circuits must tolerate bending and twisting during the product service life. By optimal design and use of flexible printed circuit boards offer several benefits ...
Reliability of Different Flex Materials in High Density Flip Chip on Flex Applications by Petteri Palm*, Jarmo Määttänen*, Yannick De Maquillé**, Alain Picault**, Jan Vanfleteren***, Björn Vandecasteele*** *Elcoteq Network Corporation Konalankuja 5, P.O.Box 8, FIN-00391,Helsinki, Finland **FCI Microelectronics 37, rue des Closeaux, 78200 Mantes la Jolie, France ***IMEC, Intec division, Thin Film Components Group (TFCG) St.-Pietersnieuwstraat 41, B-9000 Gent, Belgium Abstract This paper presents the latest results from the reliability tests of high density flip chip on flex application using unisotropically conductive adhesives. Four different types of flexible substrates and two different types of anisotropically conductive adhesives were selected. Both paste and film type of anisotropically conductive adhesive was used. Two different test devices were used. The contact areas were 50x50µm and 50x90µm. The effective pitch was 80 µm in both samples and the number of contacts was 200. The matrix of both anisotropically conductive adhesives was epoxy based and the conductive
1. Introduction Traditionally flexible printed circuit boards are used to replace rigid board and cables in applications like in hard disk drives, where the circuits must tolerate bending and twisting during the product service life. By optimal design and use of flexible printed circuit boards offer several benefits compared to rigid boards. Nowadays flexible printed circuit boards are used more and more also in high density applications like in smart cards, hearing aids, LCD modules and other modules for portable electronics. Common to all of these applications is small pitch and use of flip chip technology. The main reasons for the interest towards flexible printed circuit boards and flip chip technology in addition to high density/small pitch capability is the need to reduce weight and space, eliminate connectors and get more flexibility in design.
particles in film type were isolated soft metal-coated polymer particles and in paste type isolated silver particles. The contact resistance was measured with four-point method and the series resistance with Daisy chain method. The reliability of the flip chip interconnections was tested in thermal cycling tests. Cross section samples were made to analyze the possible failure mechanism of failed contacts. The results of this paper are partly from the FLEXIL development project that is financed by European Union.
2. Anisotropically conductive adhesives Anisotropically conductive adhesives are composite materials containing typically epoxybased matrix and small amount of conductive particles. The adhesive itself doesn’t conduct before processing unlike isotropically conductive adhesives, because the volume fraction of the conductive particles is relatively low and the particles are not in contact with each other. The adhesive begins to conduct in z-direction after the IC is pressed against the pads on the substrate. After the bonding process part of the particles are stuck and squeezed between the pad and the bump. These squeezed particles form an electrical conductive bridge between the IC and substrate [1]. Unlike in soldering flip chip process the electrical contact between the pad and bump is purely mechanical. The newest adhesives can be used in very small pitch applications.
Conductive particles are typically metal flakes or spheres made of silver of nickel (hard particle) or metal coated plastic balls (soft particle). Soft and elastic metal (Ni/Au) coated polymer particle provides reliable electrical contact between the bump and the pad since it deforms under pressure and provides large contact area [2]. Hard metal particle doesn’t normally deform so much but it can penetrate though the oxide layer on the conductor and form good contact. Typically the particle diameter is 310 µm, and the particle density varies from 20003000/mm2 up to 20000/mm2 [3-5]. The conductive adhesive is one of the most important factors that effect to the reliability of the contact. Not only the mechanical and chemical properties of the matrix polymer but also the type and the properties of the conductive particles and especially the compatibility of the adhesive and the bump and/or the substrate are important when selecting the suitable adhesive for the application. Other factors that must be taken into account are the type (paste vs. film) and the cost of the adhesive, the bonding time and temperature.
3. Flex for FCOF applications The flexible printed circuit boards used in flip chip applications have either two-layer structure or three-layer structure. Two-layer structure or adhesiveless flexible PCB consists of thin and flexible dielectric substrate and copper foil. Three main technologies are usually applied for two layers continuous films production. In the first one a thin copper layer is evaporated or sputtered onto the dielectric surface (Kapton® or Upilex®) and an additional thick copper layer is electroplated. In the second one a liquid resin (epoxy or polyimide) is coated onto the copper layer and cured. In the third one an epoxy glass prepreg is laminated onto the copper layer and cured. Three-layer structure flexible PCB consists of thin and flexible dielectric substrate and copper foil which is laminated to the substrate with thin adhesive layer. Especially in small pitch applications the most widely used substrate material is polyimide. The benefits of the polyimide (PI) are that it can tolerate relatively high temperatures, its dimensional stability is relatively good and the
chemical resistance is high. Unfortunately the water absorption and the cost is relatively high. Other base materials that are used are polyester, polyethylene naphthalate (PEN) and epoxy-glass (EG). Polyester has excellent flexibility, good electrical properties, chemical resistance and low cost but the temperature stability is poor. The thermal stability of the polyethylene naphthalate is slightly better than polyester has but also the cost is higher. The benefit of epoxy-glass is low CTE and relatively low cost but the flexibility is poor compared to other flex materials. One new and interesting base material is liquid crystalline polymer (LCP). Important factors when selecting dielectric substrates are mechanical strength, flexibility, dimensional stability, dielectric properties, thermal properties, chemical resistance, moisture absorption and cost [6, 7]. Copper that is laminated to the flex substrate is typically either electrodeposited copper film (ED) or rolled annealed (RA) copper film. In static applications, where the circuitry is bent into the position where it remains the whole lifetime of the product, ED copper is the most suitable. This type of copper film is produced by electroplating a thin layer of copper onto a stainless steel or titanium drum. After plating the copper is removed and if required, post treated to achieve robust surface for lamination. Rolled annealed copper film is manufactured by first hot rolling the copper ingots to copper sheets. To achieve desired thickness and properties for the film the copper sheet is cold rolled and finally annealed. Due to the grain structure form by
cold forming this type of copper is very flexible and suitable for applications where flexibility is required during the lifetime of the product [8, 9]. Depending on the application and assembly process the copper tracks are Sn, SnPb or Au plated. The purpose of the plating is to protect the conductor metal against oxidation, improve solderability and to ensure good electrical contact. Gold has excellent electrical conductivity and it doesn’t oxidize and due to that it is applicable to be used in anisotropically conductive adhesive process. The Au plating can be done using electroless or electrochemical plating process. Tin and Tin-lead plating is also used in some applications (e.g. TAB), but some problems with these type of platings can occur
due to oxidation of the surface. With pure tin plating also the growth of tin whiskers can cause serious problems. To avoid these special heat treatment or alloying must be used. SnPb plating can be done using electrochemical process and Sn plating either using electroless or electrochemical process. There are several different types of adhesives that are used to laminate the copper film to the base material. The most widely used adhesives are epoxies, polyesters and acrylics. Important factors when selecting the adhesive are adhesion, flexibility, thermal resistance, chemical resistance, moisture absorption, electrical properties and cost [8, 10]
Thermal properties of different materials are presented in Tables 2 - 4.
substrate
Table 2. Thermal properties of Espanex (PI) base material Properties
ESPANEX
Etch Shrinkage (%) Heat Shrinkage (%) Thermal Linear Expension Coefficient (*10-5cm/cm/°C)
0,03 0,05 2,2
Melting Point (°C)
None
Table 3. Thermal properties of PEN base material. Surface Resistance
PEN (75 µm)
Glass transition temperature Heat Shrinkage Thermal Linear Expension Coefficient (*10-5cm/cm/°C)
120 0,2
Four different flexible printed circuit boards were used in the tests. One of the materials was two layer type and three were three layer types. Two different copper thickness were used; 12 and 18 µm. All flexes were electroplated with 0,5 µm nickel and 0,075 µm gold layer. Materials are presented in Table 1.
Melting Point
266
Heat Shrinkage (%)
< 0,1%
Table 1. Flexible printed circuit boards that were used in the tests.
Glass Transition Temperature
145°C
4. Experimental Procedures
1 2 3 4
Flex material
Copper Thickness (µ µm)
Adhesive Thickness (µ µm)
Substrate thickness (µ µm)
PI PEN (1) PEN (2) EG
12 18 12 18
18 18 18
40 75 75 110
The flexible printed circuit boards were manufactured by FCI Microelectronics. The picture of the test flex is presented in Figure 1.
Table 4. Thermal properties of Epoxy glass base material. Properties
Two different types of anisotropically conductive adhesives were selected. Both film (ACF) and paste (ACP) type adhesive were used. Conductive particles in paste type adhesive was silver sphere and in film type adhesive metal (Au/Ni) coated polymer particle. In both cases the particle was isolated. The properties of the adhesives are presented in the Table 5. Table 5. Properties of the adhesives
Thickness Matrix
Unit
ACP
ACF
um
Thermo-setting epoxy Ag sphere
30 Thermo-setting epoxy Ni/Au coated resin particle yes 5 63/130 125
Particle material Insulation Particle size um CTE ppm/°C Tg °C Viscosity Pa.s Modulus
Figure 1. Flexes that were used in the tests
EPOXY GLASS (110µm)
GPa
yes 5-10 45 105 26
1,2
Flip chip components were bonded to the test flexes by using accurate Toray FC-1000 flip chip bonder. Bonding parameters were selected according to the ACA manufacturer’s recommendation.
With two-layer structure, where no adhesive used between the copper conductors and the substrate, both adhesives passed the test without any failures (1ACP1 and 1ACF1). Also PEN(2) substrate with paste type adhesive passed the test without any failures (3ACP1). With all other samples first failure happened already before 236 cycles. As can be seen from the Figure 3 with test IC2, four material combinations passed the test without any failures.
1ACF2
5. Results and discussion To evaluate the reliability of different flexible printed circuit board materials for high density flip chip on flex applications thermal shock test was done. The temperature shock test was done using two-chamber cabin. Temperature range was from –40C to +125 C, the samples were 15 minutes in both extreme temperatures and the transportation time was 30 seconds. The total number of the cycles was 1000. The contact resistance and the daisy chain resistance were measured during the test. The results of the temperature cycling test are shown in Figures 2 and 3. As can be seen from the Figure 2 with test IC1, three material combinations passed the test without any failures.
Temp. cycling test (IC1)
1ACP1 1ACF1
Failure %
100
2ACP1
80
1ACP2
Temp. cycling test (IC2)
Failure %
Two different test IC layouts were used. Both test IC’s were Au bumped and the number of the I/Os was 200. The bumps in test IC1 were in one row and in test IC2 in two rows. The contact area was 50x50µm in IC 1 and 50x90µm in IC2. The effective pitch was 80 µm in both IC’s. Both IC’s contain measurement structures to measure contact resistance (four-point probe) and daisy chain resistance. The height of the Au bumps was 20 µm and the bumping was done in Elcoteq Network Corporation.
100
2ACP2
80 60
2ACF2
40
3ACP2
20 0
3ACF2 4ACP2 0
236
571
768
1000
4ACF2
#cycles
Figure 3. The cumulative failure % of test IC2 with two different adhesives and four flex materials in temperature cycling test for 1000 cycles. Two-layer PI flex with both adhesives (1ACP2 and 1ACF2), PEN(2) substrate with paste type adhesive (3ACP1) and epoxy glass substrate with film type adhesive (4ACF2) passed the test without any failures. With all other samples first failure happened already before 571 cycles. According to the results a difference in reliability between different flex materials was notified (Table 6.). Also larger contact area (IC2) gave slightly better reliability and smaller contact resistance.
2ACF1
60
3ACP1
40 20
Table 6. Summary from the reliability test results.
3ACF1
0
IC1
4ACP1 0
236
571 #cycles
768
1000
4ACF1
Figure 2. The cumulative failure % of test IC1 with two different adhesives and four flex materials in temperature cycling test for 1000 cycles.
Polyimide
ACP yes
IC2 ACF yes
ACP yes
ACF yes
PEN (1)
no
no
no
no
PEN (2)
yes
no
yes
no
no
no
no
yes
VEP yes no
passed the test without any failures contact resistance increased >50 % during the test
According to the cross section samples three layer structure can cause problems due to sinking of conductors, as can be seen from Figure 4. The pad has sunken to the adhesive during the bonding process. This can cause variation in contact pressure and in contact resistance in different positions. Also adhesion problems between the copper conductor and substrate can occur.
(1) Polyimide flex with two layer structure gives the best reliability in thermal shock test. (2) Not only the base but also the adhesive that is used to laminate the copper foil to the substrate in three layer structure has a great influence to the reliability (e.g. PEN(1) vs PEN(2)) (3) Sinking of the conductors can increase contact resistance value and variation of values and decrease reliability (4) Larger contact area gives a better reliability in thermal shock test (5) Larger contact area gives smaller contact resistance value because of higher amount of the conductive particles between the pad and the bump (6) Full metal particle gives smaller contact resistance values than metal coated polymer particle
7. Acknowledgement Figure 4. Sinking of the pads in three layer flex. Thus the adhesive that is used to laminate copper foil to the substrate is very critical in ACA flip chip on flex process. One possibility to avoid this problem is to use two layer structure or to optimize the properties of the adhesive.
6. Conclusions The conclusions summarized as follows:
in
this
paper
are
The results are from FLEXIL project that is part of European Union IST research program and the authors would like to thank EUCommission for their funding in this project. The authors would also like to thank Ms Tanja Heimonen and Mr Pasi Perttula from Elcoteq Network Corporation for the help during the tests, the laboratory of electronics production technology from Helsinki University of Technology for carrying-out the reliability tests and Protecal Oy from Finland for the test IC and test substrate designing.
References 1) Palm P., The development and reliability if flip chip technology in electronics module manufacturing, Ph.D. thesis, TUT, Tampere, Finland, 2001 2) Määttänen, J., Anisotropic adhesive interconnection: An alternative for solder joints in high density electrical contacts, Ph.D. thesis, TUT, Tampere, Finland, 2001 3) Määttänen J., Palm P., Tuominen A., Ristolainen E., Conductivity model for metal-coated polymer particles used in anisotropically conductive adhesives, 2001 ICEP conference, April 18-20, Tokyo, Japan, 2001 4) Watanabe I., Takemura K., Anisotropic Conductive Adhesive Films for Flip Chip interconnection, Conductive Adhesives for Electronics Packaging, Edited by Liu, J., Electrochemical Publications Ltd., 1999 5) Holloway M., Crane L, Peters S., A Roadmap to Flip Chip Assembly using Advanced Adhesives, VTE Aufbau und Verbindungs Technik in der Elektronik, No.5, Oct 1999, p.81-85 June 1, Strasbourg, France, 2001 6) Sheldahl, Materials for Flexible Circuits, Printed Circuits Handbook, edit. Clyde F. Coombs, Jr., Fourth edition, chap. 40, McGraw-Hill, New York, 1996 7) Palm P., Määttänen J., Picault A., De Maguille Y., The evaluation of different base materials for high density flip chip on flex applications, 13th European Microelectronics and Packaging conference, IMAPS, May 30-
8) Sheldahl, Design of Flexible Circuits, Printed Circuits Handbook, edit. Clyde F. Coombs, Jr., Fourth edition, chap 41, McGraw-Hill, New York, 1996 9) Robert A. F., Advanced materials technology, Printed Circuits Handbook, Fourth edition, chap 9, edit. Coombs ,C.F. Jr., McGraw-Hill, New York, 1996 10) Mahesh G.V., Bousted K., Brox, B., Flexible circuit technology –an overview, IVF report 95015
