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The Self-Alignment of Reflowed Surface Mount Components

Misplacement of surface mount components by a pick and place machine or a human assembler can be inconvenient.  Fortunately, surface mount components seem to magically (and conveniently) align themselves when they go through the reflow oven. What is the cause of this favorable phenomenon? Well it’s as intuitive as being on a trampoline.

When you stand on a trampoline, the fabric bends down and exerts an upwards elastic force. You remain still because that force is balanced with the downwards gravitational force. You’ll also feel yourself leaning towards the center of the trampoline, as that is the point at which your weight would be equally distributed across the entire surface of the trampoline. Nature mostly tends towards equilibrium.  Now imagine instead of being on a trampoline, you’re on a square platform that is being balanced on four separate trampolines. Assume that none of the trampolines have a frame you could hit, and are placed symmetrically at the corners of your square platform. No matter how you jump and try to spin the platform, the corners will slide towards the center of each trampoline when you stop.

Tech Notes (The Self-Alignment of Reflowed Surface Mount Components Figure 1)

This is similar to what’s happening in the reflow oven, except instead of a platform and trampolines, it’s your surface mount component and solder deposits. When the solder turns liquid, it acts like a trampoline because of surface tension. The component is then aligned to the position which spreads its weight equally across all of the deposits. There are of course other forces at work (hydrostatic and capillary pressure, dynamic friction, gravitational), but surface tension plays the primary role in aligning the component.

When the solder becomes a liquid, it attempts to minimize its surface area (this is why it has a surface tension in the first place). The reason behind this is because all of the molecules try to have the smallest amount of free energy, which is when they are connected to/completely surrounded by other identical/similar molecules. This means that the molecules in the bulk of the material have less free energy than the molecules on the surface of the material (if the reverse were true, materials would tend towards forming surfaces rather than being in bulk). When the solder reflows, it is already inherently in favor of bonding with an identical/similar material in order to minimize the number of molecules that are on the surface and thus decreasing its free surface energy. The substrate of the QFN is made from ceramic, which holds its atoms together using ionic and covalent bonds (much stronger than metallic bonds). Trying to get the solder to bond to ceramic is like trying to mix a liquid with a solid; you might get a few drops to stick, but most of it will slide right off. On the other hand, bonding metal together is like mixing different liquids; some will separate, but most mix easily.

To what extent can the component realign itself? The answer to this question will vary depending on the geometry and size of the component. A quick experiment using mixers in 3mm (0.114”) QFN packages showed that components were able to realign themselves from up to a 9-degree offset and 0.2 mm (0.008”) horizontal translation. The next experiment showed that that a 10-degree angle could also be corrected, whereas a 12-degree angle would be more disorientated. Furthermore, any translation over 0.3 mm (0.012”) wouldn’t be realigned. What’s the significance of these values?

Tech Notes (The Self-Alignment of Reflowed Surface Mount Components Figure 2)

The value for translation is fairly easy to explain. Since the width of the pads on the QFN is 0.3 mm (0.012”), anything over 0.3 mm would mean that there is no contact between both the top and bottom pads (labelled 1 and 2 in figure 3) and the solder. Going back to the trampoline analogy, that would be like moving the platform so far to the left that the two trampolines on the right were no longer part of the system. Assuming that the platform can’t fall, it would be rebalanced on the two trampolines to left. With the QFN there are still four traces in contact with the pads, but the top and bottom pads (labeled 3 and 4 on figure 3) are not the correct ones and due to adhesive forces, pull the package off the trace on the right, and the balance is redistributed across three points of solder.

Explaining the value for the angle isn’t as simple, but still fairly intuitive. At a 10-degree angle, the surface area of the QFN’s middle pad that is overlapping with the trace is greater than the outer pad’s overlapping surface area. As a result, more of the solder adheres to the QFN’s middle pad and cohesive forces between the solder molecules pull it further into the correct position. In the case of the 12-degree angle, the overlapping surface area of the outer pad is greater than the middle pad, and so the QFN is pulled into a position that maximizes the area of contact between the outer pads and the trace.

Tech Notes (The Self-Alignment of Reflowed Surface Mount Components Figure 3)

Although we didn’t run experiments with other surface mount components, we can extrapolate from these results that self-alignment will be affected by the size of your components and traces, the number of traces required for that component, and the component’s substrate. Our QFNs have a large potential for self-correction, since they’re relatively small, have four symmetrical traces that act as alignment points, and are on a ceramic substrate that doesn’t bond well with solder. On the other hand, a larger component with only three traces on a metallic substrate would likely have a smaller potential for self-correction. What does this mean for your assembly process? If you’re not already using a reflow process, you should consider it. Not only is it forgiving in terms of component misplacement, but the tendency towards self-alignment can also reduce reflection losses from pad to trace misalignment. The relatively large margin for error shouldn’t affect the quality of your process (it doesn’t here at Marki), at least you can rest easy knowing that a small hitch usually won’t have catastrophic results when it comes to surface mount assembly.

Determining how a component will perform for your application using only a datasheet is difficult and unreliable. If you’re interested in seeing how Marki products would actually fit into your system, contact [email protected] to obtain some of our multicomponent evaluation boards. With a diverse range of configurations made up of bias tees, amplifiers, and mixers, you can find the optimal solution for your needs.

Tech Notes (The Self-Alignment of Reflowed Surface Mount Components Figure 4)

Repeatability of T3 Mixers and Other Handmade Microwave Components in Six Charts

T3 mixers are the highest dynamic range mixer available. They are also handbuilt parts, subject to unit to unit and lot to lot variability. In this blog post we attempt to quantify that variability. Our sample is 10 T3-08LQP mixers from 5 different date codes. All the date codes are separated by at least a month, totaling nearly two years.  Therefore, the variation you see in the plots below accurately represent the variation a designer could expect across two years in the life of their product. Of course there are always outliers, but the following represents typical performance variation.

Conversion Loss

T3 Conversion Loss Variability

Time domain vs. Frequency domain measurements


At Marki, as in much of the microwave industry, we tend to focus on frequency domain measurements. This includes S-parameters (insertion loss, return loss, isolation, rejection), parameters that can be calculated from S-parameters (amplitude balance, phase balance, common mode rejection ratio, directivity), and related parameters (group delay flatness, differential group delay). These are the parameters that are most interesting for applications such as radar, narrow band communications, and electronic warfare.

In the signal integrity industry, the people that design the high speed signaling hardware that powers backplanes on major computing systems powering the internet, among other applications, time domain is king. These engineers generally care about one thing: the data throughput that a system is capable of. Components are evaluated on the basis of whether they can successfully transmit the data. For this reason the primary metrics are eye diagrams, error vector magnitude plots, and bit error ratio measurements.

Each approach has its benefits and drawbacks, and it is important for a well rounded engineer to master both domains. Think of time domain as an aerial view and frequency domain as a microscope. The drawback of frequency domain measurements are that they can be too specific, and you can miss the forest for the trees. You know the 3 dB rolloff is at 7 GHz, and the return loss has a troubling hump at 3 GHz, but will that sink the ship or not?

In time domain measurements, you immediately see the big picture, but if anything is wrong it is difficult to diagnose. For example, a ringing in the eye could be from many sources, and there may be no way to isolate the source without resorting to frequency domain measurements. Here are a few eye distortions, and what the potential sources might be. First, here is what your basic eye diagram looks like, as plotted by microwave office:

basic eye

 

It is perfectly square and with minimal ringing because I selected many sample points. If you select fewer points, you get an eye that looks more realistic, like this:

undersampled eye

 

Which is the same way that it looks

Filtered Eye

 

after you filter it with a filter that looks like this

filter responseexcept with some ringing due to Gibb’s phenomenon, a result of the limited high frequency content in the signal. Ringing is among the most common phenomena experienced in a lab, but it is generally harmless, since it does not introduce noise near the sampling point. In fact, a signal can be filtered very aggressively without affecting the sampling point:

Aggressively filtered eye

as shown in this example with a 500 MHz filter (1/2 the datarate). On the other hand, high pass filtering will cause baseline wander, which shows up in eye diagrams as a split in the different levels of eyebrows that is dependent on the coding scheme of the PRBS or data sequence:

high pass filtered 128 bit eye

high pass filtered 1024 bit eye

 

In the 128 bit sequence, there are discrete levels that result from each bit sequence. If this bit sequence repeated then it would always look like that. The eyebrow is not necessarily from random noise, as it appears, but from deterministic artifacts of a given bit sequence. This is why coding like 8B/10B and 64B/66B can be used on lower bandwidth hardware: the low frequency components are not distorted because they are not present.

What we’ve covered so far is only half the story. There’s still phase/group delay and timing variations, which are usually much more important than amplitude considerations, but that will have to wait for another post.

 

 

What’s the deal with S-parameters?

As a relative newcomer to microwave and RF, there are certain things that the industry takes for granted that I find very weird when I first encounter them. It took me a long time to really understand the concepts of isolation and directivity, for example, and why do 2.4 mm connectors look just like SMA connectors when they don’t fit together? Couldn’t we color code them or something?

S-parameters are one of these things. I know from working with our datasheets and test data that the S parameters represent power loss. For example, and insertion loss of 3 dB corresponds to a power loss of ~50%. This is because converting 10 dB to linear units, you use the formula 10^(-3dB/10) = .50118723362727, the point is that 3 dB is approximately a 50% power loss, though not exact. There is no fundamental reason this is so, it just conveniently happens to be about the same.

According to our microwave bible, “Microwave Engineering” by David Pozar, the scattering matrix [S] is defined according to the forward and reverse voltage waves as [V-] = [S][V+]. A specific element of the matrix is defined as a formula, but the formula states roughly that “Sij is found by driving port j with an incident wave of voltage V+j, and measuring the reflected wave amplitude, V-i, coming out of port i. This is the definition that is used in all of the books and papers that I have seen using scattering parameters. 

Here is the tricky part, that everyone in the microwave industry takes for granted, and has been hazy for me for a while. If the S-parameters are defined in terms of voltage, than a 3 dB reduction should mean a 50% reduction in voltage, equivalent to a 75% reduction in power. A 3 dB loss (or gain) however never means a 75% reduction in power, or a 50% reduction in voltage. This is because:

S parameters in linear units always refer to the amplitude (voltage or current), while S parameters in logarithmic (dB) units always refer to power.

There, I said it. It is one of those things that is usually gets lost or taken for granted going from the classroom to the lab. It’s like amplitude balance: even though it is called amplitude balance, it refers to the the difference in power between two outputs, and it is quoted in dB. So when calculating between linear and logarithmic S parameters, you have to use 10^(X dB/20), while for power it is 10^(X dB/10). It’s weird, and you just have to get used to it.

I hope this clears things up for some poor students out there. Good luck on your finals and happy late spring break!

 

Marki Microwave Passives in a Single Chart

I was thinking about the difference between power dividers, baluns, and couplers, and realized that they could all be thought of as power splitters. The characteristics that make them different are the relationship between the outputs in terms of amplitude, phase, and attenuation between outputs. Here is a brief chart that explains them all:

Passives Overview Chart

Phase Delay vs. Group Delay

When you master phase, you become like a God, capable of performing wonders that mere mortals can only dream of. Wonders like making laser beams (using phase engineered quarter-wave reflectors), communicate tremendous information great distances through thin air (all modern communication formats use both amplitude and phase), and create amazing products (balanced amplifiers, balanced mixers, phased array antennas, Mach Zender modulators, the list never ends).

BUT…phase is the hardest thing to understand in microwaves, RF, and photonics. It is hard to measure, hard to visualize, and makes some very confusing homework problems that kept me in the late night coffeeshops of Champaign-Urbana well past my bedtime.

In this post we will make a dent in the universe of phase understanding by clarifying the difference between phase and group delay, and in the process explain why you can’t match phase with variable line lengths. When you buy a phase shifter, it is sometimes what I would call a real phase shifter, and sometimes what I would refer to as a ‘group delay shifter’. The trombone type variable delay lines (we like the ones from sage) are actually variable time delay elements, and not phase shifters.

A group delay (or time) shift is easy to understand: it is how long the pulse (or wave) takes to arrive at your measuring receiver. Differential delay is therefore the difference in how long it takes for two pulses or waves to arrive. In passive components it is just the distance divided by the speed of light (or whatever your wave is) at your frequency in your material.

Phase is much more difficult. It is the integral of group delay over frequency (plus an offset), or differently the group delay is the derivative of the phase vs. frequency. This is why filters can be used as time delays; the edges of the filter have significant phase variation that leads to significant group delay variations over a narrow bandwidth (this is called Kramers-Kronig relation).

A variable length delay line, therefore, can only change the phase by changing the group delay. But by changing the group delay, you are changing the integral (slope) of the phase vs. frequency. This means that the phase change will be different at different frequencies. This is very different than what you get from a quadrature hybrid coupler, or a balun, where the phase shift is constant across frequencies. The difference is shown below. First is a plot of the phase difference between the two outputs of a BAL-0520 Balun (180°), a QH-0226 quad hybrid (90°), a coupler plus two 37.5° Schiffman phase shifters we developed as a custom (165°), a PD-0220 wilkinson power divider (0°), and a PD-0220 with an extra .570″ adapter on one side (variable).

Phase Differential of various components

As you can see, the phase is flat across the bandwidth of the device for everything except the PD-0220 with the extra delay line (adapter). This has a rapidly changing phase across frequencies. If we take the derivative of this we should get the group delay, but instead I measured the differential group delay with the PNA-X.

Differential Group Delay of Various Components

 

Here you can see that the differential group delay between outputs for each of the devices is 0, except for the power divider with the adapter, which has a flat constant group delay (ignore the big hump, I think that is from the calculation the PNA is doing with the phase flip).

So what is the lesson? You can phase match two outputs using a variable delay line, but only at a single frequency. Otherwise you have to do it with a coupler, a balun, a Schiffman, or some other true variable phase circuit.

Moisture Sensitivity of Marki Surface Mount Components

We frequently receive inquiries about the moisture sensitivity of our surface mount components.Technically, all of our components are moisture sensitivity level 1 (MSL1) according to the  IPC/JEDEC’s J-STD-20, meaning that they can be mounted and reflowed an unlimited amount of time after they are removed from the packaging. This is not the complete story however.

Because of the unique, high frequency construction of our surface mount devices, the lid provided for them is only a dust cover, epoxied on. For this reason it is not sealed against aqueous solutions, and special precautions need to be made.

All sensitive elements inside of the units are sealed independently, and the circuits themselves are not moisture sensitive. However, during the aqueous wash process the solution will penetrate the components and therefore will require some time to dry.

For this reason, Marki Microwave suggests that all surface mount components be subject to a vacuum bake at less than 120° C for one hour.

The important element in this is the vacuum, which will cause the solution inside to evaporate and evacuate the dust cover. A lower temperature or lower vacuum can be used, but it must be subject to the process for a longer time. If the part is tested within 24 hours after an aqueous wash, then the performance will be affected by the solution still inside the dust cover.

 

Marki Microwave Surface Mount Guide

Microwave designs are pushing towards an all surface mount future, due to reduced assembly complexity and cost and reduced system size. Marki Microwave is pushing this technology by offering many of our designs in surface mount packages. The main question for a microwave surface mount package is the transition: how do you bring the signals from the board into the component, and back? The component must be on a substrate of some sort, and a straight wire up the side of the substrate will not generally be 50 ohms. The trick is to maintain a broadband 50 ohm impedance through the transition. Marki uses three different techniques to achieve this:

EZ6 T3A Top and Bottom

Image of T3A-EZ package

EZ eyelet – The EZ package uses a Marki proprietary technology we call an ‘eyelet’. This is a 50 ohm transition built into a metal carrier to allow a transition to a taller base height for the substrate. A suspended substrate is then attached to the carrier, allowing for surface mount suspended substrate mixers. This is the best method for transitioning to a suspended substrate part available.

Product Lines: Mixers (M1, M3, M4), Amplifiers, Doublers

Carrier MaterialTin/Lead(85/15) plated Brass

Max Recommended Frequency: 20 GHz

Suspended Substrate Compatible: Yes

Lead-Free Option: Because of the construction of the EZ eyelet, it cannot be built with lead free solder. For this reason the M1, M2, and M3 mixers are not available as lead-free surface mount units.

429-Edit

Image of T3-CQ package

CQ and CQG Castellation Via – A castellation via is a method of creating a transition by making a plated through hole in a circuit and cutting the through hole in half. Unlike in an EZ eyelet, it does not have a ground that transitions with it, so the transition appears as an inductive line. Further, it is limited to certain materials that are capable of supporting the construction. It is however very robust and is a visible transition, making it easier to inspect the solder fillets.

Product Lines: T3 Mixers

Carrier MaterialFR4

Max Recommended Frequency: 16 GHz

Suspended Substrate Compatible: No

Lead-Free Option: Yes (CQG)

E_sp_bt_02Bias Tee SM Package

Plated Through Hole – This is a standard way to achieve surface mount transitions. It can be performed with a wider variety of materials, including thinner and lower dielectric materials.Because it is used with thinner materials, it is typically only associated with smaller parts, to keep the board flat and prevent it from warping during assembly. Because it is used with thinner materials the transition can appear less inductive, thus performing to higher frequencies.

Product Lines: Bias Tees, Power Dividers, Diplexers

Carrier MaterialPTFE

Max Recommended Frequency: 35 GHz

Suspended Substrate Compatible: No

Lead-Free Option: Yes (SMG)

Marki products listed on Google Shopping for Suppliers!

We are happy to announce that you can now search our product catalog on Google’s shopping for suppliers at http://www.google.com/shopping/suppliers.

We are excited to be selected as a pilot company for this innovative new way to shop that we expect to be extremely useful for our customers in the future.

Challenges in Space Qualified Hardware

Frequency Synthesizer for JPL

Space is a really tough environment. Electronic parts must suffer massive shock and vibration during launch and often see wide temperature swings as satellites are heated by the sun then slip behind the earth into very cold outer space. Most space parts must tolerate a vacuum and heat must be managed carefully. Radiation hardening is critical as a spacecraft is bombarded by a merciless sea of high-energy particles. Parts must be clean and outgassing limited, to ensure that camera lenses are not clouded and there is little tolerance for repairs. Designing and building parts for space requires tenacity and a commitment to process… from managing ESD, testing, and training to handling analyses correctly and efficiently.

Procurement specifications for space are usually written for the requirement at hand. A scientific mission with a tiny budget will rely on the expertise of the manufacturer to assist in defining the technical details of the part and testing. A commercial satellite will have more detailed specification often defined at the satellite level, rather than for the particular assembly required as part of the satellite. Costs can mount as program managers sift through technical data and handle all contingencies. And missing a deadline is not an option.

Standardizing specification around existing MIL standards and specifying hardware already designed for space can reduce costs, improve lead times and overall, improve the quality of life. A paper that I delivered at PTTI in Reston, VA provides more details on the benefits of this approach. Read Paper