How to improve solder joints and avoid disasters

Whilst an obvious failure of a joint is hard enough to accept, it can at least be quickly addressed and remedied. The bigger issue can be with a less obvious, underlying fault or intermittent breakdown as it may cause the equipment to fail once it’s in service. Fortunately, the vast majority of solder joints produced are of good quality and do not pose any problems.

In this month’s edition of Hakko Tech Tips, we will look at some of the most frequently asked questions fielded regularly by our technical support team and explore solder joints in more detail, such as how a perfect solder joint should look as well as looking at skips, lifting, webbing and splashing. We will also explore the impact that soldering quality has on a PCB. So, let’s take a closer look in our five tip format below:

1) What impact does the quality of soldering have upon the impact of the PCB?

Most components on a PCB will be soldered into place. This process can be completed by hand soldering /and or machine soldering. To ensure a high reliability product, the PCB, components and soldering must be of the highest quality.

Any soldered joint provides not only an electrical connection but also mechanical strength. Poor quality soldering can produce a poor electrical connection (sometimes intermittent), low mechanical strength or any combination of the two.

2) Can you describe the perfect solder joint? (i.e. complete solid wetting, smooth and shiny surface, a neat concave fillet).

Traditionally, a perfect soldered joint should display a concave fillet, excellent wetting of both the component lead & PCB and have a nice bright, shiny surface finish. However, this was more relevant pre-2006 when most electrical/electronic soldering was carried out using Tin/Lead solders.

In the advent of Lead-Free solders, you should still strive for a concave fillet and excellent wetting but the surface of the joint will now appear dull in comparison. This is not actually a defect and can be attributed to the coarser grain structure of a Lead-Free solder joint.

The outline of the pad and lead should still be clearly visible, and there should be no holes in the solder, which should fill at least 75% of a plated through hole with wetting visible for at least 180 degrees within the hole circumference (Component Side).

3) What are skips?

When a solder joint is insufficiently wetted with solder, it is commonly referred to as a skip. A solder skip can be seen when machine soldering using solder paste. They are manifested as soldered joints with little or no solder content.

There are numerous reasons this may happen, but the root cause is usually attributable to the screen-printing process, e.g. Insufficient solder paste on the stencil, solder paste retention within the stencil or poor stencil design.

4) When do lifted pads occur?

PCBs are made from a composite material. In most common applications this would be a fibre glass resin, i.e. FR4. This provides a solid, rigid and non-conductive support layer which is laminated on one or both sides with copper tracks/pads. Under normal circumstances the bond between the fibre glass and the copper is sufficient to withstand most applications.

However, if during manufacture or rework, excessive heat is applied to a particular track or pad this will weaken the laminate bond and can lead to damage, often referred to as “lifting”. In this instance, the PCB would need to be repaired by using any approved method, i.e. Wire Links to replace a track or the fitting of a new eyelet for a Plated Through Hole.

Please note that in some high reliability applications this may not be acceptable and the PCB may have to be scrapped.

5) When does webbing and splashing occur?

Both are phenomena usually associated with machine soldering, in particular wave soldering. As usual there are many potential causes but the most likely causes of webbing are insufficient curing of the solder resist during bare board manufacture, contact with solder dross on the molten solder wave and insufficient or inconsistent flux application to the underside of the PCB. Solder splashing is usually caused by excessive flux being present when the PCB makes contact with the molten solder wave.

The contact between a cooler liquid and molten solder at approximately 270 deg C will promote a violent boiling of the flux. This in turn will cause the solder to splash or solder ball. Care should be taken to ensure that the flux manufacturers parameters are strictly adhered to, i.e. The topside PCB temperature is low enough to retain activity but high enough to evaporate any excess material. This is especially important when using modern VOC-Free Liquid Fluxes as they are water, and not alcohol, based. 

In today’s society, all of us depend upon and expect high reliability, it’s not just represented in very sophisticated systems. Our high expectation of reliability in our devices now forms part of our everyday lives, and solder, and indeed flux, are a critical factor.

Whether products are designed for the consumer electronics sector or systems used in high-reliability military hardware, advances in soldering technology have vastly assisted with these developments. Hakko is a world leading manufacturer of soldering equipment with a culture of innovation, creativity and sincerity.