How to Use Solder Paste in PCB Assembly

A crucial component in electronic devices is the PCBA board, consisting of two main parts: PCB and Components. The PCB supports and connects the components, which, in turn, provide the actual functionality. Initially, these two parts are separate, but through the assembly and soldering process, they are eventually combined into a unified whole.

In this assembly process, the use of certain key materials is necessary, and one of these is solder paste. This article will provide a detailed overview of the role of solder paste and a guide on how to correctly use solder paste in the Processo di assemblaggio dei PCB.

Solder paste printing during PCB assembly

What is Solder Paste and Its Purpose

One of the most reliable methods to connect two metals without melting them is soldering, and solder paste is essential for achieving this. Solder paste is a special metal alloy composed of metal solder particles and a unique combination of flux. During the soldering process, the distinctive properties of solder paste allow it to melt and form a permanent bond, thus establishing a reliable connection between metals.

To ensure good performance during the soldering process, solder paste must meet several important standards. Firstly, it must have a relatively low melting point, considering the sensitivity of components to temperature. If the melting point of the solder paste is too high, there is a risk of reaching the maximum temperature the components can withstand before it fully melts. Secondly, the conductivity of solder paste is crucial as it is used not only for mechanical connections but also as an electrical connection, ensuring uninterrupted flow of electrical current to the components. Lastly, to ensure safety for both humans and the environment, the raw materials of solder paste must comply with the strict requirements of the Direttiva RoHS.

In the past, approximately 40% of solder pastes in electronic manufacturing consisted of a combination of lead and tin. However, due to the environmental hazards associated with lead, and with advancements in technology and increased environmental awareness, the electronic manufacturing industry has gradually transitioned to lead-free processes. Consequently, lead-free solder paste has become the mainstream choice, reflecting a positive response to environmental concerns and industry focus on health and sustainability.

Choosing the Right Solder Paste Type

Using high-quality and the correct type of solder paste is a crucial step in ensuring the quality of PCBA boards. In assembly projects, FS Technology offers various grades and types of solder pastes to meet different requirements. You can choose based on the materials planned for soldering and the needs, or select according to the size of solder balls in the solder paste.

To meet the requirements of different projects, solder pastes are usually divided into different grades and categories. These grades are not solely based on the accurate size of solder balls but are evaluated based on around 80% of the solder balls falling within a specific size range. Here are the various grades and their corresponding solder ball sizes.

Grado di pasta saldante

Dimensione della particella

Tipo 1

Da 75 mm a 150 mm

Tipo 2

Da 45 mm a 75 mm

Tipo 3

Da 25 mm a 45 mm

Tipo 4

Da 20 mm a 38 mm

Tipo 5

Da 10 mm a 25 mm

Tipo 6

Da 5 a 15 mm

Tipo 7

Da 2 mm a 11 mm

Tipo 8

Da 2 mm a 8 mm

In addition to considering the particle size, choosing the appropriate solder paste based on the type of flux is also a viable option. Different types of Flusso di PCB can better match specific solder paste formulations to achieve better soldering results or meet specific process requirements. However, not all environments allow for the use of this type of flux, and it must be ensured that the environment is clean enough.

  • Water-Soluble Flux Paste: Composed of organic components and ethylene glycol base. Residues of water-soluble flux can be easily removed by using readily available cleaning products on the market.
  • No-Clean Solder Paste: This type of solder paste is typically used in the manufacture of no-clean flux. It has relatively low solid residues and resin content. The main advantage of using a no-clean flux is the potential cost savings on cleaning supplies.
  • Rosin-Based Solder Paste: Solder pastes of this kind are made from rosin and can be cleaned using solvents after soldering.

Storing PCB Solder Paste

Depending on the temperature and humidity of the Fabbrica di PCBA, store these ready-to-use solder pastes in specially designed packaging that keeps the product’s temperature between 0° and 30 °C (86 °F and 84 °F) for at least 48 hours.

La pasta saldante deve essere consegnata durante la notte o tramite trasporto refrigerato. In estate, il trasporto deve avvenire il più possibile di notte per ridurre i tempi ed evitare le alte temperature. L'imballaggio della pasta può includere confezioni fredde che arrivano sciolte. Se il tempo di viaggio di 48 ore non viene superato, si tratta di un fenomeno tipico e non di un danno o di una manipolazione impropria. All'arrivo, la pasta saldante deve essere trasferita immediatamente in un magazzino controllato. L'uso della refrigerazione è consigliato ma non sempre necessario.

A temperature normali (22 °C), le paste saldanti hanno solitamente una durata di conservazione compresa tra tre e sei mesi. Non refrigerato: 25 °C (77 °F) / Refrigerato: 0°-12 °C (32 °F-55 °F). Le prestazioni della pasta saldante possono essere compromesse dall'esposizione a temperature superiori a 30 °C (85 °F). Tenere la pasta saldante lontano da fonti di calore superiori a 32 °C (90 °F). Se possibile, conservare le paste saldanti in siringhe o cartucce in verticale, con la punta rivolta verso il basso. Le confezioni devono essere occasionalmente ruotate di 180 gradi se sono conservate orizzontalmente.

La pasta deve essere utilizzata prima della data indicata sulla TDS specifica del prodotto, che corrisponde alla data di produzione (DOM) riportata sull'etichetta. In un sistema FIFO con la registrazione della data e dell'ora di rimozione dal magazzino, i lotti di pasta saldante più vecchi devono essere utilizzati per primi. Non è consentito l'uso a freddo della pasta saldante. Prima di aprire la confezione di pasta, il materiale di riempimento deve essere lasciato a temperatura ambiente, ovvero 20°-77 °F (20°-25 °C). Di norma, ciò richiede dalle quattro alle sei ore. NON RISCALDARE LA PASTA CON FORZA.

La pasta saldante in barattolo deve essere mescolata leggermente e accuratamente con uno strumento di plastica per almeno un minuto dopo che ha raggiunto la temperatura ambiente. Le siringhe e le cartucce di pasta saldante non devono essere mescolate. È sconsigliato l'uso di apparecchiature automatiche per la miscelazione della pasta saldante, poiché potrebbero danneggiarla.

PCB Solder Paste Printing Process

Stencil Printing

PCB Stencil Printing

The most common practice is stencil printing, which is a special metal grid structure typically made of stainless steel. Manufacturers first create a Stencil per PCB with holes or apertures of the same size and shape as the surface mount pads in the design. To ensure accuracy, FS Technology employs laser cutting technology to process the stencil and applies a coating material on the surface to smooth the apertures and facilitate even distribution of solder paste.

The stencil is then placed in an SMT device and aligned with the PCB below. Solder paste is uniformly applied to the stencil, and a squeegee blade is used to scrape off excess paste, allowing it to drop onto the PCB surface through the apertures. The entire process typically takes about 15 to 45 seconds.

After the solder paste printing is completed, SPI testing is conducted to assess the uniformity and quality of the solder paste, ensuring each pad receives the correct coverage. Finally, the stencil is cleaned after completing a batch of PCB solder paste printing to ensure that residue from the previous printing does not impact the distribution of solder paste on the next board.

Stampa a getto

While stencil printing is advantageous for assemblaggio di PCB su larga scala, constructing the stencil takes a considerable amount of time and precise control over the solder paste quantity for each component is challenging. Therefore, Jet Printing is becoming increasingly popular.

Jet Printing utilizes a modified formula to create a more liquid-like solder paste. Inkjet printers, using the same PCB design data as stencil printing, apply a precise amount of solder paste to each interface pad on the board. Depending on the solder material required for the application, the printer can easily switch and alternate between solder paste cartridges, from which tiny droplets of solder paste are jetted out.

Jet Printing technology allows PCB assemblers to precisely control the amount of solder paste for each interface, providing them with significant flexibility. This means that solder paste quantities for each solder point can be adjusted based on specific requirements.

Manuale

In repair or hobbyist projects, the printing method is usually not employed to apply solder paste to PCBs. Instead, there are two common manual methods:

  • Using a syringe, precisely apply solder paste to the pins and then use a soldering iron to bond it with the pins.
  • Place solder wire between the two pins of the component and then use a soldering iron to heat it.
 

Regardless of the method chosen, the ultimate goal is to establish a reliable connection between the PCB and the component. After manual soldering is completed, it is advisable to perform a quality check and conduct functional tests to ensure the reliability of the connections.

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