Image may be NSFW. Clik here to view. 如果一定得要使用無鉛錫絲，則可以考慮錫銻(SnSb)合金的Sn95Sb5，其熔點約在240°C，或是Sn90Sb10熔點約在250°C。但是SnSb合金的熔點其實與現在無鉛製程的回焊最高溫(peak temperature)差不多，回焊過程中會有融錫的風險，所以最好在測溫點的邊緣輔以「高溫紅膠」來固定熱電偶線。
不過工作熊發現，英文一般會以EPad來稱呼這種QFN零件上的散熱焊墊，而EPad則是【Exposed Pad】或【Exposed central Pad】的簡稱，我們一般稱之為「外露墊」或「外露焊墊」；而EPad焊接於PCB相對應位置的焊墊大多為接地，英文一般稱之為【thermal pad】，台灣稱之為「散熱墊」，請注意：少數零件可能會有訊號從EPad傳遞，所以接的不一定都是地(ground)。
此EPad焊接空洞率(solder void ratio %)的實驗使用的BTC為48腳數且長寬為 7mm x 7mm大小的封裝，在PCB散熱墊設計有4×4個1.0mm間距(pitch)的0.3mm導通孔(via)。圖片的縱座標為熱阻θJA(thermal resistance)，橫坐標為空洞率(Solder void %)。
Image may be NSFW. Clik here to view.最後，提醒電路及電路板設計者們一件事，SMT工廠為了避免錫膏在大焊墊的融錫過程中因內聚力而群聚至某一區域而導致零件頂高，造成零件傾斜，最後形成單側吃錫不良之現象，會在開EPad鋼板時做井字形、田字行、米字形或條紋形狀的開孔，其錫膏覆蓋率大概只會有EPad面積的50～60％，如果PCB的散熱墊(thermal pad)上又有過多未塞孔電鍍的導通孔(vias)，為了避免錫膏流入到這些vias造成錫量不足以及溢流對板子另一面造成錫珠或短路等問題，在錫膏印刷時都會特意避開這些vias，這可能使得EPad焊接的氣泡空洞率增加大於50%，甚至來到70％的空洞率，那就可能使得熱阻增加、熱導率降低並影響到散熱的效果。
一般來說，片式零件在吃錫上較容易發生錫珠問題，而錫珠的發生，除了錫膏氧化之外，其最大原因就是錫膏內含的助焊劑在高溫會下迅速揮發轉變成氣體並帶著部份的錫膏往外側移動（想像氣炸的情形），於是就會在零件本體下方與PCB之間的小縫隙形成分離的錫膏區塊，回焊時零件下方因為沒有焊墊可以吸引熔融的錫膏，再加上零件本體的重量擠壓，於是分離的熔融錫膏就從零件的本體下方冒出來並在其邊緣上形成小錫珠。 Image may be NSFW. Clik here to view.
錫膏是現代電子組裝製造中不可或缺的焊接材料，而且扮演一個很重要的角色，它主要被應用於SMT(Surface Mount Technology，表面貼焊技術)中用來將電子零件焊接於PCB，讓不同電子零件的訊號可以連接在一起形成一個有效的迴路。哪你知道不論是早期的含鉛錫膏或是現在流行的無鉛錫膏及無鹵錫膏為何在使用之前都必須要先經過攪拌？退冰回溫？而開封後的錫膏也必須管控其使用期限？
錫膏使用前為何要退冰回溫？
Image may be NSFW. Clik here to view.Image may be NSFW. Clik here to view.
Image may be NSFW. Clik here to view.這是因為錫膏中含有助焊劑，而助焊劑中又含有乙醇類等易揮發的溶劑，也就是說錫膏一旦開封後溶劑就會開始揮發（其實不開蓋密封的情況下也會揮發，只是速度很慢，就類似汽水裝在寶特瓶中一段時間後也會沒氣是一樣的道理），尤其是那些已經塗抹在鋼板或是已印刷在PCB上的錫膏，其溶劑揮發的速度會更快，所以，一般有紀律的SMT廠都會嚴格管控錫膏的使用標準及壽命。
而單位質量物質由固態轉化為液態時，物體需要吸收的熱量就被稱之為「熔化熱(Enthalpy of fusion)」。熔化熱是一種潛熱(Latent heat)。同一種物質中，液態比固態擁有更高的內能，因此，在熔化的過程中，固態物質需要吸收熱量來轉變為液態。同樣的，物質由液態轉變為固態時，則要釋放相同的能量。液體中的物質微粒與固體中的相比，受到更小的分子間作用力，因此擁有更高的內能。
SMT產線為何要分長短線？其主要目的當然是為了要提高效率，這裡的長短線指的是SMT腺體的長度，也就是機台的數量，大家應該都有看過零件不到10顆零件的板子吧！像這種板子就可以使用短線SMT來作業，只要一台錫膏印刷機+一台貼片機+回焊爐就可以了。甚至連SPI及AOI都不一定需要。而長線SMT通常會放置比較多台的置件/貼片機(pick and placement machine)，適合單面零件數比較多的板子。
In the previous article, we talked about "How to Increase the bonding force for PCBA by using "Copper" base as the PCB surface finish“. Today, we will discuss further how the SMD (Solder Mask Defined) and NSMD (Non-Solder Mask Defined) pad designs of the BGA package affect the solderability and how these two types of pads affect the bonding force of PCA.
Increasing the solder contact area of a component terminal or leads will generally improve the solderability of the component, as it provides more surface area for the solder to wet and bond to the terminal. This can result in a stronger and more reliable solder joint.
However, it’s important to note that there are other factors that can also affect solderability, such as the type of surface finish on the component terminal and the solderability of the solder itself. So while increasing the solder contact area can be beneficial, it may not be the only factor to consider when trying to improve the solderability of a component.
II. Increasing the solder contact area of the component’s terminals or leads
Please note: what I mentioned here is to increase the contact area of the component’s solder terminals or leads, not to increase the amount volume of solder.
Before reading this article, it is recommended that you refer to the article of "Concept Clarification of Electronic Component Soldering Strength" by workingbear first.
Workingbear must emphasize again: "The soldering strength is basically proportional to the soldering contact area". Without considering the replacement of the solder paste formula and the surface finishes of the PCB (Printed Circuit Board), there are two key points to enhance the soldering strength of the component:
1. Increasing the solder contact area of the component’s terminals or leads. It’s not the amount volume!
If only more solder is added to the solder balls of the BGA, its solder strength will not increase basically, because its solder pad contact area remains the same. For SMD components with leads (such as QFP) or side solder terminals (such as the capacitor), increasing the amount volume of solder may increase its solder strength, because the leads of these SMD components have side terminals that may have solder climb up to higher position and increase the solder contact area.
If the solder amount can be increased to allow the solder to form a complete curve on the side of the solder foot, the solder strength can be effectively increased. Just like adding a support on the side of a tree or a utility pole to prevent it from falling down in the event of a typhoon, adding an R angle at the corner of a mechanism design is also the same principle, and the arc-shaped solder can effectively disperse stress.
Therefore, if increasing the amount of solder can allow the solder to climb up the sides of the solder terminals or leads, and even make the side solder climb higher to completely cover the solder leads, then its strength will increase significantly. This is much better than just welding at the bottom of the solder leads because you have increased the soldering area and its soldering structure has been strengthened.
Unfortunately, the side walls of most SMD component leads are not electroplated directly on bare copper due to cost considerations, which causes them to easily oxidize and cannot be soldered. Otherwise, they are electroplated, but the pitch between the lead is too small, causing the design of the PCB solder pad to be afraid of short-circuits and unable to extend so that the solder cannot completely climb up its side to form a perfect arc.
Image may be NSFW. Clik here to view.Image may be NSFW. Clik here to view.
Since climbing solder on the side walls of the solder lead can increase the solder contact area and strength, many people focus on the grounding foot (GND), such as increasing the circular or semi-circular holes on the ground pin of the B2B connector at both ends or designing it into a "U" shape, which all have the opportunity to increase its solder strength.
Since we’re talking about how increasing the soldering contact area can help to enhance the strength of the solder, we have to briefly mention the pros and cons of SMD and NSMD pad designs for BGA packaging on the PCB side.
Please note that we should not confuse the "SMD (Solder Mask Defined) pad" with the "SMD (Surface Mount Device) component" here.
Image may be NSFW. Clik here to view.
Assuming that the exposed area of SMD and NSMD pad designs is the same, the solderability of NSMD pads should be better than that of SMD pads. As mentioned earlier, this is because NSMD pads will wet the side walls of the pads during soldering, while SMD pads do not have side walls due to being covered by the solder mask. (NSMD is also called "Copper Defined pad.")
But don’t rush to change all BGA pads to NSMD designs immediately because there is no perfect solution if BGA cracking! In this fair world, even if the exposed pad surface of SMD and NSMD seems to have the same size, the actual size of the SMD pad is much larger than that of NSMD (depending on the layout design). This is because a large part of the SMD pad is covered by the green solder mask actually, and it is easy to be deceived if you don’t look carefully. Because the size of NSMD pads is relatively small, almost only slightly larger than the solder balls of BGA, the ability of SMD pads to withstand pulling force will be relatively poor. Once the BGA solder ball breaks, it is often seen that the NSMD pads are pulled up together with the solder balls.
As mentioned by the workingbear before, stress will find the weakest point to release. When the pad changes from NSMD to SMD and the soldering strength increases, the ability of the solder IMC layer to resist stress becomes greater than the bonding force of the PCB copper foil attached to the substrate (because the size of the SMD pad becomes smaller), and the breaking point shifts to between the pad and the FR4 substrate. Therefore, the workingbear still believes that if you want to completely solve the problem of BGA cracking, you should try to reduce stress, which is the only way to achieve the best improvement.
Therefore, the conclusion is that NSMD pads have better solderability than SMD pads, while the bonding strength of SMD pads is better than that of NSMD pads.
2.Using Through-Hole component to Replace Surface Mount device
In fact, no matter how much the solder strength of electronic components using surface mount technology (SMT) is improved, its ability to resist stress is limited. To further increase the strength of the solder, the stress needs to be transmitted to other structures through mechanism design. The most effective way to achieve this is to design the pins as upright plated through-holes (PTHs), so that the stress on the pins can be transferred to the hole walls of the printed circuit board (PCB) for support, which in turn can increase the strength of the solder. The common practice is to change some of the surface-mount soldering of the component pins to through-hole soldering, such as the iron frame soldering pins of Micro-USB connectors. They still go through the SMT process, but some of the pins are produced using the paste-in-hole (PIH) process. The latest Type-C connectors also have parts with a mix of through-hole and surface-mount soldering.
In addition, for ball grid array (BGA) packaged components, we can consider to layout vias on the pads, just like a rivet to fix the pad on the FR4 material. This is similar to the idea of anchoring a house with ground screws to prevent seismic damage. However, the vias on the solder pads must be electroplated and filled, otherwise, the BGA’s solder balls may form voids or bubbles, which can lead to severe defects, such as the head-in-pillow effect.
Recommended reading: Principles of via-in-pad handling.
而單位質量物質由固態轉化為液態時，物體需要吸收的熱量就被稱之為「熔化熱(Enthalpy of fusion)」。熔化熱是一種潛熱(Latent heat)。同一種物質中，液態比固態擁有更高的內能，因此，在熔化的過程中，固態物質需要吸收熱量來轉變為液態。同樣的，物質由液態轉變為固態時，則要釋放相同的能量。液體中的物質微粒與固體中的相比，受到更小的分子間作用力，因此擁有更高的內能。
Image may be NSFW. Clik here to view.首先，我們得先了解錫膏的是由助焊劑(flux)與錫粉(solder powders)各占一半的體積比所組成的混合物，而助焊劑中又包含有松香、活性劑、增稠劑與溶劑，其中的溶劑為乙醇類揮發性液體，其起到融合錫膏中各成分的作用。一旦，錫膏開罐後，其內部的乙醇類溶劑就會開始揮發（其實不開蓋密封的情況下也會揮發，只是速度很慢，就類似汽水裝在寶特瓶中一段時間後也會沒氣是一樣的道理），在乾燥及高溫的環境下更會加速乙醇類溶劑的揮發，造成錫膏乾涸。
Image may be NSFW. Clik here to view.其三，錫膏開罐後暴露於生產車間的時間又是如何管控的？我們前面已經說過，錫膏開罐後，錫膏中的溶劑就會開始揮發，所以我們不只要管控錫膏直接暴露於大氣環境下的溼度，更要管控暴露的時間，因為暴露時間越長，溶劑的揮發就越多，出問題的機率也就越高。一般我們會要求錫膏開蓋超過8H不得回收，超過12H或24H報廢，不建議自己添加溶劑，因為你不知道溶劑揮發了多少，更不知道需要添加多少溶劑，添加了溶劑後還得重新充分攪拌均勻。而對於已經塗抹於鋼板上的錫膏，其溶劑揮發的速度會比在罐子裡的更快，一般建議要在3H內用完。
Image may be NSFW. Clik here to view.導通孔(via)是用來連接及導通印刷電路板(PCB)不同層之間的銅箔線路用的。因為PCB基本上就是由一層一層的銅箔層堆疊累積而成，而上下相鄰銅箔(copper)層之間則會再鋪上一層絕緣膠片層(PP)，也就是說銅箔層彼此之間是不互通的，而不同銅箔層之間的訊號傳遞靠的就是vias。
樹脂塞孔(resin plugging)： 使用樹脂將已經完成電鍍銅的導通孔填平，然後再於其上方鍍銅變成焊墊。說起來很簡單，但樹脂塞孔時由於無法確保每次塞孔的表面都能與焊墊齊平，所以必須額外將之磨平後再執行一次電鍍銅，當然也可以選擇表面不做電鍍銅，而直接以防焊漆覆蓋。樹脂塞孔優點是表面可以製作電鍍銅及其他的表面處理金屬，不會影響到焊錫量，另外就算通孔直徑超過0.4mm也可以塞孔；樹脂塞孔的缺點是樹脂可能不太密實會有縫隙，文獻及經驗顯示，這些縫隙中的空氣在回焊受熱過程中可能進入BGA的錫球內部造成中空弱化焊錫強度，並吹脹錫球外徑與相鄰錫球短路。 Image may be NSFW. Clik here to view.
電鍍銅塞孔(copper electroplating microvia filling)： Image may be NSFW. Clik here to view.利用添加劑的特性，控制局部區域銅的生長速率，來進行填孔的目的，只是電鍍填孔的孔徑不可太大，一般只能在使用雷射鑽孔的微通孔(micro-via)中使用。電鍍塞孔有樹脂塞孔後電鍍的所有優點，但是不會有空氣藏在通孔中的問題，缺點是價格較貴，且填孔後容易出現表面凹陷的dimple，dimple如果太深。容易造成BGA空焊及孔洞問題，所以需要與PCB製造商訂定dimple深度的規格。
這份IPC-4761印刷電路板導通孔結構建保護指引(Design Guide for Protection of Printed Board Via Structures)文件其實已經是2006年的老版本了，距今沒有更新。它定義了幾種導通孔保護(via protection)的方法，也定義了6種導通孔保護的型態(type)，文件中花了很多篇幅來說明導通孔保護的諸多優點，以及導通孔保護施作時可能會遇到的一些問題。
IPC-4761文件中提及的幾種導通孔保護方法
請注意：以下的幾種導通孔保護方法的中文為工作熊自己的翻譯，目前看來並無統一的中文翻譯。
Tented via (封孔)：封孔主要以乾膜(dry film mask)架橋覆蓋於導通孔的上方，導通孔內沒有填充其他材料。其主要目的在保護導通孔內壁的導通金屬以確保孔內的導通功能可以正常運作。封孔可以在PCB同一個導通孔的單面或雙面位置上執行。可以參考下面表格中Type I-a/b的圖示。
Covered via (覆孔)：覆孔與封孔很相似，它是在導通孔被「Tented via (封孔)」或「Plugged via (塞孔)」之後再於其上面多覆蓋一層防焊或金屬材料。所以「Covered via (覆孔)」通常與「Tented via (封孔)」或「Plugged via (塞孔)」互相搭配。覆孔的目的也是在保護導通孔內的電鍍塗層，它同時也可以改善單獨使用「Tented via (封孔)」的強度。一般不建議在「Tented via (封孔)」的上方塗佈金屬焊墊，因為容易發生塌陷(dimple)甚至破裂造成焊錫不良或導通的長期信賴性問題。可以參考下面表格中Type II-a/b的圖示。
Plugged via (塞孔)：塞孔作業通常使用「液態顯影(LPI, Liquid Photo Imageable)」材料或樹脂(resin)以絲網印刷或刮刀或加壓塗抹方式來進行導通孔的塞孔作業，填充物可以是導體或非導體。依照IPC-4761文件的說明，塞孔作業並沒有要求一定要將導通孔完全填滿，而只要求導通孔位於PCB表面的位置必須塞住。可以參考下面表格中Type III-a/b的圖示。
Filled via (填孔)：填孔的作業方式與塞孔類似，但是要求導通孔內必須完全填滿。填孔作業一般使用LPI或樹脂來填孔，作業上可能會出現不完全填孔或voids的情形影響品質。可以參考下面表格中Type V的圖示。
Capped via (蓋孔)：在IPC-4761文件中「Capped via」似乎需要配合「Filled via」一起作業，而「Capped via」則是在「Filled via」的上方覆蓋一層金屬鍍層以之與「Covered via」作區別。適合用在有高密度連接(HDI)需求的PCB，特別是Via-In-Pad或Ball-on-Via焊墊可以節省空間。當填孔作業發生瑕疵不完全填孔或voids時，可能會發生焊墊表面凹陷(dimple)造成焊錫不良，造成導通的長期信賴性問題。另外，經驗顯示當Ball-on-Via使用樹脂填孔容易發voids，一旦填孔上方的金屬焊墊又不厚時，就容易在回焊的過程中讓voids中的空氣進入到錫球中並吹脹球體與鄰近錫球短路。可以參考下面表格中Type VII的圖示。
不過工作熊個人建議，不只要管控錫球的「融錫時間差(LTD)」，更要順便管控其「固化時間差(STD, Solidus Time Delay)」，其實只要稍加注意調整回焊爐溫度的設定條件兩者就可以同時得到改善，何樂不為。
