Direct Soldering of Electronic Components on Molded Devices

K. Feldmann ( I ) , M. Gerhard, Institute for Manufacturing Automation and Production Systems, University of Erlangen, Nuremberg

Received on January 10,1995

Abstract:

Using the great potential ofthe three-dimensional Molded Interconnect Devices (3-D MID) neces- sitates the adaption of the conventional soldering methods in electronic assembly. The different cha- racteristics of the soldering technologies determine the suitability of various engineering thermopla- stics as substrate material. So several investigations on application of different thermoplastic materi- als have been carried out. Furthermore the geometric design of the molded carriers and the impact on the temperature distribution on the board during the soldering process will be discussed and the possibilities and the limitations of conventional soldering equipment will be shown.

Kevwords: Soldering, Electronic, Quality assurance

1.

In conventional electronic production the material of printed circuit boards is based on epoxy resins filled with laminated paper (FR2) or glass fiber (FR4). By using these duroplastic materials the conductor pat- tern generation is limited to planar boards. The ongoing trend to miniaturize products (e.g. tele- communication, consumer electronic,..) and the pressure of cost reduction leads to new requirements for the electronic industry. An approach to this prob- lem is a new production technology -the Molded Interconnect Devices (3-D MID) /6/. MlDs are 3-dimensional molded circuit carriers made of ther- moplastics. The injection process permits optional design of the circuit boards and offers the possibility of a good adaptation of conductive pattern to the geometric form of products. This innovation has potential for product miniaturization through the injection molding of mechanical elements like inter- connection components. By cutting down the num- ber of mechanical parts the number of assembly steps as well as the manufacturing costs will be reduced. The integration of mechanical and elec- tronic functions in a 3-dimensional circuit carrier also improves the performance characteristics and increase the system reliability. In opposite to epoxy resins thermoplastic materials are flame retardant without hazardous additives, recyclable and thus environmentally friendly.

But the MID-technology also makes new demands on the assembly in electronic production. The avail- able processes like the application of solder paste or the insertion of SMD-components have to be adapted to the requirements of 3-dimensional cir- cuit carriers /2,3/. Additionally to these geometric

Impact of MID-technology on assembly of electronic products

demands the material properties have to be attended in the soldering process, because the thermal beha- viour of thermoplastics differs from epoxy resins. Therefore the suitability of different soldering meth- ods to various geometric types of circuit carriers and thermoplastic materials are investigated at the FAPS Institute. Based on these research projects methods for optimizing the interconnection technologies are deduced to guarantee good solder quality for MlDs (fig 1).

I process I ,--. applicability of different /r- .. I heating methods ii ,-== $ ,

geometric design

material thermal behaviour of thermoplastic materials

fig. 1 :

2. Soldering methods in electronic

For soldering of electronic components on circuit boards different methods of heat transfer are used. The solder technologies can be divided into the mass solder techniques (wave-, vapour phase- and Infrared soldering) and the selective solder methods

Influencing variables to solder quality of 3-dimensional thermoplastic devices

production

Annals of the CIRP Vol. 44/1/1995 19

(Laser-, fountain- and hot air soldering). Using mass soldering methods, which are characterized by high production volume, the complete board will be heated. Compared to that the selective soldering methods work sequentially and therefore cause only local thermal stress. The different thermal character- istics of the various soldering techniques are repre- sented in figure 2, where the minimum and maximum values for the peak temperature and the duration of confrontation between the substrate and the solder is shown 141.

4 temperature ["C]

C U

duration isecl L - - - ,

200 I I/ l'o $0 do 4 b 40

L,

fig. 2 : Thermal characteristics of different sol- dering technologies

3.

Using thermoplastics as a base material for circuit boards, the electrical, thermal and mechanical prop- erties have to be adapted for applications in electron- ics. In addition to that the choice of the different ther- moplastics has influence on the suitability of the dif- ferent process steps for manufacturing MIDs, such as the type of metallization of the circuit boards or the soldering method. The price of the material deter- mines the costs for the product too and is a decisive factor for profitability of a product in MlD-technol- OgY. The manufacturers of thermoplastics base material specify the thermal properties of the materials by using normed test methods. But the test conditions are not equivalent to the thermal stress for the ther- moplastics during the soldering process. To meet the real requirements of electronic assembly, the suitabil- ity of thermoplastics was tested using real test boards with electronic components. These molded boards were heated in different reflow methods.

3.1. Testing method For these investigations a special test layout (80 mm * 80 mm) was designed (fig. 3). The layout consists of different types of electronic components with vari- ous heat capacities and absorption coefficients. In addition to that the test boards allow a variation of the insertion density to obtain local different thermal stress for the thermoplastic boards. There arevarious methods for formation of conductor patterns. The test boards are manufactured in the hot embossing technology. This works by onserting a copper foil

20

The suitability of thermoplastic materials as substrate

(realizable thickness of layer 12-100 pm) on the thermoplastic board and stamping out the conduc- tive paths with a heated tool. The connection between the copper foil and the substrate is created by a heat reacting adhesive during the stamping pro- cess. After the embossing process the warpage of the pla- nar test boards are measured with a sensor based on laser triangulation (fig.3). To do this a laser light point will be projected to the object surface. One part of the laser light will be reflected and pictured on the detec- tor in the sensor. With a special algorithm the height of the object can be determined /5/. For the application of solder paste on the pads of the components a dispensing system is used. After mounting SMD components the test boards will be heated by a reflow soldering process. When using wave soldering technologyfirst an adhesive has to be dispensed on the boards. Subsequent to the inser- tion of the components the adhesive will be cured by thermal heating (about 120 "Cforfive minutes). In the following wave solder process the components on the board will be soldered. To detect the tempera- ture-time profile of the used material during solder- ing process, several thermocouples are fixed on the surface of the sample. Depending on the various adjusted process parameter the heating rates and maximum temperature a thermoplastic can with- stand, can be ascertained.

fig. 3 : Use of laser triangulation sensor for mea- surement of the warpage of test boards

At the end of the process chain the thermoplastic test boards are examined by visual inspection with a microscope to see possible deformations because of the high temperature during the soldering process. The warpage of the boards in x- and y- direction and diagonal will be measured again with the laser sensor to determine the dimensional stability after soldering (fig. 3). The results of the measurements are compared with the permissible values for war- page of boards based on epoxy resins. The norm IPC-MB-380 allowa tolerance of 1 %from the long- est side of flat boards.

3.2. Evaluation of thermoplastics for MID For investigations the molded plaques are made of different thermoplastic materials with various thermal characteristics. Corresponding to their long term operating temperature the thermoplastics can be classified in high performance poiymers (T > 1 50°C), engineering polymers (100 "C c T < 150°C) and standard resins (T < 100 "C). With the increasing thermal properties the price of the materials rises. All examined thermoplastics out of the group of high performance polymers are adapted for mass solder- ing techniques. There was no deformation after the soldering process and the measured values for war- page is within the tolerance. But the water absorption of the some polymers (PES, PEI) requires an addi- tional process step before manufacturing the circuit boards. These materials have to be preheated for one hour at 120 "C to prevent any deformation of the substrate. The group of engineering thermoplastics is of interest for MID applications because they are cheaper than high performance polymers. With polymaide (PA 66) a good solder quality is obtained using the three dif- ferent mass solder technologies. Also testboards with high insertion density, which requires high sol- der temperature (about 260 'C), have a good dimen- sional stability. Another examined thermoplastic material (PBT) is unsuitable for wave soldering, But for infrared soldering the material is suitable for sim- ple structured circuits carriers, which need peak tem- peratures less than 230 "C. Thermoplastic materials like polycarbonate (PC) or acrylonitrile-butadiene-styrene (ABS) cannot with- stand the high temperature of the mass soldering technologies. The low heat resistance of these mate- rials requires a reduction of the thermal stress by application of selective soldering methods. Using these soldering methods the complete molded boards will be dimensionally stable after the solder- ing process. The results of our investigations prove, that selective soldering techniques allow the use of thermoplastic materials with heat distortion tempera- ture of about 100 "C.

4. The effect of the design on the soldering of MlDs

The geometric forms of molded circuit carriers influ- ence the mechanism of the heat transfer during the soldering process. The knowledge of planar circuits boards has to enlarge to the third dimension, which require a lot of research projects to determine the effects of optional design on the solder process.

4.1. Soldering of electronic components on inclined plans

First investigations were performed to determine the influence of gravity on the soldering of electronic components on inclined plans /3/. Depending on the different types of electronic components the critical angle of inclination was ascertained. As seen in figure 4 some components can be soldered even on vertical position. In this context also process parameters like the amount of solder or the preheating of solder paste were tested. So the requirements on the mate-

rial and process parameters could be defined for sol- dering on inclined plans. These results are the base of advanced design guidelines for the layout of 3- dimensional circuit carrier.

Influencing variables to angle of inclination for electronic components

fig. 4 : Soldering of electronic components on vertical walls

4.2. Limitations on soldering equipment Conventional soldering methods, which are applied to planar circuit boards, are basically adapted for molded boards, but the soldering equipment is lim- ited to the different geometric forms of 3-dimen- sional circuit carriers. So standard machines of the different soldering techniques are analyzed to deter- mine these limitations. Using mass soldering technologies the designers of circuit boards have to consider these limitations of solderable geometric forms. In Infrared furnaces the product clearance is limited and varies between 20-50 mm. So the height of the molded parts has to be lower than the fixed distance between IR-emitter and transport system. To enlarge the geometric possibilities of Infrared fur- naces a special soldering system was constructed at the FAPS Institute, where the IR-emitters are fixed on numeric controlled axes. By changing the dis- tance between the molded parts and the emitters the temperature on the board can be influenced and thereby the system allow a good adaptation on the geometric form of molded parts ( fig. 5).

position of

variation of the distance of the

emitter z z

Bio, temperature - time -profile on the board

fig. 5 : Temperature controlled Infrared soldering

Because of the heat transfer method of the vapour phase soldering method there is no limitation to the geometry of circuit boards. Only for molded devices with borders around the board there is the problem,

21

that the produced condensate cannot flow off. There- fore vapour phase soldering is only available for this geometric form, if the designer of the circuit board has the possibility to integrate outlets for the conden- sate to flow off. The limiting factor for wave soldering machines is the height of the wave, which is normally between 8-10 mm. This means, that no parts on molded circuit boards (inclusive electronic components) are allowed to rise above more than 8 mm. The vertical distance between different solder areas has to be smaller than the maximum wave height. Another problem is, that the form of the wave will be disturbed by obstacles rising in the wave. Consequently solder will not cover areas near such elements because of the shadow effect. Thus the wave solder process is available only for simple geometric forms. One possi- bility to enlarge the geometric limitations of this heat transfer method is the use of the fountain soldering technique. In contrast to the mass soldering technologies the selective soldering systems have the advantage, that there is no geometrical limitation to the design of the circuit carriers. In most of the cases the soldering tools or the molded devices are handled by a flexible handling system.

4.3. Development of design rules Besides the limitations given by soldering equipment the complex geometric form of molded devices has effects on the temperature distribution on the board. At the example of a housing for a bicycle front light with electronic circuit on a cylindrical surface the tem- perature distribution is shown in figure 6.

Infrared soldering , ~ 'i ofaproduct ma-& ,' ~

-port direction

fig. 6 : Temperature distribution on a cylindrical board

The peak temperatures during the soldering process in transverse direction show the highest temperature on the border of the lamp and a temperature decrease to the middle of the light. The temperature lengthwise prove an increase from the back end to the front side of the light. One reason for this increase

22

is the shadow -effect of the wall at the back end, which is a barrier for an effective heat transfer. This not very complex product shows the necessity of creating design rules for the layout of a MID prod- uct. So investigations have to be carried out to deter- mine the effect of geometric components to the tem- perature distribution on the board. For these tests a infrared thermography system is used to make the characteristic radiation visible (fig. 7).

fig. 7 : Use of Infrared thermography to deter- mine the effect of geometric elements to the solder process

This measurement method allows a non-contact temperature measurement of any object. In contrast to thermocouples normally used in electronic indus- try, which can determine temperature values only at selected points, the infrared camera gives a plane image of the object. So the critical areas of high or low temperature on the board due to various types of components or various insertion density of compo- nents can be determined. For 3-dimensional circuit carriers test boards with various heights of walls will be examined to find out the different temperature dis- tributions near the walls. Special design rules makes a recommendation for the layout of the circuits for the minimum distance of the electronic components to the wall dependent on the height of the wall.

Literature Conway El?, et. al., 1992, SMD Reflow Solde- rin : A Thermal Process Model, Annals of the CIf% Vol41/1/1991 S.21-24 Feldmann K., Franke, J.,1991: Automated as- sembly of new 3D Molded Interconnection De- vices. Proceedings ll. IEEE International Electronics Manufacturing Technology Sym- posium, San Francisco, CA USA Feldmann K., Brand A,, 1994, Analytical and Experimental Research on Assembly Systems for Molded Interconnection Devices, Annals of the ClRPVol. 43/1/1994, S. 15-18 Flohr R., 1991, Beitrag zur optimalen Verbin- dungstechnik in der Oberflachenmontage (SMT), Carl Hanser Verlag, Munchen Mengel I?, etal.,l992, Application of Advanced Data Processing Technology for Integrated Inspection in Electrotechnical Assembly, Annals of the CIRP, Vol41/1/1992 S. 29-32 Mettler M.R., 1994, The Benefits of MID Pro- duct Design, Proceedin s MID 94, Meisen- bach Verlag, Bamberg, 8 .373 - 380