Contents:

Introduction

These Guidelines are designed to outline best practices to help minimise errors arising during the assembly process.

The world is divided into two parts one part works with the Metric system and the other with the Imperial system when defining measurements, the “µm” (micron) is 25.4 times smaller than the “Mil” (0.001 of an inch).

We decided many years ago to work exclusively in the Metric system to ensure the highest quality possible. Therefore the basis of all measurements given in this document and in our website are Metric and all Imperial values are there for clarification and general understanding ONLY.

As there is no general consensus throughout the global PCB industry on terminology, if we feel any term we use may be unclear we will try to explain it when it first appears.

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BGA/QFN/LGA

This section is under construction.

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BOM & CPL Data

This section is under construction.

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Cold/Hot Spot

#Cold/hot Spot

Hot & Cold Spots are issues during the reflow soldering process of the PCB Assembly and as the term suggests they relate to the temperature of the components or specific areas.

Cold Spot

A Cold Spot is an area that doesn’t get enough heat to activate the flux at the right time and allow the solder to reach molten stage and reflow.

The main reason for a Cold Spot is that Tin Chip components such as resistors are placed next or between large or high bodied components. this cause whats known as a shadowing effect where the larger components effectively shadow the smaller components from the heat in the reflow oven.

To avoid this type of issues ensure that smaller components are not placed too close to larger components.

Hot Spot

A Hot Spot is an area that gets too much heat and results in warped, blistered or even burnt boards.

Generally this is caused by large areas of copper in specific areas, these heat up quicker and hotter than other areas of the PCB and thus temperature across the PCB is unbalanced.

To avoid Hot Spots ensure the copper is balanced across the PCB.

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Component Clearance

This section is under construction.

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Component Pad/Footprint

This section is under construction.

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Component Orientation

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Customer Panel

This section is under construction.

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Fiducials

This section is under construction.

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Overhanging Components

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Selective Wave Soldering

This section is under construction.

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Solder Escape/Wick

#Solder_Escape/Wick

Solder Escape/Wick (also know as Solder Drainage) is where the Solder flows away from the SMD Pad towards a connected copper element, usually Via Hole. However, it may also be a copper pad or area that is not covered or isolated with Soldermask.

If a Via Hole is too close or inside SMD pad, the heat dissipation and surface tension (the meniscus effect, capillary action) may cause the Solder to flow towards the hole and away from the SMD pad.

This would then result in either a poor quality or even a dry solder joints.

Simply increasing the volume of the Solder Paste may sound like a solution but it results in either, other SMD Pads having too much Solder Paste or the need for,  stepped solder paste stencils, larger pads or larger solder paste apertures in the stencil, all of which are undesirable.

Below is a graphical representation of what happens to the Solder during the Reflow process if the Via Hole is too close to an SMD pad and there is no Soldermask web/dam between them.

Issue – Via Hole to Close to SMD Pad

Via Holes that have been placed too close to a SMD pad with No Soldermask web/dam.

Solution 1 – Via Fill with Resin

By using the Via fill with resin technique it ensures that the Via Hole is fully covered with Soldermask and this stops the solder flowing away from the SMD pad.

Solution 2 – Move the Via

It should be noted that there are various articles that discuss how far a Via should be from a SMD pad, however, the consensus seems to be a minimum of 0.250mm from edge of the SMD pad to the edge of the Via pad.

It is also important that by moving the Via to meet this minimum distance also allows for a Soldermask web/dam between the SMD pad and the Via.

Via Hole in SMD Pad

As miniaturisation of electronics continues Via Holes in SMD pads is seen as a solution to gain more real estate on PCB’s. The problem is that Solder will flow from the SMD Pad into the Via Hole resulting in a dry or weak Solder joint of the SMD pad.

Solution 1 – Move the Via Holes

It should be noted that there are various articles that discuss how far a Via should be from a SMD pad, however, the consensus seems to be a minimum of 0.250mm from edge of the SMD pad to the edge of the Via pad.

It is also important that by moving the Via to meet this minimum distance also allows for a Soldermask web/dam between the SMD pad and the Via.

Solution 2 – Via Filling with Resin

By using the Via Fill with resin technique it ensures that Via Holes are either covered fully with Soldermask or copper/final finish ensuring the Solder will not flowing into the Via Hole.

Via Holes under an IC Body

It has become quite common for IC chip manufacturers to use the body to as either a ground connection or as a way to dissipate heat or both.

To achieve this they recommend a copper area under the IC main body with a matrix of Via Holes.

The problem is the body must be soldered to this copper area and if there are open Via Holes then the Solder will escape into them and may result in a poor/weak/dry Solder joint and without X-ray inspection impossible to verify the quality of the solder joint. .

IC chip manufacturers suggest using Soldermask around the Via Holes or tenting them to stop the Solder escaping.

Whilst this sounds practical it has disadvantages, both methods reduce the area of the IC that can be soldered. This may affect the quality of the ground connection or the effectiveness of the heat dissipation.

In reality using Soldermask to stop the Solder escaping is not a practical solution in this type of design.

The only real solution is to use Via Fill with resin which will the allow for the IC body to be correctly soldered to the copper area and thus provides the most effective solution.

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Soldermask Issues

This section is under construction.

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Tombstoning

What is Tombstoning?

Tombstoning (also known as Manhattan effect or crocodile effect) usually affects surface mount passive components such as resistors, capacitor and inductors. It is where one end of the component lifts from a pad of the PCB during the soldering process.


The final angle of the component will vary from a few degrees up to 90 degrees, however, regardless of the angle the result is that one of the component terminals is NOT soldered to the PCB pad and an open circuit is created.

What Causes a Component to Tombstone?

To understand why components Tombstone we must first understand the part of the soldering process known as Wetting.

Wetting is where the Solderpaste becomes fluid or molten and is able to attach itself to the component terminal and to the pad on the PCB. The ideal situation is when all the terminals of a component complete the Wetting process at the same time creating a solid physical and electrical inter-metallic bond.

It is important as when the Solderpaste becomes fluid it applies a pulling force to each of the component terminals, known as the Meniscus Pull, something like a Tug-of-War on the component.

This pulling force has the benefit of helping self-centre the component between its pads if the Wetting process is completed at the same time.

On passive component a significant difference in the completion time of the Wetting process may result in many defects one of which is Tombstoning other include bill-boarding, misalignment, shorting etc. Basically, the pad that completes the Wetting process first will win the Tug-of-War and may pull the component vertically resulting in Tombstoning.

What it really comes down to is the difference in thermal mass between pads of the same component. This will define how quickly the Solderpaste is heated and becomes fluid then, how quickly the heat dissipates and the solder becomes solid.

Whilst there are assembly process issues that may result in Tombstoning we should accept that these are under control by the assembler.

Therefore, in this section we will focus only on the PCB Layout related issues and how to avoid them.

PCB Layouts that Cause Tombstoning

Tombstoning is related mainly to Passive Components with 2 terminals and the goal should be to ensure that the Wetting process for each of the terminals of a component is completed at the same time.

Below are the main reasons that influence the heat dissipation from a component pad and thus the Wetting process.

  • Track Size to the Pad
  • PTH Close to the Pad
  • Pads Connected to Large Copper Areas
  • Via in Pad

Track to the Pad Issues

Avoid multiple tracks to one pad and only one track to the other pad different track widths to pads of the same component, also avoid different track widths to pads of the same component.

                                   
Common Layout                                                                                              Proposed Alternative Layout

A pad with multiple tracks connected to it will dissipate the heat quicker than with the pad with a single track connected to it.

Therefore, the Wetting process will be completed at different times which may result in Tombstoning.

When a pad is connected to Power or GND it is common for larger tracks to be used as above. However, this will result in the Wetting process for the pad with the larger track being completed before the pad with the thinner track and may result in Tombstoning.

                                    

Two possible solutions are shown above.

The aim is to make the track width to each pad of the component the same width. An important point to consider is that this unified track width must be run at least 0.25mm from the pad edge before the width changes to help keep the Wetting process in sync.

PTH Close to the Pad

Any PTH (Plated Through Hole) including Via Holes that are too close to a pad will dissipate the heat quicker during the Wetting process. There should be a minimum of 0.25mm from the edge of the component pad to the edge of the actual hole.

In addition, if the PTH is too close to the component pad to ensure a reliable Soldermask Dam then the solder will flow in to the hole (known as Wicking or Solder Escape).

In both cases this may result in Tombstoning.

                                    

The graphic on the left shows a layout where the PTH is too close to the component pad, the one on the right, shows the PTH with the minimum distance of 0.25mm from the pad edge to hole edge.

The graphic above shows the Via Hole in the component pad, there are 2 possible solutions.

The first is to use Via Filling, this will help reduce the copper area of the pad however, the Via Hole itself will still dissipate heat and the Wetting process may still not be unified.

The second and more reliable solution is to move the Via Hole at least 0.25mm from the edge of the component pad to the edge of the Via Hole.

Pads Connected to Large Copper Areas

                                     

Large copper areas act as Heatsinks and therefore connecting a component pad to one as in the graphics above would most likely result in Tombstoning.

To solve this issue ensure a track of a similar size is connected both pads as in the graphic above. In addition, there should be at least 0.25mm of track before it connects to the copper area, in fact the longer the better.

If high power is involved then use a similar solution as above (Track to the Pad Issues),

Via in Pad

As miniaturisation of electronics continues so does the available real-estate on PCB to place components and run tracks etc.

One solution is to place Via Holes in component pads, however, the Solder will Wick/Escape down the hole resulting in an uneven Wetting Process.

Even if both component pads have Via Holes in them there is still a high chance of one completing the Wetting process before the other depending upon what the Via Hole is connected to and Tombstoning may still occur.

Also with Via Holes in pads there is the issue of having sufficient Solderpaste to fill the Via Hole and still attach to the Component terminals during the Reflow process.

The graphics show Via Holes in component pads, as the Via Holes are unfilled the Solder Paste will Wick/Escape in to the holes and may result in Tombstoning or unsoldered an component terminal.

Via Filling is one solution and will help reduce the risk of Tombstoning.

However, moving the Via Holes at least 0.25mm from the component pad edge to the edge of the Via Hole is a more reliable solution.

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Via in Pad

This section is under construction.

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