Package Warpage Challenges for LQFP 144 Lead CMOS 90 Device

and it's Impact to Lead Coplanarity

Teng Seng Kiong, Ibrahim Ruzaini, Kesvakumar, Foong Chee Seng Freescale Semiconductor Malaysia Sdn Bhd

2, Jln. SS8/2, Free Industrial Zone Sungei Way, 47300 Petaling Jaya, Selangor.

603-78731133

Abstract

Package warpage or generally referred to as plastic package deformation has become more challenging with larger and thinner package body sizes. The detrimental effects of the package warpage are more prominent in peripheral packages with leads such as fine pitch LQFP (Low Profile Quad Flat Package). As all the leads are embedded in the plastic body, the warpage will displace the leads in the same direction of the warp after the trim and form step. In the first half of the paper, we characterized the warpage of different LQFP packages designs using Akrometrix Thermoire. It was found that larger die size and smaller down set are favorable to reducing package warpage. In the second half of this paper, we explored various factors such as molding process parameters, post mold cure conditions, effects of direct materials properties, and package geometry to determine their impact on the package warpage and lead coplanarity for the LQFP molded package. Molding parameters such as cure time and molding temperature were evaluated in a series of DOEs in attempts to find the most promising factors that can control the warpage. Similar DOEs were also carried out in the post mold cure process coupled with various methods of clamping. As for the direct materials, studies focusing on molding compound properties such as coefficient of thermal expansion (CTE), higher filler content, higher glass transition temperature Tg, and lower mold shrinkage factor have been completed as well. From these series of analyses and experiments, we understood how the leads formed with spring back effect in a warped package will end up with poor coplanarity measures. It is also found that the combination of several material properties that are not optimal will further deteriorate warpage and cause poor lead coplanarity. These findings and results will be shared in this paper and recommendations to correct the issue are presented.

1. Introduction

Molding process of semiconductor manufacturing has become more challenging as new package designs come with more lead count, larger and thinner body size. When the package increases in size, maintaining package lead coplanarity becomes more stringent as failure to do so causes yield loss in downstream assembly processes. Package warpage is known to cause lead coplanarity. Lead coplanarity is critical for the package during customers SMT process. It is also important during manufacturing to ensure device testability.

The degree of package warpage depends on the package design, materials, and to a lesser degree, assembly processes. As such, several methods can be employed to reduce package warpage. An overview of factors affecting package warpage illustrated in fish-bone diagram given Figure 1. This paper discusses several evaluations on improving package warpage for an LQFP package.

Machine

AfterMoldCooling� /' PMCClamping

Mold Degree of Cure � /'/Temperature (Mold;ng& PMC) i? Mold /"" /'

Releasibility Mold Chase & Cavity Mold Ejector P in Action & Location

LeadframeType & "-Thickness ----.. " EMC aE MoldCompound �

Mold Machine Variation

ModulusofDie� ,,""'- EMCShinkage Thickness of Upper Die Size

Attach & leadframe " Bond line Thickness� '" EMC Filler Content

DjACure " EMCTg & Modulus Temperat� "EMCWaterAbsorption

& lower Halves of '\. the package "'" Flag Size & Type

Figure 1: Fish-bone diagram shows the causes for plastic package warpage

Lead coplanarity is measured with respect to a seating plane formed by the three lowest leads of the package during lead scanning. In this study we have relied on an ICOS lead scanning machine. These three leads have a coplanarity value of zero and all other leads are referenced off this datum plane and will have a positive coplanarity value. The final reported lead coplanarity of the package is the maximum value of the worst individual lead.

* 3 lowest leads forming the seating plane

Figure 2: Lead Coplanarity Defmition

Other than package body warpage, poor lead coplanarity in the LQFP packages could also have been caused by other factors such as poor trimming and forming, handling, or poor JEDEC tray designs. To ascertain that the coplanarity issue we were facing was indeed casued by the plastic body warpage, we compared the lead coplanarity to actual body warpage. Distribution of individual lead coplanarity for package 144 LQFP (20x20mm) which is the test vehicle for this study is shown in Figure 3. Leads at the corner of package have higher coplanarity compared to leads at center

34th International Electronic Manufacturing Technology Conference, 2010

of package. Moire interferometric scans using an Akrometrix Thermoire model PS400 was performed on the package body. Package warpage measured using the Thermoire instrument revealed that warpage is convex (live bug) as shown in Figure 4. Comparing data from Figure 3 and the Thermoire results in Figure 4, it was clear that warpage in the plastic package body caused the lead coplanarity.

Lead Coplanarity (mils) ,w ,-----------------__________________ --,

Corner 2

I

I

I

,

Corner 3

,

,

I

,

Corner 4

I

I

I

Fugure 3: Lead profile of 144 LQFP by Icas lead scanner

CurrentPhase.tif

Co planarity = 2.6 mils

_0.9 _0.7 _0.5

OA CJO.2 c::::JO.l c::::J-o.' c::::J-o.3 c:::::J-o.4 c:::::J-o.6 CJ-o.8 CJ-o.9 _-'1 _-1.3 _-1.4 _-1.6 _-1.8

Figure 4: Thermoire warpage measurement of 144 LQFP in dead bug position with lead face up

For a thin package of 1.4mm, warpage would easily translate into coplanarity level without inclusion of variation resulted in the Trim/Form process. This would mean that theoretically if Trim/Form process is perfect, produces zero coplanarity, the resultant coplanarity level will equal to the warpage level.

2. Experiments

In our study, the 144 LQFP package was selected as this is the largest LQFP package qualified in production. Three major factors affecting package warpage were evaluated; molding processing condition, package design geometry and direct materials properties.

A. Molding Processing Condition

Six different parameters have been carried out as shown below:

1) Molding Temperature: 165, 175 & 185 degC 2) Mold Cure Time: 120, 150 & 180 sec 3) Cooling Time with Air Blow: 30, 35 & 40 sec 4) Extended PMC Time: 4 and 6 hours at 175°C 5) PMC Clamp: With and Without Clamp 6) PMC Profile: Normal and With Ramp Down

B. Package Geometry Design

Two major factors in package design were considered which are leadframe flag downset and die size. Below are parameters investigated:

1) Downset Depth: 7.6 mils versus 11.2 mils 2) Die Size: 5.340 x 5.340mm and 3.862 x 3.934mm

Figure 5: Package structure of 144 LQFP

C. Direct Materials

The effects of direct material properties of mold compounds and lead frame were studied. There were 2 types of lead frame materials and 3 mold compound candidates were selected in this study. Table I and Table II shows the properties for leadframe materials and mold compounds respectively. Standard lead frame in this study is Alloy A Cu leadframe with solid flag and standard mold compound is compound C.

Table I: Leadframe material properties

Alloy Temper Tensile Strength Hardness

(kgF/sq-mm) / (N/sq-mm) (HV)

Alloy A ET/ES 51.4-60.3 I 504-590 150-170

AlloyB 1/2H 61.9-74.0 I 607-725 180-220

Table II: Mold compound properties

al a2 Flexural Filler Mold Water

EMCGrade (ppm/"C) (ppm/"C)

Tg ('C) Modulus Content Shinkage Absorption

@RT (%) (%) (%) Compound A 8 32 130 27000 90 0.07 0.13

Compound B 9 40 120 23500 87.5 0.15 0.14

Compound C 12 48 120 20000 84 0.24 0.17

34th International Electronic Manufacturing Technology Conference, 2010

D. Multi Stage Experiments

The experiments were carried out in stages, where stage A focused mainly on molding process condition, followed by stage B on package design, and finally stage C on direct material in depth study. Mold compound properties has been shown to be most prominent factor in controlling package warpage [I]. In stage A, mold process condition was evaluated. Table III and Table IV shows the matrix for the molding parameter and PMC condition studies where each cell consists of 24 units. A standard compound (compound C) was used for this study. For PMC clamping study, three

Table III: DOE for molding parameter evaluation

� 1 2 3 4 5 6 7 8 9

Mill" T!:mll!:rS!tyr!: lOCI 165 165 165 165 185 185 185 185 175

Cyr!: Tim!: I�!:'I CQllling Tim!: I�!:'I 120 30 120 40 180 30 180 40 120 30 120 40 180 30 180 40 150 35

methods were used which were i) spring clamp jig, ii) Table IV: DOE for PMC condition evaluation torque wrench clamp, and iii) weight clamp. Samples size of

30 strips were clamped in each method with interleaf paper between strips during PMC. As for temperature ramp down, the strips were kept in PMC oven for 2 hours after the PMC time complete in order for strips to cool down slowly in oven. Package warpages were measured using the Thermoire system for each of these cells.

In stage B, samples with different die sizes and downsets were prepared for molding with standard compound C and

� PM!: Dl!rS!tiQn @ 17:Z°!: (HQl!rl 1 4

2 4

3 4

4 4

5 6

6 6

7 6

8 6

PM!: !:l!rj1 !:Qn!!itiQn �tril1 !:l!rj1 !:Qn!!itiQn Normal Profile In Magazine

Normal Profile Clamp

Ramp Down In Magazine

Ramp Down Clamp

Normal Profile In Magazine

Normal Profile Clamp

Ramp Down In Magazine

Ramp Down Clamp

package warpages measured with the Thermoire system at 3. Experiment Results & Discussion room temperature.

In stage C, mold compound propeties and leadframe material effects on package warpage were evaluated. Three different mold compounds have been tested with 48 units each. As for lead frame material, standard Cu lead frame Alloy A and the harder type Alloy B have been used in the study consisting of 48 units each molded with standard compound C. All the wire bonded leadframes for this study have been prepared in a same assembly batch with die size of3.862 x 3.934mm and 0.9 mil Au wire. In addition, to further access the performance of the mold compounds, moldability data been collected and submitted for reliability study with MSL3@260°C and up to ACI44 and TCIOOO. Wire pull and ball shear for High Temperature Bake @ 150°C up to 2000 cycles been collected as well.

Package warpage for each samples was measured using Akrometrix thermoire with sample size of 6 units per JEDEC standard after PMC and trim and form at room temperature in dead bug position. For coplanarity measurement an ICOS lead scanner was used in the study with sample size of 36 units. After obtaining the warpage measurement, 6 units were randomly taken for lead scanning to measure the lead coplanarity and thus to establish the correlation between the degree of warpage and lead coplanarity.

In addition to understand the package warpage relationship with lead coplanarity, trim and form process mechanism had been studied as well. This is primarily to understand the package warpage behaviour and the spring back effect of leadframe material itself on lead coplanarity.

The results from experiments are presented in following sections.

A. Molding Processing Condition

DOE results for mold process condition consists of 3 factors of mold temperature, cure time and cooling time shown in Figure 6. None of the factors above is significant to package warpage.

A

AC

BC

B

ABC

C

AB

Pareto Chart of the Effects (response is Package Warpage (Average), A\:lha = .05)

I -

-

=r

I

4 Effect

7058

I

6

Lenth's PSE = 1.875

Figure 6: DOE results for mold process condition

PMC process condition evaluation with PMC duration, temperature ramp profile and PMC clamping using spring clamp jig results are shown in Figure 7. The results show that there is no any significant factor among the cell for package warpage. Cell 1 is the standard production practise.

34th International Electronic Manufacturing Technology Conference, 2010

Units with clamping during PMC generally have higher package warpage.

Pareto Chart of the Effects (response;; Package Warpage (Average), Alpha = . 05)

A

AB

B I C[J

ABC

BC

AC

Lenth's PSE = 2.25

8469

I I

Effect

Figure 7: DOE result PMC condition evaluation

PMC clamp using torque wrench with 4N/m setting together with control cell without clamp results are shown in Figure 8. Package warpage with PMC clamp shows higher warpage compared to the control cell. Thermoire measurement indicates that the warpage is concave which is believed due to overclamp.

Package Warpage No Clamp vs Clamp

No C"mp C"mp

Figure 8: Package warpage for PMC clamp using torque wrench and without clamp

The next clamp method using weight clamp in which the strips were stacked up and a metal plate of Skg weighs down on the stacked strips. Figure 9 shows the package warpage result, which is again shows insignificant difference among clamped and unclamped units.

Package Warpage

Wlhout C"mp WeightC"mp

Figure 9: Package warpage for PMC clamp using weight clamp and without clamp

PMC clamping is known to be effective in controling package warpage for single sided MAPBGA package. However, for double sided leaded package, PMC clamping may not be effective as revealed by the results above.

B. Package Geometry and Leadframe Design

Downset and die size are important factors for package warpage performance, as shown in Figure 10. Larger die size and smaller downset are favourable for better package warpage. It is key that the compound volume between top of die to top package and the compound volume between flag bottom to package bottom are balanced in volume. Ideally the die have to be at center of package. The warpage of the 20 mm package is four times more sensitive to changes in downset than the 10 mm package for the same package thickness [2].

57.5

55.0

52.5

50.0

B B 47.5

45.0

42.5

40.0

Package Warpage

Big Oie+7.6 Downset Big Die+ll.2 [):)wnset Slmll Die+7.6 [):)wnset Small Oie+l1.2 Downset

Figure lO: Package warpage for different down set and die size

C. Direct Materials

Two packaging materials were considered in this section which are the mold compound and Cu leadframe. Mold compound properties shown to be major factor affecting

34th International Electronic Manufacturing Technology Conference, 20lO

package warpage. In contrast, leadframe material do not

have significant difference in terms of package warpage, as

shown in Figure 11.

Package Warpage

70

60

5 ! 50

40

30 L-______ �----------------��------� Alby A !lJby B

Figure 11: Package warpage using Alloy A and Alloy B lead frame

Moldability data for 3 different mold compound types

summarized in Table V; package warpage and wire sweep

result shown in Figure 12 and 13.

Table V: TO Moldability data

Moldability EMCA EMCB EMCC

Wire Sweep Acceptable Good Good

Internal Void Good Good Good

Delamination Good Good Good

Visual Mechanical Good Good Good

Package Warpage Good Good NG

Strip Warpage Good Good Good

Resin Bleed Good Good Good

Coplanarity Good Good NG

Marking Legibility Good Good Good

Package Warpage Performance

60r------------------------------------,

55

� 50 E 2-:g, 45 e. .. � 40

35

30 � ____ �----------�----------�----� EMCC EMC B EMC A

Figure 12: Package warpage performance for different mold

compounds

Wire Sweep Performance

..

EMCA

Figure 13: Wire sweep result for different mold compounds

60 E .a.

50 >

·E .. c 40 ..

Q. 8

30 Oil QI • Package Warpage .. ..

20 e- • Coplanarity .. 3:

10 QI .. .. iI

0 .. Q.

EMCA EMCB EMCC

Mold Compound

Figure 14: Average package warpage and lead coplanarity

From the results obtained, mold compound fillers content

playa significant role in controlling package warpage. This

can be explained through the package warpage mechanism

illustrated in Figure 15. Package warpage caused by

shinkage difference between mold compound and leadframe

is mainly affected by CTE and in-mold shinkage of mold

compound. Chemical shrinkage of molding compound has

also been shown to playa certain role to reduce the amount

of package warpage [3]. As filler content increases, the CTE

and mold shinkage decreases and this will helps to lower the

shrinkage difference thus resulting in lower package

warpage. Higher filler content also helps to increase flexural

modulus which decreases package deformation. However,

higher filler content also caused significant wire sweep

during mold filling into package. From the result, as filler

content increase from 87.5% to 90%, the wire sweep

increases from average 2.5% to 4.0%.

34th International Electronic Manufacturing Technology Conference, 2010

fi, c:: G>

....I G> Q. E '"

<J)

Leadframe CTE2

Tg Heat

_______ gIE1 Shrinkage

Mold Shrinka

---�T:�r--------------------------------Shrinkage difference between compound & /eadframe

o 50 100 150

Temperature (degC) 200

Figure 15: Package warpage mechanism

Based on the TO moldability data, EMC B outperformed the other candidates and been selected for reliability study focusing on mechanical stress without electrical test to further access the mold compound capability. Table VI shows the reliability result summary for mold compound B.

Table VI: Reliability study result for mold compounds

Bilil Lift FiliI!,!r!: Reliability Test Delamination Failure (unit) Wire Pull Ball Shear

TO 0/120 0/60 0/60

MSL3 260·C 0/90 0/60 0/60

HTB 504 Hours 0/30 0/60 0/60

HTB 1008 Hours 0/30 0/60 0/60

HTB 1500 Hours 0/30 0/60 0/60

HTB 2000 Hours 0/30 0/60 0/60

AC 96 Hours 0/30 0/60 0/60

AC 144 Hours 0/30 1/60' 0/60

Bosch AC 0/30 0/60 0/60

TC 500 Cycles 0/30 0/60 0/60

TC 1000 Cycles 0/30 0/60 0/60

.. .. .. II ..

II II II II II c

Figurel6: TO CSAM images for C90 144 LQFP molded with EMC B (Top scan, Thru scan and Bottom scan)

As summary, mold compound B with higher filler content of 87.5% compared to standard mold compound of 84% able to provide lower package warpage, while at same time meet the package reliability requirement for C90 144 LQFP.

4. Conclusions and Recommendation

Proper selection of mold compound material and optimum package design are found to be critical in controlling package warpage. Mold compound with higher filler loading nonnally come with lower CTE, mold shrinkage as well as lower water absorption proved to be a key factor in providing good package warpage. For package geometry design, it is important to have optimum compound volume at top and bottom portion of package. This can be achieved by controlling the down set of leadframe either in leadframe design or in assembly control. There is direct correlation between package warpage and lead coplanarity, where reducing package warpage improves lead coplanarity.

From this study, PMC clamp has been shown to be not effective in improving package warpage for 144 LQFP. However, since clamping the units during PMC able to change the warpage, it is recommended that further evaluation to be carry on to optimise the PMC clamping for optimum package warpage response.

Acknowledgments

The team would like to thank LC Tan and SF Zhao for the support and inputs to carry out some of the experiments.

References

1. L. T. Nguyen, "Plastics in electronic packaging: Trends in technologies,"ASME Trans. Electron. Packag., vol. 115,pp. 346-355,1993.

2. G. Kelly "The Simulation of Thermomechanically Induced Stress In Plastic Encapsulated IC Packages ", pp82,1996.

3. G. Kelly, C. Lyden, W. Lawton, J. Barrett, Z. Saboui, H. Pape, and H. J.B. Peters, "Importance of molding compound chemical shrinkage in the stress and warpage analysis of PQFP's," IEEE Trans. Camp., Packag., Manu/act. Techno!. B, vol. 19, pp. 296-300, May 1996.

34th International Electronic Manufacturing Technology Conference, 2010