As the IoT market demands higher data rates and processing,the PCB technology is driving for smaller form factors,higher signal densities,and advanced material solutions. Miniaturization has been a common trend. Besides reduction in transmission-line and space widths,higher density and bump pitches down to 200 microns,as well as smaller diameter microvias down to 50 microns will be required. The finer features challenge suppliers of equipment,chemistry and materials to find solutions to support the PCB fabrication requirements. This presentation addresses the science and technologies developed in collaboration with supply partners to understand,develop and deliver a 10-layer“substrate-like PCB+” (SLP+) with50 micron diameter stacked microvias,transmission line and space of sub-25 microns and full array 200 micron pad pitch circuit technology solution to enable the next generation of IoT PCB requirements. This SLP+ technology goes beyond the MSAP/SLP technology that several suppliers are currently implementing. The sub-25 µm line and space is achieved with a modified semi-additive process (MSAP) that electroplates copper through openings in the dry-film,instead of using the dry-film as an etch resist. The initial SLP+ designs with leading smartphone customers and M2M modules are in the development phase. The extension of this technology drives feature sizes down to sub-25 micron line/space and 200 micron solder-bump pitch and below. This technology allows down to 200 µm pitch ICs to be directly attached to the PCB,thereby eliminating the need for a BGA substrate. Without the BGA substrate,reductions in z-height are achieved. The thermal resistance to the PCB plane and any rear heatsink is reduced by eliminating the BGA substrate. Similarly,signal integrity is improved by removing impedance discontinuities at the additional interfaces of the BGA substrate. Consumers will see their devices shrink beyond the current state-of-the art,and with performance improvements. The presentation will demonstrate ‘advanced high-density interconnect’ produced in high volume in collaboration with suppliers. The interconnect solutions include a single line between pads at 200 µm-pitch,and two lines between pads at 250 µm-pitch. Thermal cycling (air-to-air and liquid-to-liquid),and 10X solder reflow reliability test results will be presented.

Misregistration of holes is the maximum amount of variation between the centerlines of all terminal pads within one plated through hole in a printed circuit board. The impacts of misregistration can be very serious due to possible electrical opens caused by breakouts or a short or intermittent connection due to a violation of the minimum clearance. This study highlights the differences in registration in Printed Circuit Board (PCB)samples made by the same supplier at two geographical locations. The intent is to determine a statistical correlation between the populations of registration error measurements taken on samples,from layer-to-layer or across layers,and reliability or performance risk. The study includes comparisons between PCB samples made in both the locations. The focus of the comparison includes studying whether both populations reflect the same statistical results,trying to understand where the difference in population occurred and maybe answer the question -which population has the lower amounts of misregistration between layers. Misregistration occurs during board fabrication and is directly attributed to problems involved with the production of the artwork,with artwork materials,with the setup procedures during lamination,and/or with the dimensional instability of the laminate materials used.

Sixteen simple inorganic salts were separately dissolved in water to a specific concentration,applied to separate Surface Insulation Resistance (SIR) coupons,dried and then the coupons were subjected to common SIR testing. A correlation between the final SIR resistance readings and the hydrated radii and ionic charges of the salts has been found. Squares of bare FR4 were immersed in more concentrated solutions of the same salts,rinsed,dried,ground up,leached and the concentrations of the liberated salts were obtained by ion chromatography and inductively coupled plasma optical emission spectroscopy. The results show some discernible correlations with the SIR results.

Rosin-based no-clean solder pastes are the most widely used solder pastes in the world and dominate consumer electronics assembly. The most common rosin-based no-clean solder paste reflow environment is air,which has some obvious advantages such as no-cost and tombstoning mitigation. However,there are a number of manufacturers that use a nitrogen reflow environment for reasons ranging from “shinier” solder joints to head-in-pillow (a.k.a. head-on-pillow) defect mitigation. Most solder paste manufacturersperformJ-STD-004 qualification tests in air unless the solder paste specifically requires a nitrogen reflow environment. Of the J-STD-004 tests,the Surface Insulation Resistance (SIR)test is designed to predict the electrical reliability of a no-clean flux residue. In this test,solder paste is printed onto a test coupon and reflowed. This reflow is typically conducted in air,especially for a rosin-based no-clean solder paste. Knowing that the SIR values are typically the result of a reflow heating cycle conducted in air,some have asked if and to what degree would the SIR values be different if the reflow heating cycle had been conducted in nitrogen. This paper will attempt to address this concern. For this work,three commercially available SAC305 Type 4 rosin-based no-clean solder pastes were used: a halogen-free (ROL0) paste with a traditional residue; a halogen-free (ROL0) paste with a residue optimized for pin-probing; and a halogen-containing (ROL1) paste with a traditional residue. These three different solder pastes were used to see if different chemistries respond differently to a nitrogen reflow environment. These solder pastes were subjected to a total of four different reflow profiles: a nitrogen ramp-to-peak (RTP) profile,a nitrogen soak profile,an air ramp-to-peak (RTP) profile,and an air soak profile. Per J-STD-004B,IPC-TM-650 2.6.3.3 and 2.6.3.7 are the instructions used for SIR testing. In these procedures,solder paste is stencil printed on to IPC-B-24 SIR boards using a .006”/150µ thick stencil. Each B-24 SIR board has four SIR patterns; A through D. Three SIR boards are prepared per paste per scenario. Also,two controls are prepared. The SIR data from these boards will be shared and compared.

Prior to any conformal or chemically vapor deposited coating process,specific areas and components-often called “keep out zones”-on the printed circuit board (PCB) assembly are generally masked to shield them from exposure to coating materials. There are several approaches to masking and,traditionally,this has been achieved through the use of tapes (applied manually),UV curable masking materials,latex-based products and,more recently,a dispensable,peelable hotmelt material. Masking materials,however,can leave residues. These residues may have to be cleaned as there could be a risk of corrosion,electrochemical migration and/or parasitic current leakage. This paper will evaluate polyimide tape,UV curable,latex-based,and hotmelt masking materials and compare residue production of each,in addition to testing the products in relation to performance with no-clean solder paste residues. Results of surface insulation testing (SIR) per IPC TM-650 2.6.3.3 along with Fourier Transformed Infrared Spectroscopy (FTIR) analysis of material residues will be presented.

Since the introduction of lead-free soldering technology,voids in solder joints have been a topic of intensive investigations and discussions. One issue of detailed investigations is the formation mechanism of voids in solder joints. The objective of these activities is the assessment,if and how voids can be reduced to a low or at least acceptable level. The second question for deeper analyses is the effect of voids on solder joint reliability. If the potential for elimination or reduction of voids in solder joints is found to be low,this question comes automatically into focus. Especially within the automotive industry or other business sectors with high reliability requirements for electronic assemblies the relevance of voids for product reliability is still a matter of debate. This is why this paper tries to give a general overview of mechanisms governing void formation,an assessment of the relevance of different influencing parameters on void formation as well as an assessment of the effects of voids on solder-joint reliability. Widely known facts or findings from other studies are combined with additional studies on issues of special interest. The whole paper wants to explain current understanding of solder joint voiding,primarily as an insight on this topic from the perspective of an automotive supplier,but in a next step also as a statement and input for further discussions within scientific and technological working groups dealing with this issue.

Voids are a plague to our electronics and must be eliminated! Over the last few years we have studied voiding in solder joints and published three technical papers on methods to “Fill the Void.” This paper is part four of this series. The focus of this work is to mitigate voids for via in pad circuit board designs. Via holes in Quad Flat No-Lead (QFN)thermal pads create voiding issues. Gasses can come out of via holes and rise into the solder joint creating voids. Solder can also flow down into the via holes creating gaps in the solder joint. One method of preventing this is via plugging. Via holes can be plugged,capped,or left open. These via plugging options were compared and contrasted to each other with respect to voiding. Another method of minimizing voiding is through solder paste stencil design. Solder paste can be printed around the via holes with gas escape routes. This prevents gasses from via holes from being trapped in the solder joint. Several stencil designs were tested and voiding performance compared and contrasted. In many cases voiding will be reduced only if a combination of mitigation strategies are used. Recommendations for combinations of via hole plugging and stencil design are given. The aim of this paper is to help the reader to “Fill the Void.”

Silicone thermal interface materials (TIM) are used to bond heatsinks on many critical components in server card assemblies in typical industry systems. Silicone TIMs have excellent high temperature mechanical stability and resilience. Applications with operating temperatures above 90 degrees Celsius (C) can exhibit mechanical degradation with some acrylic or epoxy TIM adhesives. Silicones also have a low elastic modulus which enables reliable bonding of materials with differing coefficients of thermal expansion (CTE’s). Silicone TIM adhesives generally require thermal cure activation,and as such are becoming increasingly difficult to use on some applications with temperature sensitive components (TSC) on printed circuit board assemblies (PCBAs). Future applications are driving the need to find the minimum cure temperature coupled with the minimum acceptable cure strength. With a silicone thermal interface material adhesive,one can optimize the cure strength based off the combination of cure temperature and surface roughness. This work focuses on varying multiple thermal bonding contributors in order to find the best combination for specific applications. One goal of this work was to provide a roughness parameter that can be used to offer optimal heatsink surface characteristics for consistent TIM bonding. Copper and aluminum are the most common heatsink materials,therefore silicone TIM was applied between these material surfaces and bond strength was evaluated by shear testing. The parameters evaluated that affected bond strength had a range of surface roughness,cure temperature,and cure time settings. Surfaces were prepared with sandpaper of various grit values. The resulting surfaces were characterized to determine average surface roughness (Ra) and peak count (RPc). Surfaces were also evaluated using scanning electron microscopy. All of the data shows that roughness has the greatest effect on resulting bond strength,followed by cure temperature. Cure time had a minimal effect on the final results as long as the cure time was greater than the minimum recommendation provided by the TIM supplier. The results show that samples with a coarse grit sanded finish did not perform well,despite the relatively rough appearance (and high Ra value). The smoother appearing,fine grit sanded finish provided the best bond strength. The smoother grit finish had a higher micro-roughness (as measured by the RPc value) than the coarse grit. In addition,a higher cure temperature produced a higher bond strength,as expected.

Knowing how package warpage changes over temperature is a critical variable in order to assemble reliable surface mount attached technology. Component and component or component and board surfaces must stay relatively flat with one another or surface mount defects,such as head-in-pillow,open joints,bridged joints,stretched joints,etc. may occur. Initial package flatness can be affected by numerous aspects of the component manufacturing and design. However,change in shape over temperature is primarily driven by CTE mismatch between the different materials in the package. Thus material CTE is a critical factor in package design. When analyzing or modeling package warpage,one may assume that the package receives heat evenly on all sides,when in production this may not be the case. Thus,in order to understand how temperature uniformity can affect the warpage of a package,a case study of package warpage versus different heating spreads is performed. Packages used in the case study have larger formfactors,so that the effect of non-uniformity can be more readily quantified within each package. Small and thin packages are less prone to issues with package temperature variation,due to the ability for the heat to conduct through the package material and make up for uneven sources of heat. Multiple packages and multiple package form factors are measured for warpage via a shadow moiré technique while being heated and cooled through reflow profiles matching real world production conditions. Heating of the package is adjusted to compare an evenly heated package to one that is heated unevenly and has poor temperature uniformity between package surfaces. The warpage is measured dynamically as the package is heated and cooled. Conclusions are drawn as to how the role of uneven temperature spread affects the package warpage.

This paper focuses on three different coating material groups which were formulated to operate under high thermal stress and are applied at printed circuit board manufacturing level. While used for principally different applications,these coatings have in common that they can be key to a successful thermal management concept especially in e-mobility and lighting applications. The coatings consist of: Specialty (green transparent) liquid photoimageable solder masks (LPiSM) compatible with long-term thermal storage/stress in excess of 150°C. Combined with the appropriate high-temperature base material,and along with a suitable copper pre-treatment,these solder resists are capable of fulfilling higher thermal demands. In this context,long-term storage tests as well as temperature cycling tests were conducted. Moreover,the effect of various Cu pre-treatment methods on the adhesion of the solder masks was examined following 150,175 and 200°C ageing processes. For this purpose,test panels were conditioned for2000 hours at the respective temperatures and were submitted to a cross-cut test every 500 h. Within this test set-up,it was found that a multi-level chemical pre-treatment gives significantly better adhesion results,in particular at 175°C and 200°C,compared with a pre-treatment by brush or pumice brush. Also,breakdown voltage as well as tracking resistance were investigated. For an application in LED technology,the light reflectivity and white colour stability of the printed circuit board are of major importance,especially when high-power LEDs are used which can generate larger amounts of heat. For this reason,a very high coverage power and an intense white colour with high reflectivity values are essential for white solder masks. These "ultra-white" and largely non-yellowing LPiSM need to be able to withstand specific thermal loads,especially in combination with high-power LED lighting applications. The topic of thermal performance of coatings for electronics will also be discussed in view of printed heatsink paste (HSP) and thermal interface paste (TIP) coatings which are used for a growing number of applications. They are processed at the printed circuit board manufacturing level for thermal-coupling and heat-spreading purposes in various thermal management-sensitive fields,especially in the automotive and LED lighting industries. Besides giving an overview of the principle functionality,it will be discussed what makes these ceramic-filled epoxy-or silicone-based materials special compared to using "thermal greases" and "thermal pads" for heat dissipation purposes.