25 May Printed Circuit Board Fill and Aligning Expectations
In this series of blogs, we look at some of the questions that are often asked by customers, dispel some of the myths about the electronics manufacturing industry, and provide examples of problem solving.
This piece will look at why 100% through-hole filling is sometimes unachievable and how components can affect this.
The need for 100% through-hole filling
100% through-hole filling is a technique that many of our customers think is a necessity. In actual fact 100% through-hole fill is often unachievable and unnecessary. Initially, the desire for 100% filling came from the late 60s, when the early solder standard MIL-STD-454 Req 5 came into force. The standard set certain rules, some of which that are still enforced today. These rules include the 25% recession rule, which provided clear guidance on tackling a critical reliability concern by ensuring that there was 360o wetting at the upper edge of the hole wall.
There was a good reason for this. During soldering, the PCB expands in the Z direction, and pushes upwards on the top side pads. Those pads could then give way at the knee bend of the pad-to-hole barrel, which would cause a separation or crack. During the soldering process, the crack would thermally isolate the top side pad from the soldering heat and stop the solder filling the hole. The pad-to-barrel crack would then reconnect when cooled that would, over time, leave a potential open circuit condition. The standard mandated 360o solder wetting at the upper edge of the hole to guarantee solder healing by filling any knee bend cracks that could potentially isolate the pad from attached traces.
The solder wetting on the upper edge of the hole can be visually verified by ensuring that there is a metallic lustre surrounding the hole, providing a level of confidence that any potential cracks have been healed. Today, the standards only enforce the requirement for a 25% recession, but the issue of through-hole to trace open circuit, can still be a reliability concern to customers.
Ensuring optimal solder connection
To ensure the optimal solder connection, parts and components shall be mounted such that they do not obstruct solder flow onto the solder destination side lands of plated-through-holes (PTHs) required to be soldered.
A good solder joint will have a concave surface of solder at the intersection of the metal surfaces of a solder connection, for example between the component lead and the edge of the hole. This is called the solder fillet. There are several events that can cause the top side fillet to not form correctly, including poor lead wetting, bad component mounting, and flux vapours blocking solder flow up the hole. However, the most common cause of the issue is through-hole components that have built-in stand-offs – the way that some components are manufactured can mean that the component leads are solder coated up to the stand-off, but not beyond. The area beyond the stand-off is referred to as non-wettable
The issue is caused by a combination of wetting and surface tension. Surface tension is an elastic energy on the surface of the solder. It pulls the components that have been placed slightly off-pad back into alignment during the soldering process. Conversely, soldering can also cause some components sit out of alignment due to poor pad design. In the context of through-hole soldering, it is the elastic energy of the surface tension that produces the final solder fillet shape. The vertical wetting dictates whether a top side fillet is formed or not, and poor hole-fill is worse when excellent soldering is achieved without the top side fillet.
To understand the phenomenon more clearly, we need to step through the various stages of the soldering process relative to surface tension. As the pre-heat and fluxing stages are irrelevant to the current discussion, we will ignore them. After those stages, the actual wave soldering process fits into three key stages within the liquid solder wave.
- Entrance, often called the wash region
- Calm region – where wetting takes place
- The peel back region – where the solder fillet is formed
- In the wash region, the solder wave flows fast, and solder is pumped up through the hole. Here, the surface tension fights with the pulsating pressure
- In the calm region, wetting takes place, and the surface tension tightens as the solder wets up the lead and to the edge of the pads, pulling the solder taut
- In the peel back region, the solder is drained pulling down the top side solder and stretching the top side fillet towards the through-hole. This process builds up latent elastic energy from the surface tension, which is released when the solder escapes the wave, pulling excess solder from the bottom side into the through hole. It is the vertical wetted lead that holds the solder for the top side fillet, stopping it from being drained down the through hole.
To achieve a top side solder fillet, you need both horizontal and vertical wetting. When one or the other is not present, solder will be drained down the hole with no restrictions, apart from the hole wall wetting and the fluidity of the molten solder. Unfortunately, this lack of restriction can produce a solder hole fill significantly less than the target 75% fill.
- The 25% recession rule was intended to heal any knee bend cracks, which might potentially electrically isolate any traces attached to the
- Typical component with built in stand- The rational for the stand-off is to allow flux gasses in the hole to escape, allowing solder to flow through to the topside.
- The image above shows this non wet area clearly. You might think this is not an issue, however the physics of through hole wetting is dependent on vertical wetting
- Surface tension is the elastic energy of a liquid
To describe how surface tension works, we will use the example of erecting a circus tent –
The tent is flapping in the wind as it’s being pulled up the pole.
Once erected and taut the tent is secured.
Basically, a tent without a pole is not a tent.
About Dynamic EMS
At Dynamic EMS, we understand that no two-electronic equipment manufacturing customers are the same. Their products, their supply chains, and their markets all differ, which is why Dynamic EMS offers tailor-made, customised electronics manufacturing services to customers with complex, highly diversified business.
From design to distribution, we enable our customers to be more competitive by bringing innovative solutions to market faster, with a commitment to quality in everything we do. Dynamic EMS – Your Product Solutions Architect.
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Find out more about Dynamic, www.dynamic-ems.com.