Investigating the self-alignment of chip components during reflow soldering

Since the application of lead-free soldering has become obligatory in the electronic industry by the 1st of July 2006, the in-depth analysis of the soldering process is more important than ever. The small chip components of the present days demand very accurate component placement machines to prevent common reflow failures such as skewing or tombstoning. The ability of components to be self-aligned during soldering works against these failures, therefore it matters to what extent the solder promotes this effect. Dynamic behaviour of SMT (Surface Mount Technology) chip components during lead-free reflow soldering will be demonstrated in the paper. A force model has been introduced with the five main forces which determine the movement of the chip component during reflow soldering, namely: the force originating from the surface tension, the forces originating from hydrostatic and capillary pressure, the force of gravity, and the force of dynamic friction. The self-alignment of 0603 size chip components has been investigated by real experiments. SM chip components are misplaced intentionally by semi-automatic pick & place machine, and before and after reflow soldering the exact location of components was measured. The results have shown that the self-alignment of components does occur even in the case of 400-500 \mu m lateral misplacements. The explanation of the equations of the applied theory and the results of the experiments are presented in the paper in details

Újraömlesztéses forrasztás stencilnyomtatási folyamatának modellezése

Napjaink elektronikus áramköreinek szerelésénél az egyik legkritikusabb lépés a forraszpaszta stencilnyomtatása, melynek alapos vizsgálata, pl. numerikus modellezéssel elengedhetetlen. Jelen cikk a stencilnyomtatási folyamat numerikus modellezésének lépéseit részleteiben tárgyalja. A peremfeltételek kritikus részeit, a geometriai valamint az anyagparamétereket egyaránt részletesen ismertetem. Bemutatom, hogy milyen módszerrel lehetséges a stencilnyomtatás numerikus modelljének validálása, a modell érvényességének ellenőrzése. Megmutatom, hogy milyen mértékben csökken a stencilnyomtató-kés szöge a nyomtatás közbeni késerő hatására. Ismertetem, hogy milyen mérési módszerrel mutatható ki a forraszpaszták viszkozitásának csökkenése a stencilnyomtatási folyamat során. Ugyanezen mérési módszerrel kimutatható a különböző szemcseméretű forraszpaszták reológiai tulajdonságai közötti különbség is. A reológiai tulajdonságok modellezésével kapcsolatban taglalom az elterjedt, Cross és Carreau-Yasuda anyagmodelleket. Végezetül ismertetem a gépi tanulás lehetséges alkalmazását a stencilnyomtatási folyamat optimalizálására

Analyzing the overfitting of boosted decision trees for the modelling of stencil printing

Stencil printing is one of the key steps in reflow soldering technology, and by the spread of ultra-fine-pitch components, analysis of this process is essential. The process of stencil printing has been investigated by a machine learning technique utilizing the ensemble method of boosted decision trees. The phenomenon of overfitting, which can alter the prediction error of boosted decision trees has also been analyzed in detail. The training data set was acquired experimentally by performing stencil printing using different printing speeds (from 20 to 120 mm/s) and various types of solder pastes with different particle sizes (particle size range 25–45 μm, 20–38 μm, 15–25 μm) and different stencil aperture sizes, characterized by their area ratio (from 0.35 to 1.7). The overfitting phenomenon was addressed by training by using incomplete data sets, which means that a subset of data corresponding to a particular input parameter value was excluded from the training. Four cases were investigated with incomplete data sets, by excluding the corresponding data subsets for: area ratios of 0.75 and 1.3, and printing speeds of 70 mm/s and 85 mm/s. It was found that the prediction error at input parameter values that have been excluded from the training can be lowered by eliminating the overfitting; though, the decrease in the prediction error depends on the rate of change in the output parameter in the vicinity of the respective input parameter value

X-ray imaging in pin-in-paste technology

Today, pin-in-paste technology is used extensively. With the help of pin-in-paste, through-hole devices can be soldered using a standard surface mount technology (SMT) process and hereby a reduction in wave soldering is possible. This can result in cost savings and a decrease in production cycle time. To ensure successful pin-in-paste soldering the following steps must be taken: solder paste volume calculations for through-hole components; stencil aperture design for the pin-in-paste application; solder paste deposition through stencil printing, application of solder volume increasing techniques after printing; reflow profile optimization; inspections using special methods for each individual process; final tests. The paste is printed into the through-holes. For high quality pin-in-paste solder joints a sufficient volume of paste is a fundamental requirement. Nevertheless, after printing the through-hole-filling is usually unknown. In this paper a new method is described how to accurately determine the volume of solder alloy in solder paste that is present in the through-hole, using X-ray measurements, image processing and calculations. In addition, a method is suggested to determine the measuring characteristics and gray-scale linearity of the X-ray machine

Elektronikai gyártástechnológiákban alkalmazott stencilnyomtatás optimalizálási és modellezési aspektusai

Az elektronikai eszközök automatizált gyártásának legelterjedtebb technológia az ún. újraömlesztéses forrasztás, melynek az egyik legkritikusabb lépése a stencilnyomtatás; a forrasztási hibák akár 60%-a erre a folyamatra vezethető vissza. Jelen cikk célja, hogy bemutassa ezen folyamat optimalizálási és modellezési lehetőségeit irodalmi forrásokra alapozva. Az optimalizálási metódusok közül a régebbi források a DMAIC (Define, Measure, Analyze, Improve and Control) vagy Taguchi illetve RSM (response surface methodology – válaszfüggvényre illesztett felület) módszereket alkalmazták a stencilnyomtatási folyamat tekintetében, mely kutatások eredményeit a 2. fejezet mutatja be. A 3. 4. fejezet a stencilnyomtatás modellezési lehetőségeit ismerteti, valamint ahhoz kapcsolódóan a forraszpaszták reológiai tulajdonságait leíró anyagmodelleket. Végezetül az 5. fejezet ismerteti a stencilnyomtatási folyamat legújabb modellezési és optimalizálási lehetőségeit gépi tanulási módszerek alkalmazásával. Az ún. „zero-defect”, nulla-hibás gyártás eléréséhez elengedhetetlen a gyártási folyamatokhoz kapcsolódó optimalizációs módszerek folyamatos vizsgálata, elemzése, ehhez kíván a jelen cikk hozzájárulni

Gépi tanulási módszerek optimalizálása a stencilnyomtatási folyamat vizsgálatára

Jelen cikk célja, hogy bemutassa az újraömlesztéses forrasztási technológia kritikus lépésének, a stencilnyomtatási folyamatnak a gépi tanuláson alapuló modellezési lehetőségeit. Ismertetjük az egyes gépi tanulási módszerek alapjait, majd pedig a következő gépi tanulási módszerek optimalizálását a stencilnyomtatás tekintetében: mesterséges neurális hálózat, neuro-fuzzy rendszer, szupport-vektor gépek, boost-olt döntési fák. A módszerek vizsgálatához és optimalizáláshoz kísérleti úton nyertük a tanító adathalmazt, melynek bementi paraméterei a forraszpaszta szemcseméretének tulajdonságai, a stencilapertúra mérete és a nyomtatási sebesség. A folyamat minőségét jellemző kimeneti paraméterek pedig a forraszpaszta-lenyomatok területe, magassága, térfogata. Az egyes, gépi tanulási módszerek becslési hibáját az átlagos abszolút százalékos hiba (MAPE – mean absolute percentage error) értékével jellemeztük. A vizsgált gépi tanulási módszerek teljesítményét összességében megfelelőnek találtuk (az átlagos becslési hiba 5% alatti), kivéve a neuro-fuzzy rendszert, melynek alkalmazását nem javasoljuk a stencilnyomtatási folyamat modellezésére

Investigation of the Microstructure of Mn-Doped Tin-Silver-Copper Solder Alloys Solidified with Different Cooling Rates

Due to the moderate price and the non-toxicity, manganese is considered as an ideal dopant for the SAC (SnAgCu) solder alloys. Manganese refines the grain of solder joints, yielding better thermomechanical properties. In present research, the microstructure of the manganese-doped alloys solidified with different technological parameters had been investigated. Sn/Ag0.3/Cu0.7 based solder alloy with three different Mn content (0.1, 0.4, 0.7% wt%) were reflowed on a copper substrate with tempered hot plate. They were solidified with different cooling rates from 0.3 to 4.5 K/s. Cross-sections have been prepared from the solder samples and the metallographic properties of the solder samples was investigated with optical and scanning electron microscopes. The characteristic features of the samples have been compared to conventionally used SAC305 (Sn/Ag3/Cu0.5) solder alloys, solidified with the same rates of cooling. Results showed that besides the grain refinement, the Mn content might also have effect on the evolution of intermetallic layer between the substrate and the solder alloy. The IMC grains of the layer were more elongated and more spalled grains had been observed close to the layer. However, independently of the cooling rate, the microstructure of the Mn containing solder alloys remained the same. This suggests that the macroscopic properties are also expected to be less sensitive to the cooling rate of the solidification

Characterization of Tin Pest Phenomenon in a Low Ag Content SAC Solder Alloy

In the electronics technology, the metallic β-Sn (white tin) is the basic material of solder alloys and surface finishes. The “tin pest” phenomenon is the spontaneous allotropic transition of ß-Sn to the semiconductor α-Sn (gray tin) under the temperature of 13.2°C. In this work, the tin pest susceptibility of the widely used Sn99Ag0.3Cu0.7 solder alloy was investigated and compared to Sn99Cu1 alloy (as a well know reference). Bulk solder bars were prepared by metal casting and the samples were inoculated by InSb, CdTe and α-Sn powders to enhance the allotropic transition process. The inoculator materials were pressed onto the surfaces of the samples by a mechanic lamination. The samples were stored at -10 °C temperature for 8 weeks. The allotropic transition was monitored by optical inspection and by electrical resistance measurements. The microstructural changes of the samples - caused by the transition of crystal structure of Sn – were investigated by metallurgical cross-sections. The results showed, that in all cases the Sn99Ag0.3Cu0.7 solder alloy is much less susceptible to tin pest development than the Sn991Cu, which might be explained by the suppression effect of the Ag alloying. Furthermore, it was found that the process of transition highly depends on the applied inoculator material

Whisker Development from SAC0307-Mn07 Solder Alloy

In this study, the whisker growth from SAC0307-Mn07 solder alloy was investigated. To get fast information, an ultra-thin film was vacuum evaporated from the examined solder alloy onto Cu substrates. The evaporated layer was ~100-150nm thick in average with the same grain size. The layers were kept at room temperature for 28 days. Whisker growth was monitored by scanning electron microscope with some days frequency, and the layer structure under the whisker was examined on focused ion beam cuts. The large mechanical stress on the Sn layer due to the intermetallic layer growth initiated the whisker growth right after the layer deposition. However, it stopped after 2 weeks since the intermetallic grains consumed the ultra-thin Sn layer. The whisker density was extremely high (~24000 pcs./mm2). Most of the whiskers were the short nodule and hillock type, which does not mean risk in microelectronics. Therefore, the alloying of Mn into the high Sn content solder alloys have a positive effect on the reliability of the solder joints

Numerical investigation on the effect of solder paste rheological behaviour and printing speed on stencil printing

Purpose of this paper was to investigate the effect of different viscosity models (Cross and Al-Ma’aiteh) and different printing speeds on the numerical results (e.g., pressure over stencil) of a numerical model regarding stencil printing. A finite volume model was established for describing the printing process. Two types of viscosity models for non-Newtonian fluid properties were compared. The Cross model was fitted to the measurement results in the initial state of a lead-free solder paste, and the parameters of a Al-Ma’aiteh material model were fitted in the stabilised state of the same paste. Four different printing speeds were also investigated from 20 to 200 mm/s. Noteworthy differences were found in the pressure between utilising the Cross model and the Al-Ma’aiteh viscosity model. The difference in pressure reached 33–34% for both printing speeds of 20 and 70 mm/s, and reached 31% and 27% for the printing speed of 120 and 200 mm/s. The variation in the difference was explained by the increase in the rates of shear by increasing printing speeds. Parameters of viscosity model should be determined for the stabilised state of the solder paste. Neglecting the thixotropic paste nature in the modelling of printing can cause a calculation error of even ~30%. By using the Al-Ma’aiteh viscosity model over the stabilised state of solder pastes can provide more accurate results in the modelling of printing, which is necessary for the effective optimisation of this process, and for eliminating soldering failures in highly integrated electronic devices