Special aspects of the metallographic preparation of electronic and microelectronic devices Katja Reiter, Mario Reiter, Each individual step in the prepara- ten made to achieve a good prepa- Thomas Ahrens tion from cutting, through grinding ration result by, for example, polis- Fraunhofer Institut für to the final polishing stage is of sig- hing for longer times. However, Siliziumtechnologie, nificance. Mistakes made in the even if a scratch-free and appar- Modulintegration, first stages of preparation can only ently well-polished surface has been D-25524 Itzehoe, Germany be corrected during subsequent achieved, this is often at the ex- steps with considerable difficulty or pense of edge retention and sample Contents not at all. For each step, the rate of planeness. The objective is always 1. The problem material removal and the deforma- to prepare a flat sample exhibiting 2. Typical materials and their uses tion layer which remains are impor- a high degree of edge retention. 3. Preparation and microstructure tant factors. development The polishing parameters used 2. Typical materials and 4. Applications (grinding surface, type of abrasive, their uses 4.1 Ceramic composites grain size, lubricant) must be care- The materials most commonly used 4.2 Circuit boards fully selected on the basis of the in packaging and interconnection 4.3 Silicon and glass diodes physical properties. The general technology (electronic packaging) 4.4 Capacitors and resistors rules for the preparation of solid are listed in Table 1. 4.5 Gold wire bonds: preparation of materials are that soft and medium- Electronic components are most a transistor hard materials should be plane commonly constructed from combi- 5. Artefacts ground using SiC papers, whereas nations of the following materials: 6. References hard materials require resin-bonded alumina, fritted-glass-filled or metal-bonded diamond grinding duromers, tin-lead solder, circuit Preface discs. In the composite materials boards (epoxy-fibreglass sheet) and In Structure 32, we reported on the described here, one is dealing with copper. techniques used for the analysis of both extremely hard components, microstructures and materials in e.g. ceramics such as Al2O3, and Each of these materials has a differ- the electronics packaging industry. very soft materials, e.g. tin-lead sol- ent characteristic property. The In this article, which can be re- der. In such cases, preparation must Al2O3 ceramic is extremely hard and garded as a continuation of our ear- seek a compromise. brittle, the tin-lead solder and the lier article, we are concerned with copper are very soft. The soft solder the metallographic preparation of If only grinding papers are used to microstructures are easily ‘washed individual samples and the possible grind hard materials, then edge out’ during polishing, and the cop- sources of erroneous results that rounding and relief formation set in per tends to “smear”. The epoxy one should be aware of. Recom- at an early stage. Plane grinding resin matrix from which the circuit mended processing techniques should be performed using the abra- board is made has a similar hard- which avoid preparation artefacts sive most suited to the sample ma- ness to that of copper. However, the will also be presented. terial. The finest grain size should 1. The problem be chosen which Material Common applications Universal Electronic and microelectronic de- will still allow the in mounting and hardness vices are complex material compos- sample to be connection technology [HU] ites. Imaging and analysis of the ground flat and Al2O3 ceramic carriers (hybrid technology), various material microstructures, smooth in an ac- body of components 17000 layered structures and interfaces ceptable time. Silicon semiconductors, transistors, ICs 9300 are necessary if the quality of prod- The removal rate NiP resist layer, metallization 4000 ucts is to be assessed. The dimen- when using SiC sions of the individual features wet grinding pap- Kovar (FeNiCo) lead frames, circuit board core 1900 range from fractions of a micron ers on soft mate- Invar (FeNi42) lead frames, lead wires, 1800 (micrometre) to several centimetres. rials is consider- housing covers Due to the close packing of the vari- able, but these Aluminium housing, heat sinks, capacitor foils 1300 ous materials within a small vol- grinding papers Copper-silver solder solder (brazing) 1100 ume, microstructure analysis is cause a high Gold surface finishing layers faced with the problem of simulta- degree of surface (connectors, solder, adhesive and neously imaging different materials deformation. In wire bond contacts) 500 which have very different proper- such cases, an Tin-silver solder soft solder 300 ties. Sample preparation for micro- attempt is still of- structure investigations therefore Copper metallization on circuit boards, 300 lead frames involves the simultaneous process- ing of hard, often brittle, materials Table 1: Epoxy-fibreglass sheet circuit board material 280 Materials used in and soft materials exhibiting plastic electronic mounting and Tin-lead solder soft solder 230 deformation. connection technology 12 glass fibres break-out easily on orientation of the features to be problem with epoxy is its modest grinding. In the next section, two imaged and is often marked on the hardness (approx. 280 HU) com- preparation methods for developing assembly plan. If the defect site has pared to ceramics (17,000 HU) microstructure are introduced. This been located then this will deter- which means that long polishing is followed by a description of appli- mine the grinding plane. times will inevitably lead to edge cations of these techniques to vari- rounding in the ceramic parts of the ous combinations of materials. II Sampling and mounting sample. Another side effect is a This stage of the preparation was comparatively long curing time of 3. Preparation and micro- treated in detail in Part 1 (see about twelve hours. structure development Structure 32). It is worth noting Metallographic preparation can be once more that the sample must re- III Grinding and polishing divided into the following steps: main wholly undamaged after sec- The two grinding and polishing I Optical inspection tioning and that when mounting, no techniques particularly well suited II Sampling and mounting heat must be transferred to the to the preparation of composite ma- III Grinding and polishing sample as even small quantities of terials are listed in Table 2. IV Assessment of microstructure heat can cause changes in the by light microscopy microstructure. Components In preparation method 1, the sam- mounted on a circuit board are best ple is fine ground in several stages I Optical inspection sectioned with a simple fret saw, using graded SiC papers down to a The initial stage involves the mac- whereas ceramic composites will re- 1200 grit size and is then polished roscopic visual inspection of the quire using a diamond cutting using diamond suspension on silk structure of the assembly. This al- blade. If the test piece has aged, it cloths. In method 2, the sample is lows critical sites to be located and is advisable to impregnate the sam- ground plane using SiC paper with the locations of both obvious and ple before cutting. This should al- a 180 grit size, fine ground on a possible defects to be marked. Typi- ways be performed under vacuum to Struers MD-Largo disc with dia- cal sites include: poor solder joints, prevent cavities remaining below mond suspension as abrasive, and cracks in components, or defects in the component. finally polished using diamond sus- the circuit board material. These pension on silk cloths. In both faults are then investigated more Because of its low curing tempera- methods, final polishing with an closely using other techniques. Sub- ture and its low viscosity, epoxy OP-S suspension is performed. surface defects such as electrical resin is particularly suitable for discontinuities or short circuits can mounting electronic material com- Which method is applied depends often be analysed non-destructively posites. A further advantage is the upon the material combinations by ultrasound microscopy or x-ray transparency of the embedding me- present. Polycrystalline diamond radiography. The orientation of the dium which helps when performing suspensions are used. The rate of grinding plane will depend upon the the various preparation steps. One stock removal depends upon the fol- lowing factors: the mixing ratio of diamond Preparation method 1 Grinding Polishing grains to solvent, the degree of wear of the grinding Step1-45678 paper or polishing cloth, and Grinding surface Grinding paper Plan-O-Grip DP-Dur DP- Dur OP-Chem the cleanliness of the cloths used. Abrasive SiC diamond diamond diamond OP-S The type of lubricant employed also Grit/grain size # 180-1200 6 µm 6 µm 1 µm 0.25 µm affects the quality of the prepara- Lubricant Water Blue Red Red tion. Alcohol-based lubricants in- Rotation speed [rpm] 300 250 250 150 150 crease the removal rate but also re- sult in a greater deformation depth. Time [min] until plane 10-20 5-10 2 0.5 With oil-based lubricants, removal rates and deformation depths are Preparation method 2 Grinding Polishing both lower. Step 1 2 3 4 5 It is therefore advisable to use an Grinding surface Grinding paper MD-Largo DP-Dur DP- Dur OP-Chem alcohol-based lubricant for rough polishing and an oil-containing Abrasive SiC diamond diamond diamond OP-S lubricant for the final polishing Grit/grain size # 180 9 µm 6 µm 1 µm 0.25 µm stages. Lubricant Water Blue Red Red Grinding quality is also affected sig- nificantly by the amount of abrasive Rotation speed [rpm] 300 300 250 150 150 Time [min] Until plane approx. 5 approx. 1 approx. 0.5 0.5 Table 2: Preparation methods 1 and 2 13 used. The polishing cloths should be Figure 1: Cross-section through kept rigorously clean and should a power module not be made too wet by applying too much abrasive or lubricant.