Why HASL Surface Finish for Flexible Circuit Boards Is Not Recommended

Do you know why HASL surface finish for flexible circuit boards is not recommended now? We have discussed all notable points behind this fact in this article.

Before digging deeper, you must better understand the HASL PCB surface finish and its pros and cons. Then it will be easier for you to know why it is intelligent to avoid the HASL process for FPCs.

Different Surface Coatings for Flexible PCBs

HASL Surface Finish

HASL creates a shield layer over the PCB surface through Tin or Lead.

HASL is an affordable and easily accomplishable surface coating in the PCB world. This is a traditional PCB surface finish, actually the 2nd oldest among other surface finish processes. But its popularity fades with the advancement of PCB technology.

As HASL remains backdated and can’t meet up the goal of Green Technology, manufacturers prefer other options as a substitute for it. But still, Hot air Solder Levelling is compatible with some sort of rigid or flexible circuit boards, which is not so remarkable.

HASL Processing Types: Vertical vs Horizontal

In the case of equipment processing used in HASL, you can find both vertical and horizontal configurations. When you are dealing with vertical designs of HASL equipment, you need to immerse the circuit board inside a solder solution completely.

The vertical arrangement ensures a premium finish on the flexible printed circuit boards. But here, a gravitational impact on the PCB solders occurs, which is not desired at all during surface finish processing.

Vertical Processing in HASL PCB

On the contrary, the horizontal HASL process involves immersing the flex PCB in solder using rollers and nozzles. These materials get attached to the upper and the lower PCB panel. As no gravitational impact is found in solder distribution, the solder thickness becomes more uniform compared to vertical processing.

How to Process HASL

HASL processing is simple and manageable. The entire HASL process combines the following steps-

• Washing
• Application of solder flux
• Use of molten eutectic solder

Here, manufacturers take a jar filled with eutectic and molten solder and place the circuit board inside the jar. Then, they pass the flexible printed circuit boards through the air knives with high pressure and two hot breaths of air. Keeping the solder temperature lower than the hot air knife temperature is highly recommended. Otherwise, knives get melted.

Thus, hot air and pressure blow away any excess solder from the circuit board’s surface. Next to this, manufacturers conduct a micro etching to ensure no oxidation occurs on the PCB surface. Usually, a semi-concentrated etching solution is used in this case.

Further, they apply a solder flux solution that wets all remaining coverless copper features. Sometimes, PCB materials placed on the surface suffer from thermal shock created by the solder solution. Manufacturers apply heat on the board before using the solder flux solution to keep the flex board safe.

HASL PCB

So, this is the way you can use HASL on the flexible PCB surface. After finishing this, manufacturers apply hot air knives or bursts to eliminate the residual tin or lead from the circuit board. When the solder gets solid and almost permanent, wash the board clearly so that no residue is found on the surface.

Transformation of Tin-Lead HASL to RoHS HASL: A Last Try to be Competent in Market

Lead-based Hot air Solder Levelling is rejected by the most top-rated PCB makers due to its limitations in following RoHS regulations. So, some companies intend to transform HASL processing in accordance with RoHS guidelines and upgrade the features.

Then the phenomenon of Lead-free or RoHS-compliant HASL surface finish comes to the industry. This surface coating is better than its former version in terms of coplanarity and flatness of the PCB surface. Besides, the solderability and tolerance to thermal shock are more impressive in a Lead-free HASL finish.

RoHS-compliant HASL Surface Finish

But it requires excess heat during the processing because it lacks lead materials. Besides, RoHS-HASL causes a bridging effect that degrades the electrical performance of flex PCBs to a large extent.

As it is compatible with SMD components and supports Green Technology, its demand is increasing to a certain extent. But HASL with Lead-Tin alloy has minimum usability in the present PCB market.

Features of HASL Surface Finish

● Cost-efficiency

HASL is comparatively less expensive than other nature-friendly PCB surface finishes like ENIG, ENEPIG, etc. This is the prime cause to keep it relatable in recent times.

● Coating Thickness

HASL is supportive of maintaining uniformity in the case of coating thickness. The remaining solder flux volume on the pad doesn’t get disturbed when hot air pressure is applied to the PCB surface. That’s why HASL ensures such high-class evenness of layering of the eutectic mixture of tin and lead.

● Reworkable

This is another fascinating feature of Hot Air Solder Levelling. You can rework this surface coating without eliminating it. This property brings good value from the manufacturer’s perspective.

Reworkable HASL Coating on PCB

● Easily Manageable

This surface coating is free from involving high-volume pieces of equipment and large flex boards. That’s why the processing is comparatively more flexible than other contemporary surface finishes.

Besides, you can conduct HASL within the manufacturing without disturbing the schedule. It becomes possible due to having an extended period for HASL processing. This is massive support for manufacturers, isn’t it?

● Aesthetically Sound

Hot Air Solder Levelling brings out uniformness in the PCB surface, which makes the device more attractive from an aesthetic point of view.

● Soldering Process

In some cases, solder fluxes don’t cover the gaps between layers. It causes dewetting of solder flux in the HASL surface coating process.

● Placement of Hot Air Knives

Hot air knives are placed in such a way during HASL processing that sufficient solder flux remains absent on the solder pad. It makes the PCB surface finish vulnerable to thermal pressures and harsh atmospheres.

Use of Hot Air Knives in HASL Processing

● Copper Surface Preparation

This feature is not up to the mark in HASL surface finish. That’s why the flexible printed circuits with HASL finish can’t deliver complete wetting of solder flux in high temperatures.

Why HASL is not Good for Flexible Circuit Boards

HOT Air Solder Levelling brings some positive vibes to flexible circuits. Despite this fact, manufacturers never recommended HASL for FPCs (Flexible Printed Circuits) because of some notable negative issues.

Have you any idea why manufacturers reject HASL for flex PCBs?

Let’s find out.

Substandard Thermal Management

Flexible circuits with HASL PCB finish are highly vulnerable to extreme thermal pressures. Usually, flex PCBs contain Epoxy or Acrylic adhesives to bind the layers and base materials (Polyimide or FR4) together. But these adhesives used in Flex circuit boards remain at risk when high thermal shocks are applied.

As the adhesive’s operating temperature is lower than the base materials working temperature, it must be handled safely during HASL processing. These temperatures are often altered, delamination occurs, and HASL coating can’t withstand thermal pressures.

Besides, insufficient copper preparation makes the coating exposed to thermal pressures that significantly hampers the shelf life of the HASL process.

There is another issue that makes it clear to you that HASL offers poor thermal management in flexible printed circuits. It is the positioning of Hot Air Knives that obstructs the solder pad from holding adequate solder flux.

PCB Surface Finish with Low Thermal Stability

That’s why flex PCB surfaces with HASL coating can’t tolerate extreme thermal forces. Lead-free HASL coating requires more thermal pressure during the processing.

That means it is more endangered by temperature compared to the lead-based HASL processes. Instead of these surface finishes, manufacturers prefer ENIG (Electroplated Nickel Immersion Gold) or ENEPIG  (Electroplated Nickel Electroplated Palladium Immersion Gold) for better thermal management.

Consisting of Harmful Elements

Lead is a harmful element to nature. Consisting of Lead-based materials is a complete violation of RoHS guidelines.

That’s why manufacturers stop using HASL in flexible circuit boards. But nowadays, manufacturers prefer lead-free HASLsurface finish, which is more nature-friendly and follows the RoHS regulations.

Time-consuming Process

HASL processing requires a significant amount of time due to its lengthy pre-baking.

When you intend to apply Lead-Tin Solder on the flex circuit board, you need to place the flex board in the molten solder jar. Usually, FPCs consist of Polyimides as base materials, of which the moisture absorption rate is almost 2%.

But you must use a moisture-free Polyimide in flex circuits to get the best result from the HASL surface finish. That’s why the flex circuit must undergo an evaporation process that bakes the Flex circuit surface before applying HASL.

This baking runs for almost 6-8 hours, concerning the flex circuit design and the number of layers. You have to arrange a large oven where you can put the flex circuit materials for baking.

You must ensure that all moisture is removed from the Polyimide materials. Otherwise, the steam caused by baking delaminates the flex PCB overlays, conductive, or substrate layers. This delamination can hamper the durability of flex circuit components in the long run.

Low Mechanical Strength of Flex Circuits

As mentioned, flex PCBs are kept inside a pot containing a eutectic molten solder mixture. There, hot air knives and high thermal pressures are applied to eliminate excess flux from the surface.

But unlike rigid PCB materials, flex circuits consist of bendable elements which are not reputable for their mechanical strength. That’s why flex circuit materials can’t withstand these extreme pressures caused by hot air knives.

To handle this issue, manufacturers tend to keep the flex circuit boards inside a support frame. But all go in vain in the long run. Do you know why?

It happens because flexible circuits don’t have high rigidity like rigid circuit boards.

Flex PCB panels, irrespective of whether mounted or unmounted on the support frame, can’t resist thermal pressures and quickly get deflected because of a lack of mechanical and thermal stability. Besides, it hampers the solder surface’s evenness, damaging the whole HASL processing.

Moreover, this mounting of flex PCB panels takes a lot of time which makes the job difficult for the manufacturers to follow the lead time.

Poor Wetting

This is another disadvantage of HASL processing that causes the PCB makers to choose alternatives. Hot Air Solder Levelling offers substandard wetting. This poor wetting causes delamination to the overlays and hampers the performance of flex circuit boards.

Poor Wetting is a Blackspot on HASL

Besides, copper processing is not so good in HASL surface finish. That’s why the flexible printed circuits with HASL finish can’t deliver complete wetting of solder flux in high temperatures. This is also a reason for dewetting in the HASL PCB surface finish.

Less Compatibility with Fine Pitch and SMD Components

Hot air solder Levelling surface finish is incompatible with the flex circuit designs and components used in the present day.  Especially, HASL can’t match the surface topology used in FPCs.

In the case of surface mount components and fine pitch elements used in flex circuits require a certain amount of solderability and a flat surface. But you never expect such even PCB surface from the HASL process.

Besides, advanced flexible circuits consist of ZIF (Zero Insertion Force) contacts. Here, you can’t apply high thermal shock as desired. Moreover, ZIF connectors demand a coplanar surface with the highest flatness. But can HASL PCB surface finish deliver these features in flexible circuit boards?

The answer is No. That’s why HASL surface finish for flexible circuit boards is not recommended by manufacturers.

HASL vs. Other Surface Finishes

Now, look at the performance comparison between HASL and other current PCB surface finishes to understand the differences clearly.

We hope you have already realized the reasons for choosing ENIG, Immersion Gold, or ENEPIG, etc rather than using HASL for flex circuits. You must make your choice intelligently while choosing a PCB surface coating for your projects.