PCB News - Acceptance specification for lead-free soldering of circuit boards

Acceptance specification for lead-free soldering of circuit boards

With the emergence of new construction methods in circuit board manufacturers, those who need to perform PTH wave soldering on the sockets on the board surface may be replaced with lead-free solder paste printed in the through holes, and then the pins are squeezed in, so it only needs to be melted After soldering, the post and pins can be welded firmly at the same time. However, this new attempt is still being gradually developed.

The new addition in the preface also explains the visual inspection specifications for the appearance of lead-free solder joints, which are different from those with lead (in fact, a lot of relaxation). In order to reduce controversy, in the D version, a number of color patterns are specially attached for comparison, and the lead-free logo with a red circle on a black background and a white letter is added to the lower right corner to show the difference. In addition, two black dots are listed in the article, clearly pointing out the characteristics of lead-free solder joints (actually the shortcomings), and the other qualities are the same as the acceptable specifications for lead. The two articles are as follows:

1. Rough surface (granular or gray)

2. The connecting angle and Shu angle become larger

1. Rough surface of lead-free solder joints
The main interpretation of the appearance of orange-peel-like grains of lead-free solder is tin-silver-copper SAC305 or 405. It is difficult for these ternary alloys to achieve the ideal state of eutectic eutectic composition during the process of hot melting or cold solidification. The temperature-time curve of general welding operation, the heating and cooling of the peak temperature welding section, when the normal condition is maintained, the tin that accounts for the most in the SAC solder during the cooling will take the lead to cool down by itself and become dendrites (the rod-shaped protrusions), The rest is still in the liquid eutectic part, and will be cooled to become a smoother interval part. Therefore, there will be many granular protrusions in the overall appearance, and the pure tin dendrites can also be clearly seen in the micro-sections. The phenomenon of uniform distribution. In addition, the formation of Ag3Sn white slab-shaped IMC is also obvious to all. In fact, the granular pure tin dendrites in the structure have little negative impact on the strength and reliability, but the microcracks in the eutectic zone and Ag3Sn The IMC is the origin of cracking during aging.

2. Acceptance specifications for abnormal lead-free solder joints
If the plated through-hole pins have blown holes after wave soldering, or the SMT solder joints show bottomed pinholes, or the appearance of depressions, as long as the solder joints can still meet other quality requirements, Class1 can be accepted. However, Class 2 and 3 must be regarded as "process warnings". Readers, please note that from the principle of quality control and improvement, when a process warning occurs, the customer must see the improvement plan and implementation determination before they can consider accepting the current product. Therefore, the "process warning" has become the opposite. A more serious overall issue.

Interpretation of insufficient fluidity of liquid lead-free solder:

If the soldering peak temperature is interpreted as "firepower" or fluidity with respect to the temperature drop above the melting point of the solder, the average firepower for tin-lead fusion soldering is 42°C, and the average temperature of tin-lead wave soldering can reach 67°C. However, since the melting point of lead-free solder has risen by 34°C or 44°C compared with tin-lead, in order to prevent components and plates from being scalded by strong heat, the firepower of lead-free welding and wave soldering has to be reduced to 28°C and 48°C . As a result, when the firepower is not strong, the fluidity slows down and the viscosity increases, of course, many shortcomings of bridging and short circuits are prone to occur.

Moreover, in the tin pool of lead-free wave soldering, once the copper contamination exceeds 0.1% by wt, the m.p of the solder in the pool will rise by another 3°C. When the peak temperature is not relatively high, the firepower required for lead-free soldering is of course even more insufficient. The sluggishness of this kind of liquid material movement will inevitably lead to the phenomenon of muddy water, so all kinds of embarrassed tin bridge tin nets, and many serious shortcomings such as not being clean and tidy, all appear one by one. When the copper cannot be removed, pure tin can be added to dilute the copper pollution.

The soldering temperature of lead-free wave soldering is as high as 265-270 degree Celsius, which is extremely harmful to various copper parts on the PCB surface. Due to the faster rate of melting (dissolving) copper and the increase of copper pollution, the melting point rises and the fluidity becomes slower. Not only is the board surface covered with broken and residual tin, but also the copper in the solder joints and the solder pool, but also Will produce Cu6Sn5 needle-like crystal IMC. After this kind of foreign matter is brought out by the motor's rising waves, it often makes the board surface full of thorns and needle-shaped crystals, which will cause more disasters in the future. It seems that lead-free wave soldering is about to come to an end, so I can't play it anymore.

When the solder is no longer a component of the eutectic eutectic, a slurry state will definitely appear during the hot melting and condensation process. This state of coexistence of solid and liquid phases is actually quite unstable. Once the automatic conveying is disturbed by external forces such as vibration and jitter, not only the local solder (referring to the pure tin part) will quickly solidify to form bone-like dendrites, or appear Obvious appearance such as stress streaks; those whose surface has the appearance of green tendons exposing vascular protrusions are specially called "scrambling welding". The original surface of the lead-free solder joint is not smooth enough, but if there are too many stress streaks and excessively obvious, it is still caused by scrambling, and the third-level board is still regarded as a disadvantage.

As for the cracking of the surface mount solder joints, the reason is mostly due to the impact of excessive stress during the period of weak slurry state, which causes the lead-free solder joints to crack after cooling. All three-level boards are regarded as shortcomings. These shortcomings of mud and water in the current tin-lead solder, occasionally occur on the board surface after wave soldering, mainly due to insufficient firepower, slow fluidity, and increased viscosity.

Due to the coefficient of thermal expansion (CTE) in the Z-direction of PCB thickness, it averages 55-60 PPM/ degree Celsius under the strong heat of lead-free wave soldering, while the CTE of lead-free solder itself is only 20-25 PPM/ degree Celsius, so It was cracked by the difference of CTE when it was not welded firmly. If lead-free or bismuth pollution occurs again in the lead-free solder joints, it will be even more miserable. Sometimes the outer edge of the copper ring is pulled up even if the solder joints are stronger. For version D, the third-level board can be accepted for lead-free wave soldering on the motherboard surface floating up.

In fact, the various specifications of IPC do not involve the quality of single-sided boards. The main reason is that the two major industries of PCB and PCBA in the United States have no longer produced single-sided and single-sided assembly products for many years. American brands in need only need to send to Asia The industry only needs to purchase the final complete machine to become its own brand. As a result, the technical and quality documents required for single-panel technology have always been absent.