Land Pad Research Articles

Analysis of SMT component land pad discontinuity effect on the overall transmission line impedance in high-speed applications

Abstract High-speed and high-frequency PCB interconnect solutions with the slightest discontinuities in the physical geometry along the microstrip transmission line can significantly degrade the integrity of the signal being transferred. This paper presents an analysis of SMT component pads and their impact on the successive transmission line impedance. During 0805, 1206 and 2512 package size SMT component pad discontinuity evaluations it was spotted, that the increasing deviation from target impedance affects the microstrip impedance segment following the discontinuity. A 25 Ω impedance offset at the SMT pad reduces the line impedance afterwards by up to 7 Ω. A 37 Ω impedance offset results in a microstrip impedance reduction by up to 15 Ω, whereas a 45 Ω impedance change leads to a reduction of successive segment impedance by up to 30 Ω. This effect is also present in multiple evenly-spaced discontinuities. But a single reference plane cut-out, which is twice as wide as the SMT component discontinuity, substantially improves the overall impedance of the transmission line, including the successive line segment impedance drop.

Reduced-Reflection Multilayer PCB Microstrip with Discontinuity Characterization

In the era of technology and communication, printed circuit boards (PCBs) can be found in a myriad of devices—from ordinary household items, to state of the art custom metrology equipment. Different types of component for wireless communications are available and come in various packages, supplied by multiple manufacturers. The signal landpads for some high-frequency connectors and components, encapsulated in larger packages, are usually wider than the controlled impedance trace, thereby introducing unwanted impedance mismatch and resulting in signal reflections. The component land pad and microstrip width a discrepancy issue can be found in both complex high-density industrial devices and system-level academic research papers. This paper addresses the topic of compensating discontinuities, introduced by signal pads, which are wider than the target impedance microstrip, characterizes the difference between the compensated and uncompensated microstrip with discontinuity, and proposes a generalized guideline on compensating for the introduced impedance change in multilayer PCBs. The compensation method is based upon carefully designing the stackup of the PCB allowing for a reference plane cutout under the discontinuity to even out the impedance mismatch. A 6-layer PCB with IT180A dielectric material containing three structures has been manufactured and characterized using an Agilent E8363B vector network analyzer (VNA). A 4–12 dB improvement in S11 response in the whole frequency range up to 10 GHz, compared to that when no compensation has been applied, was observed.

The effect of PCB surface finish on lead‐free solder joints

PurposeTo study the behaviour of voids in PBGA solder joints and their influence on the lifetime of lead‐free solder joints.Design/methodology/approachThe behaviour of voids was studied using micro via and land pad PWBs, PBGA components, and by measuring voids in the solder joints. The lifetimes of solder joints were tested using accelerated temperature tests.FindingsNumber of factors affecting the solder joint lifetimes were found. The voids were discovered to have a significantly large influence on the solder joints.Practical implicationsThe findings can be used to achieve better soldering results, methods, and designs.Originality/valueIn this paper, the effect and the behaviour of voids were studied profoundly. The findings can be valuable to researchers and process personnel.

Lift-off phenomenon in wave soldering

The mechanism of lift-off, which is the serious defect formation that the solder fillet is peeled off from a Cu land pad on a printed wiring board in wave soldering, has been examined primarily by using basic Sn–Bi alloys (Sn at 2–5 wt%) and solidification simulation. The probability of lift-off increases with increasing Bi content. Lift-off results from the segregation of Bi, which diffuses in a short distance, in the interfacial region between a Sn–Bi alloy and a Cu land pad during dendritic structure formation. A higher fillet results in a more severe lift-off—the top side of a printed wiring board, where the fillet is low, seldom shows lift-off. The solidification simulation indicates that the solidification of the solder fillet rapidly propagates from a lead wire to a fillet edge. A Cu land acts as a heat sink, with heat flowing from the inner through-hole to the land pad. Both rapid cooling and annealing at high temperature can effectively prevent lift-off.

Reliability studies of surface mount solder joints - effect of Cu-Sn intermetallic compounds

Cu-Sn intermetallic compounds (IMCs), formed at the interface between the solder and the copper substrate are found to play a dominant role in determining the thermal fatigue life of surface mount solder joints fabricated from a conventional infrared reflow process. In order to predict the growth of this IMC layer during the operating life of the solder joint and its effect on the thermal fatigue life, the formation characteristics of the IMC's in 0805 and 1206 LCCC solder joints are systematically studied in this investigation. Only the stable Cu/sub 6/Sn/sub 5/% /spl eta/-phase intermetallic compound was observed in all as-solidified solder joints as confirmed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). The mean layer thickness was found to increase almost linearly with reflow time up to about 200 s. The thickness of the interfacial IMC layer increased with increasing reflow temperature for 0805-type solder joints up to around 250/spl deg/C and reached a saturated thickness of 2.5 /spl mu/m beyond this temperature. Additional intermetallic formation due to higher reflow temperature or longer reflow time would appear as Cu-Sn whiskers in the bulk solder of the joint. The copper land pad size and quality of component lead metallization were also found to greatly affect the formation of Cu-Sn IMC in surface mount solder joints, and hence its reliability in terms of thermal fatigue life and mechanical properties.