC C

2012 IEEE 14th Electronics Packaging Technology Conference 543

Figure 2: Viscosity of EMC A & C arSpecification

Mold ability data from the qualification

C shown in Table IV. As a control, EMCwell. All the 3 EMC C lots showing good reEMC A. Table IV: Mold ability Result for Qualificati

In order to improve wire sweep level bec

viscosity of EMC C, wire bond loop height average 8 mils to 7 mils, wire sweep result3.. All lots passed the quality requiremdelamination on flag and minor void, C-SMSL3 260°C shown in Figure 4. In adcomparable to current performance cviscosity for EMC C. Other mold responbleed, war page and marking legibility wfound comparable to existing.

Figure 3: Wire Sweep for EMC A (control)

re Within Current

n build using EMC C A is included as esult comparable to

ion of EMC C

cause of the higher was reduced from

ts shown in Figure ments with minor SAM result at post ddition, pad tilt is

oncerning higher nses such as resin were assessed and

and EMC C

Figure 4: C-SAM Result post Scan and Thru Scan)

Figure 5: Package War page forMaximum War page Value of 1

Reliability electrical test resthe lots passed without electric96 hours. Those reject at T0 With this, EMC C passed Cu w

Table V: Electrical Test Result

B. Compound Robustness Mold compound adhesion a

factors impacting manufacture test and shearing release abilityresults of EMC A and C are shBoth adhesion and release abicomparable to EMC A, thiscontinuous molding up to 500compound supplier and in actresponse such as stain on incomplete filling were checkehas comparable or better perfoEMC A.

MSL3 260°C of EMC C (Top

r EMC A & C using The moiré, 16 micron

sult is presented in Table V. All al failure particularly in HAST testing are functional rejects.

wire package reliability.

for Qualification of EMC C

and release ability are important ability. Button shear adhesion

y test have been performed and hown in Figure 6 and Figure 7. ility test of EMC C are found s was further evaluated with 0 shots performed by the mold tual manufacturing line. Mold package, mold sticking and

ed and confirmed that EMC C ormance compared with current

544 2012 IEEE 14th Electronics Packaging Technology Conference

Figure 6: Adhesion Test for EMC A and C

Figure 7: Shearing Release ability Test for E

Design of Experiment (DOE) was perfoto ensure an optimum molding parameter volume manufacturing. Several mold parevaluated and mold responses are shown result is encouraging; however there is nothat impacting wire sweep and the currentcan be applied for EMC C.

Figure 8: DOE Result of EMC C and Graph

EMC A and C

ormed for EMC C window for high

rameters had been in Figure 8. The

o significant factor t molding window

ical Analysis

Figure 9: Cu Wire Bond BReliability Result

Figure 9 shows the pH ver

the reliability data for Cu wrepresents pass and red dot repelectrically. While the boxesproposed pH and Cl- window For this mold compound, thecapabilities and finally fixedspecifications, which is pH fromof 20ppm (green box).

Conclusions Mold compound properti

contents are proven to affect Csemiconductor manufacturers,compound suppliers [3]. It icompound to have a specific pHas compound formulation is uniand resin type found to be domcompound pH and Cl- level, cCu wire is the future trend formold compound introduction tbe thoroughly characterized tolong term Cu wire package relsuccessfully certified a moldchanges in compound formulatiimplemented in high volume m

Acknowledgments The authors would like tpersons below for their help aduration of this work: Ms. SGuarjardo for valuable recommThe authors also acknowledgeacompound supplier. We woulstaffs in particular from the Pafor the support and developmoptimization and certificationmanufacturing.

References 1. Hidetoshi Seki, Louie Huan

Evaluation of Effects of MoMaterial Properties on RelComponents”, Conference oTechnology Conference, 20

Biased HAST 130°C/85%RH

sus Cl- graph plotted based on wire packages. The green dot presents failing HAST 96 hours s in different colors are the

for Cu wire mold compound. e supplier had assessed their

d the compound pH and Cl- m 6.0 to 7.2 with maximum Cl-

ies particularly pH and Cl- Cu wire package reliability by research centers and mold is suggested that each mold H and Cl- for Cu wire packages ique for each grade. Ion catcher minant factor in affecting mold coupled with pH adjuster.. As r semiconductor industry, new to manufacturing line needs to o ensure its compatibility and liability. In this study, we have d compound with appropriate ion for the Cu wire devices and

manufacturing.

to express great gratitude to and support through the entire Sheila Chopin and Mr. James

mendations and technical advice. a good collaboration from mold ld like to thank the Freescale ackaging Solutions Department ment in the mold compound n for Cu wire high volume

ng, Andy Tseng, Peng Su “An olding Compound and Substrate liability of Copper Wire BGA on Electronic Components and 12, page 1110-1116

2012 IEEE 14th Electronics Packaging Technology Conference 545

2. Hidenori Abe, “Cu Wire and Pd-Cu Wire Package Reliability and Molding Compound”, Conference on Electronic Components and Technology Conference, 2012, page 1117-1123

3. Seki, H. et al., “Study of EMC for Cu bonding wire application”, 2010 IEEE CPMT Symposium Japan, Aug. 2010, Tokyo, Japan, pp. 1-3

546 2012 IEEE 14th Electronics Packaging Technology Conference